Category: Tech Tips

When we install systems, we should have three main goals in mind: maximizing longevity, efficiency, and capacity. We want our units to work as long as possible, use the lowest amount of energy, and move the greatest amount of BTUs possible with a proper mix of sensible and latent.

Unfortunately, some installation practices can severely reduce these three qualities. Even small installation mistakes and oversights can affect a system’s performance, from evacuating without a micron gauge to forgetting to check for more leaks after finding one. 

This article will take an in-depth look at some of the worst installation practices that affect residential systems. We’ll also offer alternative best practices to help you with your future installations.


1. Improper brazing, flaring, and leak testing

Unsurprisingly, a good chunk of the system’s functionality relies on its piping system. Brazing, flaring, and leak testing are the main procedures we perform on pipes, and there are plenty of mistakes that can make you either create or fail to detect leaks.

Brazing: the basic mistakes

In HVAC work, brazing copper is a critical skill that takes lots of time and practice to hone. As a result, some common beginner mistakes can lead to poor installs. The inexperienced tech may move the torch around too much and may not heat the copper enough to draw the alloy into the joint. In cases like those, it’s best to focus on being steady and deliberate, which will come naturally with practice. Overheating of valves and components is also a common issue and is easily resolved by being more deliberate in prep to keep cold rags or heat blocking putty on components we are working on. 

You can avoid longevity and efficiency issues by sanding copper before cutting it. As always, you’ll also want to make sure to clear your workspace, slip on some gloves, and wear properly tinted safety glasses before you start brazing. 

To prevent contamination, seal off open tubing and don’t allow burrs to fall into the copper. The best thing you can do is hold the tube sideways or tilt it down during deburring or reaming. It also helps to have some practice under your belt and a good deburring tool.

Brazing: torch mistakes

You’ll want to heat the copper between 1200° F and 1300° F, and it will likely be a dark to medium cherry color. You can see a color chart below to determine the corresponding colors for temperatures. (Note: this chart is for pure copper. Mixtures may deviate from this table.)

When it comes to the actual torch, you’ll want what’s called a carburizing flame. The flame has layers, which we call “feathers.” All flames will have at least two feathers, but a carburizing flame has three. You can see that in the image below.

As you can see, the carburizing flame has an acetylene feather, which is an aqua-colored middle layer between the innermost white cone and the main blue feather. Even a neutral flame without an acetylene feather is okay, but you don’t want an oxidizing flame (no acetylene feather and a shorter cone). When copper oxidizes, it creates weaker bonds.


Creating flares with proper depth requires lots of skill and practice. If you don’t have a lot of flaring experience and don’t feel confident with the procedure, we recommend using a modern flaring tool to help you.

As with brazing, proper deburring is also a concern. Instead of worrying about copper pieces falling into the pipe, you must be careful not to overdo the deburring and thin out the line. Of course, you also want to make sure you deburr in the first place; otherwise, the flare might not seal well.

Cross-threading is common with inexperienced techs. The only way to overcome cross-threading is with patience and practice. Get used to pushing the two surfaces together gently and threading on carefully unit it’s aligned properly and threads with ease, not just jamming and cranking. 

Over- and under-tightening are both quite common mistakes, especially among new techs. We highly suggest using a torque wrench to help you feel out the tightening and locate a sweet spot. 

All you need is a little dab of assembly lubricant, so you don’t need to seal the threads with a big gloop of sealant. Put just a little on the back of the flare and the surfaces you’ll be connecting. Some technicians don’t use thread sealant at all, but we find that assembly lubricant (particularly Nylog) works wonders at helping to get a better connection. 

Leak detection: pressure testing

Proper nitrogen pressure testing is a crucial part of good leak detection. The most prevalent mistakes tend to happen because techs get impatient and move too quickly missing leaks.

For example, some techs don’t hold the pressure test for at least 20 minutes. Honestly, 20 minutes isn’t even enough time to find small leaks in most systems; we recommend holding a test under nitrogen for at least an hour, even up to a day or two for very large piping systems like VRF and Grocery. Twenty minutes is the absolute bare minimum for a small residential system with a very accurate digital gauge. 

We don’t recommend using gauge manifolds, as they may leak and lead to confusion. Instead, we suggest using probes to reduce leak points and minimize confusion during the testing phase.

When you perform pressure tests, be sure to do it at low side test pressure. Installations typically require you to test the line set and evaporator coil, so you’ll want to check your equipment and test the recommended low side test pressure. Many techs test at lower pressures than recommended, such as testing at 100 PSI for every system (even though the specs may list 300-350 PSI). When you test at lower pressures, it takes longer to find leaks and increases the probability that you’ll walk away from the job with undetected leaks. 

Electronic leak detection

The previous section on pressure testing dealt heavily with nitrogen. It’s worth noting that the electronic leak detectors we’re about to cover CANNOT detect nitrogen. It will only respond to whatever it’s supposed to react to, which is usually refrigerant. If you want to detect nitrogen, you’ll have to either mix some trace refrigerant into the line (per EPA standards) or perform a bubble test. (Note: if you do a bubble test, make sure to remove those bubbles before going through with an electronic leak detector.)

The first electronic leak detection mistake is pretty self-explanatory: moving too fast. When you perform any task too quickly, you may skip over critical information. When you use an electronic leak detector, you must move very slowly over the coils, pipes, or whatever you’re testing. 

There are also a few places where techs can get confused. For example, some techs don’t remember the differences between the heated diode and infrared detectors. These detectors work differently, and you may miss a leak if you treat an infrared detector the same way you’d treat a heated diode one. Heated diode detectors continue to go off when you stop them over a leak, but infrared detectors’ signals will shut off when you stop moving them; you must continuously move an infrared detector over the leak.

Some techs also mistake port and gauge leaks for system leaks, especially when they detect leaks on the outdoor condenser coil. Refrigerants are naturally heavier than air and will settle in those areas. You’ll get a false positive test for a leak if you aren’t aware of your equipment and can’t distinguish a port or gauge leak from a system leak.

You also want to make sure that you don’t test for leaks while your indoor fan is running. The coils and lines should be as still as possible, as additional movement from the fans can make it difficult to locate leaks.

You can avoid these mistakes by purchasing a good leak detector (prepare to spend at least $300), knowing your equipment, and being diligent as to avoid making oversights that muddle your test results. Also, don’t walk away after finding one leak! There could be more in the line, so check the whole line slowly.


2. Failing to flow nitrogen

Oxidation can also occur when techs braze without nitrogen. Cupric oxide forms when oxygen and high temperatures mix, and the cupric oxide can fall away from piping and clog valves or be an all-around nuisance. 

Before you flow nitrogen, be sure to purge the lines. Moisture in the lines may condense during pressurization, which isn’t good for the system. After you remove your cores, you’ll use relatively high-pressure nitrogen to purge the lines before you flow.

As a best practice, we recommend flowing nitrogen at 3-5 standard cubic feet per hour (SCFH) while you braze. This pressure is quite a bit lower than purge pressure.

Bert made a highly informative video about flowing nitrogen on the HVAC School YouTube channel, which you can watch here. It reinforces the information in this article while providing some extra tips and footage of real-life field experience.


3. Poor Evacuation

Core removal is necessary for proper brazing, so this is not an issue if you keep the cores removed. However, some techs add them back before evacuating and don’t take them off again. Evacuation is a lot harder with Schrader cores still in, and they make your evacuation setup heat up pretty quickly. Overall, it’s best to keep the cores off until the system has a refrigerant charge.

