Category: Tech Tips

I remember it like it was yesterday… It was my first day of work as a trainee at my first technician job, just a wet behind the ears kid fresh out of trade school.

It was a Monday morning and technicians and I were standing in the dusty warehouse surrounded by stacks and stacks of brand new condensing units drinking the nasty warehouse coffee…

and I was LOVING IT

Finally, I had made it, one of the guys, listening to the war stories and well-natured ribbing and getting a caffeine fix for the day.

One of the senior techs was telling a story of low suction pressure and he said “So I figured it has to be the wrong sized piston” and he stopped and looked over at me and said “you know what a piston is….. RIGHT”

It seemed like an eternity passed as the whole group stared at me, I mumbled “a piston sure” and gave a weak nod hoping that “LIAR” wasn’t emblazoned on my forehead for All to see.

The tech turned and finished his story and my mind raced….

Of course, I knew what a piston was in an ENGINE or even a reciprocating compressor but I had no clue that the little hunk of brass with a hole in it that we called a “fixed orifice” in school was called a piston.

Later I learned all there was to know about sizing and replacing pistons. The installers I worked with often forgot to put in the correct size.

In case you are like I was, a piston is a fixed orifice metering device used in systems for many years. They are especially in residential heat pumps and straight cool systems. Even now that TXVs and EEVs are becoming more popular you will still see pistons in many new Carrier models being used outside as the heat mode metering device.

Piston Facts

There are three common piston designs I see regularly and while different manufacturers may use them I will group them by the manufacturers I know them by.

Lennox / Rheem Type

The piston shown above is the Lennox / Rheem style. It is directional, meaning it can only be installed one way with the cone (tapered side) pointed at the evaporator and the other side pointed at the liquid line. This type uses seals toward the end of the cone to help prevent refrigerant bypass and it also uses an o-ring to seal the “chatleff” style housing.

Carrier Type

Carrier used to call their pistons “accurators” and maybe still do although I haven’t heard that term for years. These pistons can be installed in either direction but still use the same “chatleff” style housing as Lennox

Trane Type

The Trane style has a much smaller size and is directional. The Trane housings do not use o-rings.

Piston Size

The physical exterior dimensions of the piston must be the same as all the others for that brand/series otherwise it will not fit properly. It is only the internal bore diameter that changes.

Pistons are sized in decimals of an inch like a gas orifice, usually from the 40’s up to the low 100’s. When a piston is described as being a “65 piston” that means it is 0.065 of an inch and a “104” would be 0.104 of an inch.

Check Flow Operation

In a heat pump system, every metering device needs some method of bypassing the metering device when the refrigerant flows in the opposite direction. This is done in TXVs by means of an internal or external check valve but with a piston, the piston itself is allowed to slide in the housing allowing restricted flow in one direction and unrestricted flow in the other.

This is actually where a piston gets its name, because like a piston in an engine it is a cylinder within a cylinder that can slide back and forth.

Any carbon, wax or other solid material that gets into the piston housing can cause one of three undesired conditions

Piston Restriction in the Desired Mode

If something gets into or covers the orifice bored into the piston it can cause a restriction resulting in low evaporator pressure, low suction, high superheat and normal to high subcool. When a piston is restricted and the system is a heat pump with a liquid line filter/drier properly installed, we will often alternate the system into cool and heat and see if that will break free the contaminants and catch it in the line drier. Otherwise, the piston should be removed, inspected and cleaned or replaced and a new line drier installed.

Keep in mind that some systems have a screen built into the piston housing inlet that can also block up. Look for this once the piston housing is disassembled.

Piston Bypassing (Overfeeding)

If the piston fails to seat properly it can overfeed the evaporator in the same way it would if the system had a larger bore size than it should. This will result in high suction pressure, low superheat and low subcooling. In these cases, the piston should be removed and inspected for proper bore size and signs of contamination around the outside or near the seal surfaces of the piston and the housing.

Opposite Mode Piston Restriction 

In some cases, a heat pump piston may fail to fully unseat in the opposite mode. This will result in a pressure drop and an undesired restriction similar to a clogged liquid line filter drier.  In this case there will be a clear temperature drop across that piston when there should be little to none.

For example, if you are running a system in cooling and you notice frost starting to form on the liquid line side of the outdoor, heat mode piston housing, you can be sure it is restricting in the opposite direction. Sometimes this can be resolved by switching back and forth from heat to cool a few times but often it will require disassembly and inspection.

This condition is similar to what happens when an external TXV check valve fails.

In Closing

A piston is a simple little hunk of brass, it drives me nuts when a tech incompletes a call so that someone can “replace a failed piston”. A piston doesn’t just fail, if one does have an issue it’s either the wrong size or something got into it and got stuck in it or caused it to stop seating properly. Many of these issues lead back to improper vacuum, failing to flow nitrogen, getting copper shavings or sand in the system etc…

Every good residential tech should have a little plastic container with various brands and sizes of piston in it in case you find one that is the wrong size or worn down from improper seating. I may be a little late to the game here since pistons are a dying breed but they are simple enough that a return trip for a “failed piston” seems like a huge waste.