Some techs say that they don’t need a micron gauge because they can read the suction gauge. While suction gauges can technically give you an idea of your vacuum, the scale is far too large to provide you with an accurate idea of your vacuum. It’s easier to have a more precise understanding of the vacuum when you use a micron gauge, so don’t toss it aside. The best way you can use a micron gauge is to attach it at the system’s furthest point, not the pump.

Connecting and taking care of your vacuum pump

The vacuum pump does all the heavy lifting for you. However, you can still make some mistakes that slow down your evacuation. Some errors can also result in leaks and take their toll on your vacuum pump’s health and longevity.

We don’t recommend evacuating with a manifold because it may make the process take longer, and manifolds often leak. Using a manifold is technically not incorrect, but it can present you with avoidable problems. If possible, it’s best to use core tools instead and hook the system directly up to the vacuum pump. 

It’s also not a great idea to use refrigerant hoses for evacuation. We use refrigerant hoses to transport the refrigerant, and it’s not a good idea to get oil, moisture, and other gases into those hoses. They are also relatively prone to leaks. Instead, use dedicated hoses (the shorter and larger, the better).  

Leaving a pump open to the atmosphere when it’s not in use presents a contamination risk to the oil inside the pump. Luckily, this blunder is easy to avoid if you stay attentive. You just cap the pump or shut it off when it’s not in use. Of course, be sure to perform proper pump maintenance as well.


4. No Airflow Setup

As many of you already know, maintaining proper airflow isn’t as easy as replacing the filter once every few months and making sure the duct design is ideal. You must make sure you have an appropriate fan and that the static pressure stays in an acceptable range. We see many mistakes that result from substandard airflow setups (or simply not making an effort to check the airflow in the first place).

While duct design may be the most obvious indicator of poor airflow, the filter also has a massive impact on airflow. The main mistake that technicians (and ordinary homeowners) make is assuming that a filter works because it fits. A filter’s suitability depends on more than just the MERV rating and size dimensions (though those are also important). 

To combat these challenges, choose your filter wisely as not to be too restrictive. It also helps to be fluent in the airflow rate/initial resistance charts. That way, you can understand how much of a pressure drop you can expect from a filter with a given airflow rate. I’ve attached one of those charts from a MERV 11 filter below.

CFM targets

CFM (cubic feet per minute) targets are moving targets. The mere thought of having a “target” CFM is a point of contention among scientists and educators, but that’s not what we’re looking for. Many techs can benefit from at least being aware of a typical CFM target range under design conditions.

Some techs assume that 400 CFM/ton is a rule of thumb, but it’s not. On a typical Florida summer day, our targets are typically around 350 CFM/ton, but that changes by model and by season. 

Take some extra time to assess the operating conditions, desired temperature, and ambient temperature to calculate the desired BTU output. You can get a more precise target CFM if you do the math, which will help you understand and achieve your target total external static pressure. (Of course, we think the industry would see some significant improvement if manufacturers could be more transparent about the targets. However, that’s just a dream right now.)

Total external static pressure (TESP): installation and testing 

An alarming number of techs don’t bother checking the TESP and adjusting the blower speed accordingly. In many cases where systems have incorrect airflow, it’s been wrong since the beginning. 

When you install a system, you’ll want to make sure the blower speed and airflow are ideal from the get-go. According to one of our articles written by Neil Comparetto, you can determine the blower speed by measuring the TESP and cross-referencing it to the manufacturer’s literature.

You’ll want to measure static pressure drops across multiple parts of the system. TESP is NOT an especially helpful measurement by itself because it won’t tell you where the source of the problem is. So, we’d consider it a mistake to not take  static pressure readings in the return/supply individually as well as across the coil, filter, and air handler/furnace. The solution, of course, is to take those measurements. That will be harder than you might think, but the best thing you can do is approach each job with a trusty manometer in hand.

As a general best practice, it also helps to ensure that the wiring and settings are correct before you start looking for pressure drops all over the system.

It’s also worth mentioning that some techs mistakenly assume that the design TESP will always be 0.5”. That’s not true for every case, so you’ll want to be familiar with your system and understand its unique TESP needs.


5. Compressor overheating and flooding

Mistakes on other parts of the system may have a catastrophic impact on the compressor. Nobody wants compressor failure, so we’ll talk about the errors primarily responsible for fatal overheating and flooding.

We just talked about airflow and won’t go back into it, but poor airflow can damage the compressor over time via poor oil return and flooding. We’ve already described the common airflow mistakes and best practices, so please consult section #4 for those.

Some techs don’t weigh their charge in or out of the system. If they don’t use scales, they have no way of keeping an accurate account of the system’s charge. Both excessive and insufficient amounts of charge strain the compressor over time. To avoid this issue, simply weigh the refrigerant charge in and out. That way, you’ll know what’s in the system and if it’s an appropriate amount or way out of line.

Flooding: crankcase heaters and superheat

Flooding occurs when the refrigerant migrates to the crankcase during the off cycle and condenses to a liquid. When you start up the compressor again, the refrigerant boils off with the oil, foaming it off and removing it from the system. It’s horrible for the compressor and leads to failure a lot more quickly than normal operation does.

The manufacturer may recommend using crankcase heaters to keep your compressor crankcase warm during the off cycle and prevent flooding from the liquid refrigerant. We’ve included a picture of your standard bellyband type of crankcase heater below.

Manufacturers may also recommend using liquid line shutoff solenoids and hard shutoff TXVs. All of these can be good defenses, but it’s best to check the manufacturer literature to see what they recommend for their systems.

Low superheat can also indicate an increased flooding risk. The superheat gives you a clue about the TXV condition, and the TXV is one of the barriers against flooding. Sometimes, the TXV’s bulb will be improperly mounted, and the TXV can flood back if that’s the case.

Overheating: temperature, airflow, and hard starts

There are three main high temperatures you must look out for: ambient, superheat, and suction line.

A system running through a hot attic will be hotter than one kept in a relatively cool closet. Obviously, the system in the attic will be more prone to overheating. High ambient temperatures make it easier for a compressor to overheat.

Low superheat indicates an increased flooding risk, but high superheat can indicate a compressor overheating problem. It could be set too high and demand too much work from the compressor, or maybe the TXV could be set improperly. Either way, it will strain your compressor and reduce its lifespan and efficiency. Compressors are refrigerant-cooled, and they need the proper temperature to function correctly.

You’ll ideally want to keep the suction temperature around 65° F. There will be cases when it’s higher, such as during a hot pull-down, but it shouldn’t otherwise exceed 65° during normal operation.

Another way to prevent overheating is to use the factory hard starts where recommended. Many techs don’t use those hard starts, especially in long-line applications. If you don’t use them when your compressor is under load, it may not start at all, or it could go into thermal overload. 


Even though we’ve only listed five install mistakes, it feels like we’ve covered a lot more than that. There are so many things to consider to avoid making mistakes, and we understand that it can be overwhelming to take all of this in at once. 

Nevertheless, we hope this article can help you perform some self-reflection and guide you in practicing better installation habits.

In our line of work, it’s all too easy to let your bad experiences inform your present and future experiences. This is especially true if you have a high emotional range. Having “emotional range” is a polite way of saying that you’re a bit up and down (maybe even neurotic).

We don’t believe that sensitivity is a weakness or something that should be judged. In fact, being in-tune with your senses and emotions can give you an advantage in almost any career. However, having a high emotional range can also make it easier for you to feel annoyed with your work, avoid people you don’t like working with, and take criticism personally. It’s easy to let your past frustrations and failures seep into your decision-making skills and outlook on work.

The key to keeping the past from getting in the way is to manage your emotional range. It takes a lot of practice, self-awareness, and patience, and it can be a frustrating process in its own right. Nevertheless, we’ve put together a few things you can do to turn your emotional range into an asset in your HVAC career. 


Recognize your emotional responses

People with a high emotional range have strong emotional reactions to the people and things in their environment. Of course, not everybody responds the same way. Some people become visibly bothered and may handle parts more roughly or unknowingly slam doors. Other people may hold their feelings deep inside and become more sluggish as their emotions weigh them down over time.

There are plenty of other reactions, and all are valid. Still, you have to recognize how you respond to your environment before learning how to manage your emotional range.

For example, you can place customers or coworkers into categories of people you wouldn’t want to work with in the future. There are benefits and drawbacks to this type of emotional reaction. While you develop a strong sense of what you like and dislike, you also create new “walls” and start to feel exceedingly unwilling to do specific jobs or work with certain people. That’s just one way that the past can impede your job performance and satisfaction.

You may make money from all of the jobs you perform for those problematic customers, but the frustration and difficulty no longer seem worth it.

There has to be a healthier way to approach these difficult people and situations. While work is a necessary part of life, its emotional impact on you should not be so strong that it interferes with your relationships, personal life, and overall well-being. 


Look for opportunities without judging motives

It’s natural to approach potentially unpleasant work situations and social interactions with your guard up, especially if a coworker or client has been problematic before. You don’t want them to tear you down, so it’s normal to judge others’ motives so you can prepare for their worst and protect yourself.

When you work with a hypercritical coworker, you may think, “How is he going to nitpick my work this time?” Or you may drive up to a difficult customer’s house or business and think, “I hope she doesn’t blame me for something that’s not my fault again and then complains about the prices being too high.” When you already have possible outcomes in mind, it may seem easier for you to numb yourself to them, at least on the surface. 

But does that really put you at ease?

No, it just has you in fight mode before you even get started.

This piece of advice is MUCH easier said than done, but hear me out: you can approach each situation and look for opportunities to make it a win-win situation.

The key is to look for opportunities for growth or learning and decide beforehand that you will focus on the result and do your best to ignore the noise that distracts you from your mission.


Acknowledge that criticism may hurt, and that’s okay

Criticism is a part of any career field, but it’s especially prevalent in HVAC and construction. So many elements of our work are open to criticism: craftsmanship, efficiency, precision, customer service skills, and so on.

Unless you’re a glutton for punishment, the criticism will probably sting at least a little bit. Some people may want to hide, and many more will want to defend themselves. 

Regardless of how you respond, there’s a near 100% chance that your reaction won’t change the critic’s mind. No matter how hard you fight back or how gracefully you take the criticism, the critic has their opinion and probably doesn’t care about your point of view very much. 

You can’t convince people that you’re something different than they think you are. That may sound a bit depressing on the surface, but it can also be a freeing sentiment. In the end, you’re not totally responsible for other people’s opinions of you. When you let go of the need to protect your image and ego, you can be fully present to help and support others who will value what you bring to the table and you can just view everything as an opportunity to learn. 


Release your worries about the past and future outcomes

I know, I know. This piece of advice may seem about as useful as telling a clinically depressed person to stop feeling sad. 

The entire process of releasing your worries doesn’t happen overnight. It also doesn’t happen just because you may tell yourself, “Stop worrying about what this person will say to me.” It’s gradual, and it takes some self-reflection and acknowledgment of what’s in your control and beyond it.

Instead, you might consider entertaining the possibility that the people who hurt you in the past were in a lot of pain themselves. People who are in emotional pain sometimes look for outlets for their pent-up frustration and aggression. 

Let’s say that a customer’s EC motor failed. They have an old A/C unit that doesn’t have replaceable parts readily available, and they’ve been roasting like a rotisserie chicken in their house for a couple of days. When you finally get there, they lash out at you because you don’t have a suitable aftermarket fan motor replacement.

Does it feel bad? Probably. But was any of that your fault? No! 

The verbal abuse was completely unwarranted, and it’s a shame that it happened. However, that customer wasn’t actually fed up with a problem you caused. It may not feel good that they took out their frustrations on you, but you might feel some peace in knowing that the outburst was their problem. You couldn’t have done anything in your power to stop it.

Should you ever work with that customer again, you might be able to empathize with their first situation and offer support if they need it the second time. If they’re still nasty, you can dismiss their negativity as their problem and not yours.


Accept that we’re all a little crazy

Nobody really wants to deal with “crazy” customers. Working with a “crazy” coworker might also sound interesting at first, but their recklessness, obnoxiousness, or other glaring flaws will start to bother you after some time.

So, how do we avoid crazy people?

We CAN’T avoid crazy people. WE are crazy people!

That’s right. Each person reading this has their own brand of crazy. Somewhere, somehow, somebody will find every single one of us difficult to work with for some reason. We will probably feel the same way about everyone else out there. 

That seems like another grim acknowledgment, but it can truly help us work with our difficult coworkers and customers. If you can acknowledge a person’s difficulties and “nuttiness,” you can navigate their personality and help both of you find a win-win situation. You don’t judge their motives. You don’t judge where they’re going or what they might do. All you can do is try your best to find something that works for both of you.


We hope that these tips will help you look beyond those painful past experiences. You may even become a better, more productive HVAC technician. Most of all, we hope you can feel happier and less stressed by the crazy people at work.

Full disclosure, as a technician I was guilty for many years of setting the fan to “on” at the thermostat. I never really thought of any of the negative impacts that could happen.
I wanted to circulate the air and to keep air moving through the high-efficiency air filter that most of our houses had. Later I learned that in many scenarios fan “on” is not a good idea.

For this discussion, I will be talking about the cooling season in a humid climate. Many adverse impacts may occur in the heating season, depending on the region.

Things to be aware of when running the fan “on”.

Condensate on the coil after a cooling call with the fan running will evaporate back into the living space. Some thermostats combat this by having a fan off period at the end of a cooling call to let the coil drain.

If the ducts are in unconditioned spaces, outside the thermal envelope of the house, the sensible heat will be added back to the space. If the ducts are warmer than the air traveling through them there will be a transfer of heat. If there is any duct leakage latent heat (moisture) will be added as well. Latent heat gains do not only apply to return duct leaks. Supply air leakage can also contribute to this.

It is common that the HVAC system can cause the house to go under negative pressure. When this happens sensible and latent heat will be added. A common cause of the pressure imbalance is when the duct system is in an attic or crawl space, and the return duct has fewer connections than the supply ducts.

Since the supply has more connections than the return there is more of a potential to leak air. If the supply air leaks into the attic or crawl space this can cause the living space to go under negative pressure. The leaked air is replaced with either attic, crawl space, or outside air. One CFM(M3/h) in = one CFM (M3/h) out.

Ducts in conditioned spaces with panned returns can add latent and sensible heat as well. This happens when the panned joists and studs are not sealed by the HVAC contractor on all six sides. Joists and studs are part of the building network that when not air sealed during construction, by the builder, they will communicate with the air outside the building envelope. With blower door testing, and air changes per hour requirements now code in many jurisdictions houses are being built much tighter. In an older home, with panned returns, expect to be bringing in some outside air.

Even if the ducts are sealed, and 100% in the conditioned space it still costs money to run the fan. Running a PSC motor 24/7 can be costly. (ECM motors on a properly-sized duct system do have considerably lower operating costs when compared to PSC motors.)

The duct leakages and pressure imbalances mentioned above will also occur during a cooling call. Most of the time these issues can go unnoticed because of the ability of the HVAC system to overcome or mask them.
The goal is to get maximum customer comfort with minimum power usage and maximum system longevity. In many cases, the fan being left in the ON position detracts from these goals.

Hopefully, this Tech Tip will make you think twice about running the fan “on”. Every situation is different. I encourage you to think outside the box, if you are not already.

— Neil Comparetto

Ohm’s Law is pretty straightforward; you multiply ohms by amps to get the voltage. Using variable E to represent voltage, variable I for amps, and variable R for ohms, the equation for Ohm’s Law looks like this: 

E = I × R 

You can figure out the number of amps in a system using basic algebra to turn this multiplication equation into a division one. Divide both sides by R to isolate the amperage (E/R = I). From there, you’d take the voltage reading and divide it by your ohm reading. Your equation should yield the amperage.

However, if you’ve tried doing that and then comparing your answer to the actual amperage measurement, you’ll know that there’s a lot less current than the equation would lead you to believe. Ohm’s Law appears to be inaccurate most of the time, and it’s a bit frustrating because there’s such an emphasis on it in electrical education, but it doesn’t seem to work in the field. Why do we even learn about it in the first place?

The truth is that Ohm’s Law is still valid and works just fine. It’s merely impractical for many of the alternating current (AC) components we work on. That’s because the ohms we measure don’t account for all resistance types that make up total impedance. Inductive reactance is one of those types of impedance, and our multimeters and ohmmeters can’t pick it up. It also happens to be a byproduct of the inductive loads we regularly use.


Maybe I’ve gotten a bit ahead of myself by tacking “inductive” onto words without explaining them. So, what is a load, anyway?

Simply put, a load is a component that does something in an electrical circuit. For example, a lightbulb is a load because it lights up when it receives power. In terms of the work we do, motors and transformers are loads that we regularly use.

There’s quite a difference between lightbulbs and motors or transformers. They each belong to different load categories; lightbulbs are resistive loads, while motors and transformers are inductive loads. Resistive loads have a heating component (toasters, oven coils, and electric heaters are also resistive loads), and inductive loads have an electromagnetic element. 

We see plenty of inductive AC loads in the work we do. (AC refers to alternating current. Inductors don’t have a significant effect on DC circuits.) Inductive loads facilitate magnetism and (usually) mechanical movement.

Transformers are the exception to the movement rule. Transformers only transfer electric energy via electromagnetism and don’t have any moving parts. Still, the point stands that magnetism is the core trait of inductive loads.  


I don’t want to dwell on electromagnetism for too long. Still, I think we should have a solid grasp of its fundamentals before we discuss inductive reactance. 

When current travels through a wire, it will make a small magnetic field. It stands to reason that a coiled wire over a small area would create a larger, stronger magnetic field. After all, the current runs through the wire several times in the same small space. 

The magnetic field expands as the current runs through the coil, and electrical energy accumulates as magnetic energy when the field is at its maximum size. When the current stops flowing, the field shrinks until it disappears entirely, returning all the stored energy to electric energy. It takes a bit of time to store and release the power, so you’ll always see a lag in the current. 


Magnetism vs. heat

As we just explained, inductive loads hold their energy in magnetic fields. This energy storage method is why resistive loads heat up quickly, but inductive loads do not.

The magnetic field’s energy storage impedes the current. The current doesn’t travel from point A to point B in the circuit without experiencing that delay. The delay reduces the total power delivered, so inductive loads don’t heat up to the same degree as resistive loads. 

On the other hand, voltage and current peak simultaneously in resistive loads, which allows all the power in the circuit to be delivered. Resistive loads heat up much more quickly because their circuits don’t have the same delay as inductive load circuits. That’s why solenoids, relay coils, and motors don’t act as heaters that overload constantly. 

Reactance and impedance

Reactance is a component’s opposition to the current flow, just like the lag we talked about in the previous sections. As this description suggests, reactance is a form of resistance. 

Reactance is a type of resistance called impedance. (Remember when I said that the energy storage delay impedes the current?) As such, inductive reactance is impedance from inductive loads. Although lightbulbs and other resistive loads present some form of resistance while operating, the resistance from inductive loads is significantly higher.

The total impedance is a combination of reactance and resistive ohms, so they both make up the total number of ohms.

Like other sources of resistance, we measure reactance in ohms. However, as I said earlier, you can only use your multimeter or ohmmeter to measure resistive ohms. An ohmmeter can’t measure reactance, so there’s no way you can measure inductive reactance beforehand. As a result, Ohm’s Law can’t be used to find current by measuring ohms in most of what we do.

The design of electrical components dictates the resistance and impedance within them. The wires’ winding affects the behavior of inductive reactance, ohmic resistance, and current, as you’ll read shortly.


Inductive reactance and current

You may have noticed that motors draw higher current upon startup. Many people call this the inrush current, which will typically be 4-6 times higher than the standard running current. 

The current is strongest at the start because it takes a little bit of time for the impedance to push back against the current resistance. That usually happens after the motor starts spinning. However, once the inductive reactance has established itself, it strongly resists the current and reduces the amperage as a result. 

If you use Ohm’s Law to find the amperage and yield a number that’s much higher than your ohmmeter’s reading, that’s because you haven’t accounted for the effect that inductive reactance has on current. Again, inductive reactance won’t show up on ohmage readings, but it still impedes the amperage and results in a much lower amperage reading than expected.  


Winding: inductive reactance and transformers

We’ve already established that transformers are the odd ones out because they lack moving parts. Instead, transformers transfer electrical energy from one circuit to another via electromagnetic induction.

Transformers have two sets of windings: primary and secondary. The primary winding connects directly to the AC supply, and the secondary winding connects to the load (output terminal). A magnetic core binds the primary and secondary winding.

When a transformer has no load on the secondary winding, it draws almost no current on the primary winding. That’s because the impedance on the secondary winding is extremely high, and it becomes a near-perfect inductor when there is no load. 


Did we really go through all this information just to prove that Ohm’s Law is not a sham? Absolutely. Our ohmmeters and multimeters only give us part of the picture, and it’s unfair (and inaccurate) to judge the validity of Ohm’s Law with our limited measurements. 

As you can see, the electrical world is complicated, and there’s a lot more to resistance than the ohms we can measure with our devices. Just remember that the amps don’t magically disappear; they get impeded by inductive reactance.

The easy answer is 32v. Class dismissed.

I’m only joking, of course. Finding the difference between 208 and 240v power supplies may sound quite simple, but there are some pretty sharp fundamental differences.

Apart from the obvious differences in overall voltage, 208 and 240v power supplies use the electrical company’s power differently and the motors see different sine waveforms. The potential across these waveforms varies, which is where we get the different voltages from. We also tend to use 240v power in residential applications, while we use 208v in light commercial applications.

This article will go over the differences between single- or split-phase 240v motors and 208v ones. It will also discuss the applications of each power source type and why it may not be a good idea to use 208v motors for 240v applications.


Visualizing voltage

Before we dive into the differences between voltage types, we should probably talk about the way we visualize voltage and how it relates to motor movement.

As you know, electric motors rotate in a circular motion. We don’t view the corresponding voltage as a circle, though. We visualize and map voltage patterns as sines, which are wavy lines that rise and fall at a regular rate on a chart. 

A sine wave takes the rotation of a circle (in degrees) and compares it to unit time. Look at the picture below.

As you can see, the sine wave “rolls out” the circle by plotting the changes in its degrees as time progresses. When the motor rotates 30° from a starting point of 0°, you can see that rise on the sinusoidal waveform. The sine wave peaks at 90°, as that’s the highest point on the rotation. It also bottoms out at 270°, which is the lowest point. You can see all of those on the left circle. 

Each power leg has its own sinusoidal waveform, and that’s where you will see the differences between 240v single-phase motors and 208v motors.


240v single-phase and split-phase

We sometimes use the terms single-phase and split-phase interchangeably, and it can be confusing. Although they roughly mean the same thing, they sound a bit contradictory, so we’ll explain the names.

“Phase” refers to the use of the original power from the utility. The power company offers three legs of power, so a single-phase system only derives its power from one of those legs.

We also refer to single-phase power as split-phase power because they split into two separate, opposing phases. If you look at a waveform, you’ll notice that there are three lines: two opposite sines and one flat line. The flat line is the center tap, and the sines are the split phases. 

The split phases are 180° directly out of phase and always have directly opposing values. If you think about our earlier circle, 90° and 270° are 180° apart from each other, and they are the peak and trough of the wave, respectively. Therefore, when one of those split phases reaches its highest point, the other phase will bottom out. However, they cross once they reach the zero point (on the center tap). That’s because 0° and 180° are on the same horizontal line in a circle. You can see all of this in action below.

Single- and split-phase motors are usually 240v or 230v. Some split-phase power sources may be 220v, 120v, 115v, and so on, but higher-voltage applications are more common nowadays. You’ll probably see 240v the most often, but you may see some even higher voltages, like 245v or 246v.


So, where does the 208v come in?

As you could probably assume, power sources with 208 volts are NOT really single-phase. They require two legs of power from the power utility, whereas 240v motors and appliances only use one.  

You will commonly see 208v present in three-phase buildings when “single-phase” equipment (like an AC condenser) is wired to two legs of three-phase wye power. 

Here’s where some people get confused: if a split-phase motor splits a single power source into two 120v phases, then shouldn’t a motor that uses two legs of 3-phase power also equal 240v?

No, and here’s why:

Remember when I said that the sine waves of split-phase motors are 180° out of phase? The power legs of 208v motors are actually 120° out of phase. The distance between the sine waves isn’t totally opposite. They can’t reach their full potential if they don’t directly oppose each other, and the voltage is lower overall as a result. 

You can especially see this when you look at a graph. The sine waves look more like they wind around each other, not cross over a fixed center point. The maximum potential between the points is lower overall. You can see this in the image below.

Can you use 208v motors instead of 230v/240v?

You may notice that some single-phase motors are rated for 208/230v. So, it may seem as though the motor will find ways to make up for the lower voltage. That’s not quite how these motors work. You’ll see some differences in performance if you use a 230v motor for a 208v application, even if it’s rated for both.

You can’t expect the same degree of performance when you use 208v on 230v (or higher) applications. For example, when you use a 208v motor in a residential condensing unit that typically requires 230v, the unit won’t run at its full capacity.

Some people say that you can expect a higher current when you use 208v in place of 230v, but that’s quite rare. The motors don’t magically makeup in current what is lost in voltage, even if it might make sense to see higher current from the rated watts or horsepower. Instead, when a typical 208/230v motor is connected to 208v, it usually slips more and runs at a lower output than on 230v or 240v. This results in lower wattage used at a lower efficiency, which adds up to few BTUs produced.

230/208v  also have a harder time starting up, and they may produce a problematic voltage drop more often when connected to 208v. Voltage drop usually isn’t an issue for split-phase 240v systems. However, on 208v, the motors start with lower voltage. Therefore, the same amount of drop will have a greater effect on the system.

In short, just because a 208v motor may be designed to run on 230v or 240v systems by using two power legs from the utility source, that doesn’t mean that it will provide the same output efficiency.  You will see differences in performance if you try to use 208v when 240v is typically required.

Recommended Duct Velocities (FPM)

Duct Type Residential Commercial / Institutional Industrial
Main Ducts 700 – 900 1000 – 1300 1200 – 1800
Branch Ducts 500 – 700 600 – 900 800 – 1000

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, induct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 500 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s register specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw and more mixing will occur via entrainment but also the noisier the register will be.

— Bryan

Service calls about condensing fan motor failure are quite common. Even though fan motor replacement is a standard procedure for residential split systems, it requires careful attention to safety and detail. 

This article will give you a step-by-step guide to replacing condensing fan motors. Along the way, we will also explain a few best practices for safety and enhanced understanding of the process.


1. Diagnose the REAL problem

One of the reasons why motor failure is so common is because there are many ways that it can fail.

Before you replace a motor, you should know what caused it to fail (and that you’re dealing with a motor failure in the first place). Some causes may include failing shorted, windings failing open, and bearing failure. The image above shows highlighted bearings, which often fail due to improper lubrication or environmental contamination.

Remember: just because the motor isn’t running, that doesn’t mean it failed. There are a few other conditions that can cause a motor not to run. For example, wiring problems and failed capacitors are other issues that may cause the motor to stop running. Replacing the fan motor is not going to help anything in those situations.

Heat pump systems also stop the fan motor on defrost, which is not a motor problem at all. It’s important to know what kind of unit you’re working with so you can diagnose and troubleshoot effectively.


2. Find a suitable motor replacement

This best replacement motor is an original equipment manufacturer (OEM) motor. OEM motors are specifically designed to fit the unit’s specs and are adapted to its model’s blade, static pressure, and size constraints. 

However, there will be times when you won’t have an OEM motor and will have to use an aftermarket motor on your truck. During this step, you must consider a few different criteria for finding a replacement motor.

Size and frame

Clearly, you’ll want to use a motor that fits the system. Frame and size are going to be the two most obvious factors to consider when selecting a motor. 

Size is pretty intuitive. You can’t install a motor that doesn’t fit. However, depth is what you really want to pay attention to. Even if the new motor positions the fan blades just a little deeper or shallower than they initially were, the blade placement may adversely affect the airflow. You’ll want the fan blades to remain as close to their original position as possible to prevent airflow and high head pressure complications from occurring.

While you technically can modify the application to fit a motor’s frame, we don’t recommend it. If the new motor fails due to an incorrect installation, the original motor type may no longer fit because of the modifications you made for the aftermarket motor. 

Specifications to check

When replacing motors, you’re going to want four other values to be nearly the same as the old motor: RPM, voltage range, horsepower, and amperage. 

RPMs, or revolutions per minute, should be an exact match, if not extremely close. RPM is usually related to the number of poles, which is important for the motor’s form and function. It’s worth noting that some manufacturers may list 1075 RPM as 1100 RPM or vice versa. Those work fine interchangeably, as they are both six-pole motors. However, it is not okay to use a motor with 1075 RPM to replace a motor with 875 RPM; the latter is an eight-pole motor, so the two are incompatible.

You’ll also want the voltage range to be an exact match. For most residential and light commercial applications, the voltage range on single-phase motors will be 208-230.

Horsepower and amperage measure roughly the same thing: power at a given voltage. It’s best to find a replacement with the same horsepower and amperage, but some technicians replace motors with slightly higher horsepower and amperage, which is fine within reason. However, we NEVER recommend going below the original horsepower.

If you want to learn more about replacement fans and motors, check out our articles on aftermarket motors and fan blade depth.


3. Pull the condenser fan

BEFORE you pull the condenser fan, pull the disconnect and use a voltmeter to make sure there’s no potential present. Check leg to ground on each side and leg to leg.

Take the top off and set it on the ground, fan blades up. This is a good time to inspect the blades. Check for damage, corrosion, or other conditions that will affect its performance. If any blades need to be changed out, go ahead and do that. Three things should match the original fan blade: the pitch, the number of blades, and the diameter. 

If you’re going to reuse the original blade, pull the set screw out before removing it. As you can see in the picture above, we like to clean the blade shaft with a lubricant. We then scrub it away with sandpaper or a wire brush and remove the blade. The blade should slide off more smoothly when the shaft has as little grime and mechanical wear as possible.

Another trick to help remove the fan is to hold a crescent wrench on the backside between the motor and the fan blade. Then, use your hand to rotate the blade a little bit in each direction. As you hold the end shaft in place with the wrench, the opposing rotational forces may loosen the blade up and make it easier to remove.


4. Remove the nuts and the failed motor

To access the motor, you’ll have to remove the nuts that bolt it to the top of the unit. You will need to replace these later, so take note of the nut type; they may be acorn nuts or open-back nuts. Regardless of what they are, remove them.

Once you remove the nuts, you may start replacing the motor. Flip the top over to expose the motor. At this point, you can go ahead and remove the failed motor.


5. Replace the motor

During this step, you’ll want to pay attention to the parts you have and their orientation.

Pay attention to the wiring on the new motor. You don’t want the wires to be pinched or awkwardly placed within the unit. In the picture above, you’ll notice that the wires have been correctly aligned with the conduit (highlighted in white).

When you put the motor in place, turn the top back over. You may notice that the studs are quite long and won’t allow the cover to fit back on. You’re going to cut them, but be sure to screw the nuts back in first. If you try to put the nuts back on after you’ve guesstimated the stud length, you might not be able to align the threads properly.

In many cases, you’ll need to cut the studs on the bottom side by the shaft as well.


6. Consult manufacturer literature for drainage port information

The use of drainage ports (or weep holes) varies by manufacturer and model. However, some units can fail if you don’t unplug these ports and allow condensate to drain out. Read the unit’s manual or look up the manufacturer’s recommendations to see how many drainage ports you should unplug for optimum performance.


7. Tighten down the fan

Regardless of how many set screws your fan blade has, you’ll only want to tighten them down on flats. If you only have one flat, you only tighten one screw. Be careful not to overtighten the set screw. The set screw should be snug, but overtightening could cause it to break or create a bur on the shaft that will make the blade hard to get off later.

Check that the fan’s rotation is correct. This can be tricky, but we recommend placing the top back on the unit with the rotation wires sticking out of the top of the fan. Ensure that the blades spin in the correct direction for the unit and that they don’t hit anything.

While the rotation wires are exposed, we recommend encasing them together with some heat shrink tubing. Then, use tie wires to fasten the insulated rotation wires to the inner side of the unit’s top. We secure the wires to that part of the unit to keep the rotation wires out of reach without interfering with the fan blades. When the rotation wires are exposed, they may attract children or animals and shock them if they touch the wiring.


8. Wire in the motor

This is one of the more critical parts of the process.

First, you’ll want to route your wires. Ideally, your wires have already been aligned with the conduit or channel since step #5. Now’s the time to run them through the conduit or channel and into the electrical area.

NOTE: We are referring to wire colors here on universal motors that are by far the most common in this application. Colors can vary, so always read the info on the box or data tag on the motor.

You’ll make your connections in the electrical area. There are four wires that we use in those connections: a white or yellow wire, a brown wire, a black wire, and a brown-and-white wire. You can make a connection using one of two configurations: 3 wire and 4 wire.

It’s worth noting that the brown-and-white wire is the same as the white/yellow wire, as they connect inside the motor. That’s why you can let the brown-and-white wire go unused in a 3 wire configuration.

In a 3 wire connection, the white/yellow wire connects the condenser fan motor to one side of power on the contactor (the terminal side doesn’t matter). It’s also jumped to one side of the fan capacitor. The connection occurs at T1. The black wire connects the condenser fan motor to the other side of power on the contactor (T2). The brown wire connects to the other side of the capacitor from the jumped side, and you can cap off the brown-and-white wire, as it will remain unused.

In a 4 way connection, you use all four wires. The white/yellow wire connects the condenser fan motor to one side of power on the contactor (this is T1; again, the terminal side doesn’t matter). You DO NOT jump the wire to one side of the fan capacitor. Instead, the brown-and-white wire connects to one side of the capacitor. Just like before, the black wire runs from the condenser fan motor to the other side of power on the contactor (T2), and the brown wire connects to the other side of the capacitor.

As always, make sure to tidy up your wiring when you’re finished. We know you don’t like it when other techs leave you a clump of spaghetti wires, so please don’t do that to other techs (or yourself).

For a more comprehensive review of the wires and connection configurations, check out this article.


9. Perform a final inspection

Even though you’ve been working through each step carefully up to this point, you still want to make one final check before you test the voltage of the new motor. We’ve put together a checklist for you:

  • The blade doesn’t hit any other parts
  • Everything is wired properly
  • The blade spins freely
  • Appropriate weep/drainage ports have been removed
  • The blade should be at an appropriate height in the shroud
  • The top is securely mounted and fastened
  • The rotation wires have been insulated and fastened securely


10. Run test the unit

When you test the unit, you’ll check the voltage and amperage to make sure that your new fan motor is running properly.

Check the voltage at the load side of your contactor (as seen above). Make sure that the applied voltage is in the proper range and doesn’t drop more than a few percentage points (which would be very rare).

Next, you’ll check your amperage. You measure the current on the black (common) wire as it feeds power to the condenser fan motor. It should operate in range, but it might be difficult to determine the amperage if your multimeter’s resolution is poor. You must know your meter and take the amperage measurement in a location far enough away from other wires to get a precise reading. Otherwise, the other wires may interfere. (Your brand new motor might not be overamping; it’s likely that the compressor wire’s amperage interfered with your reading.)

Of course, if you continue to pick up high readings, double-check your motor to make sure that the RPM and voltage match the original.

Once you’ve taken the readings and confirm that the new motor is running properly, all you have to do is make sure the unit isn’t making any strange noises. If you don’t hear anything out of the ordinary, then the motor replacement is complete.

Go ahead and take the time to check the full system operation while you are there to make sure everything is working properly.

If you don’t have a gas furnace or fireplace in your home, your unit’s reversing valve is probably your best friend during the winter months. 

As their name suggests, reversing valves reverse the refrigerant flow to send the hot, compressed vapor to the indoor coil instead of the outdoor coil. The system releases heat into your home, which keeps you comfortable in the winter. 

This article will go over the form and function of reversing valves on air-source heat pumps. We’ll cover the differences between air-source heat pumps and ordinary air conditioners, and we’ll discuss the forces and components involved in reversing valve operation.


What the heat pump reversing valve does

In a typical air conditioner, the refrigerant absorbs heat inside the home in the evaporator coils. Its temperature and pressure rise in the compressor, and it moves to the condenser coils to reject its heat outside.

A heat pump reversing valve simply reverses the process by cycling the refrigerant the opposite way. It makes the condenser function as the evaporator and vice versa. You can see the heating cycle in the image below.

Even though it may feel cold outside in the winter, the refrigerant can still absorb outdoor heat. The outdoor coil functions as the evaporator coil and absorbs that heat. As usual, the refrigerant proceeds to the compressor before it condenses within the indoor coil. When the refrigerant condenses on the indoor coil, the heat gets rejected to the home. The toasty air that comes out of the grilles is really just heat taken from the outdoors (no matter how scarce it may seem). 


Differences between air-source heat pumps and A/C units

Air-source heat pumps and standard air conditioners have the same fundamental parts: compressor, condenser, evaporator, and metering device. Below, you can see a picture of an air-source heat pump in standard cooling mode. Take note of some differences between air-source heat pumps and air conditioners.

An air-source heat pump can run in two separate modes: heating mode and cooling mode. As a result, it has two metering devices: one indoors and one outdoors. An air-source heat pump uses the outdoor metering device for heating mode, and it uses the indoor one for cooling mode. As you can see in the image above, a check valve on each metering device determines which expansion device to use and which one to bypass.

Then there’s the obvious answer: air-source heat pumps have reversing valves while basic A/C units do not. The reversing valve does its job by diverting the refrigerant flow in the suction and discharge lines. The reversal process isn’t as simple as turning the flow backward. The process involves rerouting the suction and discharge lines with moving parts.


“Reversing” parts of a reversing valve

A reversing valve primarily uses a sliding mechanism to divert the refrigerant flow, but a few different parts make this process possible. 

The actual part that slides to redirect the refrigerant is simply called the slide. As you can see in the image below, the slide is a mini cylinder that moves back and forth inside the reversing valve. Its location determines if the system is in heating or cooling mode.  Below, you will see a reversing valve in heating mode. The slide is the silver part on the inside of the tube where the compressor discharge feeds into the reversing valve.

That reddish part within the slide looks kind of like an upside-down canoe. So, we tend to call it a “canoe” informally. This part fits over two of the bottom lines and redirects the suction, as you can see by the big blue arrow. 


The electromagnetic solenoid

An electromagnetic solenoid valve allows the slide to move and switch operation modes. This solenoid is usually connected to the thermostat control by two wires: a blue (common) wire going to one side and an orange wire going to the other side. With that wire configuration, the system can use the orange wire to energize the reversing valve in cooling mode. You can see where the wires connect in the image below.

However, it’s worth noting that manufacturers Ruud and Rheem don’t use an orange wire to energize their systems’ reversing valves in cooling mode. Instead, they use a B-terminal or blue wire to energize the solenoid in heating mode.


Redirecting the flow (cooling mode)

The electromagnetic solenoid does not directly cause the slide to move and reverse the refrigerant flow. It merely acts as the first falling domino in a short series of actions.

The solenoid connects to a pilot valve, which acts like a mini reversing valve that causes the slide to move. When the solenoid is energized in cooling mode, it slides the pilot valve. The reoriented pilot valve can then redirect the refrigerant flow and create a pressure differential on the opposite side of the reversing valve. The pressure differential forces the slide to move to the other side, which sets the system back into cooling mode. You can see this in action by looking at the sequence of smaller red arrows in the image below. (The system is in cooling mode, as the pressure differential is on the right side and has pushed the slide over from its position in the previous cutaway image.)

However, the pilot valve itself does not create a pressure differential. The compressor creates the pressure differential when it pumps vapor. If you have a weak or faulty compressor, it might not create a strong enough pressure differential to switch between heating and cooling mode effectively. 

Look at the common discharge line in the image above. The red arrows feed into it, but they also travel to the pilot valve. They flow through the pilot valve and create the pressure differential that pushes the slide to the right. If the vapor pressure is too low in the discharge line, it may not be strong enough to push the slide in either direction.

Since the reversing valve requires a pressure differential to switch operating modes, the reversing valve cannot work when the system hasn’t had power for a little while. It may still slide over and change modes when you’ve just turned the unit off, though.


Suction and discharge lines

Keeping track of the suction and discharge lines can get a bit tricky. Four lines meet at the reversing valve: one on top and three on the bottom. (That’s why the reversing valve is also called a four-way valve.) 

It gets easier to keep track of them when you can recognize the common discharge line and the common suction line. The common discharge and suction lines will always be used for either discharge or suction exclusively. They don’t change their function regardless of the operation mode.

The common discharge line will always route from the compressor to the side of the valve all by itself. This is usually on the top but not always.

The common suction line is the middle line on the bottom of the reversing valve. No matter what the operating mode is, the suction line will always lead to the compressor.

The two lines beside the common suction line may switch between discharge and suction. Their function depends on how the reversing valve routes the refrigerant. The canoe loops the suction line back to either of the surrounding lines. Look at the lower line of blue arrows in the image below to see this in action. (This system is in cooling mode, as the pressure differential is on the right side.)

The outer line that has not been trapped by the canoe will serve as the discharge line. 


In the end, just remember that reversing valves work by shifting the path of the refrigerant. The solenoid doesn’t directly cause the pressure differential that makes the shift possible, but it sets the process in motion. The activated solenoid moves the pilot valve, which opens the paths for the high-pressure vapor to flow to one side of the slide or the other.

Recovering refrigerant sounds like an easy task at first, especially with digital recovery machines and large hoses at our disposal. Unfortunately, we all know that filling the tanks is more complicated than it seems. 

If you want to recover refrigerant and store it in a tank safely, you’re going to have to do some math. The math isn’t easy, either. You must pay careful attention to the numbers on your tank, understand the density differences between refrigerants at a given temperature, and make sure you only reach 80% capacity. All of those variables fit into an equation, and it can get confusing very quickly.

There is some good news, though. The HVAC School mobile app has a feature that does the math for you. All you have to do is plug in some numbers and select your refrigerant from a drop-down menu, and our calculator will give you the total weight and maximum refrigerant fill.

This article will walk you through the Recovery Tank Fill tool on the HVAC School app. Before we dive straight into the app usage, we’ll cover the safety basics of recovery tanks.


Tank weight and capacity

Tank weight is perhaps the most critical element of safe and effective refrigerant recovery. The keys to evaluating tank weight and capacity are usually stamped at the tank’s top rim or handle, as you can see in the image above. Those values are tare weight (TW) and water capacity (WC).

Tare weight (TW) is the weight of the empty tank. The total weight of the tank is everything inside the tank plus the tare weight. Scales only measure the total weight, so you must remember to include tare weight when you determine your maximum recovery fill.

Water capacity (WC) is the total weight of liquid water that fills the tank to 100% capacity.

As you can see in the image above, the tare weight is 16.6 lbs, and the water capacity is 47.6 lbs. However, that does NOT mean that our maximum tank weight should be 64.2 lbs.


Refrigerants and temperatures

There’s one glaring problem with using water capacity to determine the maximum refrigerant fill. We aren’t filling the tanks with water.

You’ll most likely fill your tank with R-410a or R-22, though all refrigerants are possible. These refrigerants have different densities under variable temperature conditions, so it’s incorrect (and potentially dangerous) to use water to make those calculations.

The other major issue with water capacity is that it represents 100% tank fill. When you recover refrigerant, you don’t want to exceed 80% capacity because hydrostatic pressure can build up and explode the tank if it has nowhere to go.

The tricky thing about achieving 80% capacity is that the refrigerant capacity may fluctuate as the ambient temperature affects its density. Most tanks are rated at 77° F, and AHRI bases their refrigerant fill guidelines on 77° F. However, the backs of our vans can get a lot warmer than 77° throughout the year. We base our calculations on 130° F out of an abundance of caution. 

In short, calculating the maximum tank weight is not as easy as adding the WC to the TW. You also have to factor in the refrigerant’s specific gravity (the density at a given temperature, which you must multiply by the WC). You also have to multiply that product by 0.8 to determine an 80% fill. Then, of course, you add the tare weight to get the maximum number you should see on your scale.

Smash all those together, and you’re working with an equation that looks like this: 

0.8 x WC x SG +TW

Again, WC and TW stand for water capacity and tare weight, respectively. SG represents the refrigerant’s specific gravity under the temperature conditions.


How to use the HVAC School recovery tank fill calculator

We understand that equations make some people’s heads spin. It’s also a bit of a hassle to look up the refrigerant’s density and temperature to find the specific gravity. Luckily, the HVAC School app’s Recovery Tank Fill calculator allows you to plug in a few numbers and not worry about doing math.

With the recovery tank fill calculator, all you need to do is supply the tare weight and water capacity, adjust the maximum temperature to your conditions, and select the type of refrigerant you’re using from a drop-down menu.

If you haven’t installed the app yet, you can download it for free from the iOS App Store on iPhones and the Google Play store on Androids. It’s also a good idea to check if you have the latest version of the app or need to update it.

1. Open the HVAC School app and go to “Tools”

When you open the app, you’ll see the main menu with four options. The “Tools” option is the third one down, and it has a wrench beside it. Select that option from the menu.

2. Scroll through the menu and go to “Recovery Tank Fill”

The HVAC School app has several different calculators built into it, and recovery tank fill is one of them. It’s the last option on the menu. Scroll down to it and select it.

3. Type your corresponding numbers into the first three fields

We only require you to type in three values: tare weight (TW on your tank), water capacity (WC on your tank), and the ambient temperature (default is 130° F). For the sake of an example, I have used the tare weight and water capacity listed on the example tank we used earlier (TW – 16.6 lbs, WC – 47.6 lbs). I have left the temperature at our default value.

4. Select your refrigerant type from the drop-down menu

The default refrigerant listed is R-22, but you can select all sorts of refrigerants by pressing the drop-down menu. If we don’t have your refrigerant listed, you can select the “User Defined” option and input the density yourself. 

The right side of the image above shows the expanded drop-down menu. For the sake of an example, we have selected R-410a.

5. Select “Calculate” to receive the total weight and refrigerant weight

When you finish inputting your information and select the “Calculate” button, you will receive your recovery tank fill result. You will see two different values that are slightly grayed out. 

The top value is the total weight, which is your fill weight plus the tare weight. That’s the highest number you should see on your scale. According to our example, our R-410a recovery tank’s total weight should not exceed 49.97 lbs on a scale.

The lower value is the refrigerant weight. That’s just the fill weight and tells you how much refrigerant you can store in the tank to reach 80% capacity at the temperature you provide. In terms of our example, we can keep up to 33.37 lbs of R-410a in our tank.


A few other things to mention about tanks and recovery

Another thing we should mention about tanks is that they can only handle so much pressure. Like the tare weight and water capacity, the service pressure should also be stamped near the tank’s top. Look at an example below:

If you’re having a difficult time seeing it, the stamp says DOT-4BA400.

The stamp represents the service pressure standard. The Department of Transportation (DOT) regulates and oversees the production and handling of tanks; that’s why the service pressure stamp begins with DOT. You’ll also want to look at the number at the end. The 400, in this case, means that the tank has been designed to withstand a service pressure of 400 PSIG. However, it has been tested to withstand double that: 800 PSIG.

Tanks can explode under excessive pressure, so be sure to avoid high-pressure conditions or extreme temperatures with high-pressure refrigerants. It’s also a good idea to maintain tanks well and discontinue use if there are dents or other signs of damage.


In the end, be safe and sensible with your tanks. Read the tanks and pay attention to them, but don’t get too stressed out by the math involved in tank weight and refrigerant capacity. The HVAC School app can take care of that for you.

This article deals heavily with entropy. Entropy is not a simple topic, so we highly recommend checking out HVAC School’s Entropy in Refrigeration and Air Conditioning for some background information.

Describing isentropic compression is a daunting endeavor.

We all recognize the term compression, and I’m sure most of us can deduce that isentropic has something to do with entropy. But what exactly does isentropic compression mean? How and when do we see it at work?

I’m not an engineer by any stretch of the imagination, but I’d still like to explain what isentropic compression is and why it matters to HVAC/R techs.


The rules of entropy

Before we dive in too deep, let’s get some basic entropy rules out in the open. 

Simply put, entropy is a state of disorder when molecules get more disorganized and spread out. It is different from enthalpy, which measures heat exchange in our HVAC/R systems. We can treat entropy as a measure of wasted energy potential.

Entropy can only increase overall. We see an entropy “decrease” in the refrigeration cycle, but it refers to entropy within the system. When entropy appears to decrease in the system, the overall entropy increases outside the system. Entropy can theoretically stay the same. However, physical processes like friction will always add a little bit of entropy.

Within the refrigeration cycle, temperature and pressure directly affect the entropy in the system. Rising temperatures will increase the entropy, and increasing pressure will decrease the entropy. Compressors raise the temperature and pressure of the gaseous refrigerant.

It would make sense for entropy to increase when the temperature rises because molecules move more quickly in higher temperatures. If there’s such a rapid temperature increase in the compressor, shouldn’t the entropy skyrocket with it?

No, and here’s why:

Compressors heat the gaseous refrigerant while applying lots of pressure. Adding pressure decreases entropy because it rapidly decreases the gas volume. That gives the gas molecules less space to move around. When the gas molecules have less space to zoom about, there’s less disorder overall.

The entropy changes that occur due to increasing temperature and pressure will ideally cancel each other out during 100% efficient compression.


What does isentropic compression mean?

We’ve already established that isentropic has to do with entropy.

Isentropic has the prefix iso-, which means “same.” Basically, isentropic compression is compression in which the entropy stays the same within the system. It does not increase or decrease.

Look at the standard T-S diagram of the refrigeration cycle below.

A T-S diagram of the refrigeration cycle in which the compression stage is highlighted.


You will notice a vertical line during compression. I’ve highlighted it. The temperature (T value) increases, but the entropy (S value) stays the same. That’s what isentropic compression looks like on a graph. You’ll see that vertical line on most T-S diagrams of the refrigeration cycle.

However, that diagram is only theoretical and represents ideal conditions. In reality, the compressor will never be 100% efficient, as I said earlier. Friction and other sources of mechanical inefficiency will always occur and increase entropy as the system performs work.

But that begs the question: if isentropic compression is physically impossible, why should we care about it?


Isentropic efficiency

Isentropic compression in HVAC and refrigeration is a pipe dream. I’ll admit that much.

Being the perfect human being is also a pipe dream. It’s human nature to make mistakes on occasion. But does that stop some people from striving for perfection? No!

It may be helpful to think of isentropic compression as a type of “perfection.” Even though isentropic compression may not be physically possible, it can help us set a standard for combatting inefficiency in the compression process.

As we already said, entropy indicates inefficiency. If we want to reduce inefficiency, isentropic compression is a condition that we can aim to emulate as closely as possible. It’s a standard to which we can (and do) measure the efficiency of real compressors. 

To compare the actual compression to isentropic compression, we use an equation for isentropic efficiency. This equation is a ratio that tells you how a compressor’s efficiency compares to isentropic compression. You divide the isentropic compression work by the actual compression work.

We can use enthalpy to calculate the work performed. Using the variable h to represent specific enthalpy, I will show you how to compare isentropic compression to actual compression. In the equation below, h1 represents the specific enthalpy of gas that enters the compressor, h2 represents the specific enthalpy of the gas that exits the compressor, subscript s signifies the isentropic conditions, and subscript r indicates the real conditions.

The formula for determining isentropic efficiency.

Even though you will never see pure isentropic compression in the field, it is an ideal we can strive for as we limit entropy. The closer the ratio above can get to 1, the more efficient a compressor will be.


In conclusion, truly isentropic compression is only theoretical, but that doesn’t mean we should disregard it. 

Think about it this way: we don’t stop trying to better ourselves just because perfection is impossible. The same applies to isentropic compression in a world dictated by natural physics. We can strive to make our systems more efficient by making the compression process as close to isentropic as possible.

In other words, those vertical lines on T-S diagrams aren’t real, but they can serve as a standard for comparison.


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