— Bryan

I hear the following phrase a lot

It’s the amperage that kills you not the voltage

While there is truth to the statement it is sort of like saying “it’s the size of the vehicle not the speed that kills you when it hits you”…

OK so that’s a pretty bad example, but hopefully, it gets the point across. BOTH of them are needed to cause injury or death and in the case of voltage and amperage the higher the voltage the higher the amperage.

This statement about amperage being the real danger as led to many people inaccurately believing it is the size of a panel or the gauge of wire that makes something more or less dangerous… which is 100% incorrect.

Let’s take a quick look at OHM’s law –

Amps = Volts ÷ Ohms 

The resistance (ohms) of the human body depends on a lot of factors including things like the moisture content of the skin, what other objects the current path is traveling through, what path the current is taking through the body etc…

While the resistances vary based on these factors Ohms law still holds true that when you increase the voltage you ALSO increase the amperage.

Take a look at this chart from the CDC

Effects of Electrical Current* on the Body [3]
CurrentReaction
1 milliampJust a faint tingle.
5 milliampsSlight shock felt. Disturbing, but not painful. Most people can “let go.” However, strong involuntary movements can cause injuries.
6-25 milliamps (women)†
9-30 milliamps (men)
Painful shock. Muscular control is lost. This is the range where “freezing currents” start. It may not be possible to “let go.”
50-150 milliampsExtremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.
1,000-4,300 milliamps (1-4.3 amps)Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.
10,000 milliamps (10 amps)Cardiac arrest and severe burns occur. Death is probable.

*Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.
†Differences in muscle and fat content affect the severity of shock.

Let’s say that a particular shock is traveling through a 20 KOhm (20,000 ohm) path in your body

At 120V this would produce a 6mA shock

At 240V it would be 12mA

At 480V it would be 24mA

It becomes clear pretty quick that higher voltage does lead to more dangerous shocks as does the resistance of the path.

High Resistance and Low Voltage = Safer

Low Resistance and High Voltage = Danger

This is why working around live electrical should only be done with insulated tools, proper PPE and in dry conditions. These all serve to keep the resistance up to reduce the likelihood of a fatal shock. The higher the voltage the more diligent you need to be.

Some people may bring up high voltage shocks from a taser or static electricity as proof that “voltage doesn’t kill”.

In these cases, the power supply is either limited, intermittent or instantaneous. This means that while the voltage is high it is only high for a very short period. Unfortunately in our profession, those sorts of quick high voltage discharges aren’t the big danger we face, most of the electrical work we do is on systems that will happily fry us to a crisp before the power supply cuts out.

A circuit breaker or fuse will never protect us because we draw in the milliamp range when we are being shocked as almost all fuses or breakers don’t trip or blow until much higher levels are reached.

Be safe around high voltage and keep your resistance high.

— Bryan

 

 

 

As HVAC/R techs we don’t do a lot of soldering generally unless you are in a shop that has embraced Stay Brite® 8 from Harris.

There are several aluminum repair products on the market that also use an indirect soldering type technique so this is is a general and generic overview of some best practices. As always, follow the manufacturer’s instructions for best results.

 

Prep the Work Area

When soldering you will want to get everything as clean as possible before you start. You can begin with brushes or Emory cloth to get the big stuff off then go to alcohol and a lint-free cloth at the end to get off any residue or silica particles. Just make sure any alcohol is completely evaporated before using a torch.

Another nice trick for tight work on aluminum coils is using a wire wheel on a Dremel to get the area clean. I had luck with this when repairing a microchannel coil.

 

 

Use Lower Heat Than Brazing

Often soldering is best done with an air-acetylene or MAPP gas torch rather than a typical oxygen rig especially when you have room to work. If you are working a tight space you may opt for a small oxy/acet tip like the one shown above but be VERY careful. The flame may be small and therefore put out less BTUs than a larger flame but it will still be a much hotter temperature than air-acetylene or MAPP.

Work Indirectly 

When working with solders or lower temperature base metals like aluminum it is generally best to heat around the repair or joining area with your rather than right on it. The goal is to allow the heat to gently conduct into the area ESPECIALLY when working with the hotter oxygen flame. With brazing, we can almost put the heat directly on the rod as we work and for most of these products, this won’t work at all.

Watch the Flux

Flux not only acts to keep oxides away from the work area, but it also gives us a visual indication of when the work area is at the right temperature to apply solder. If we underheat the work area the solder won’t flow int the joint and if we overheat the work area we will burn the flux and the solder won’t flow into the joint.

Another note on flux is we only want to apply it to the male end when joining and we don’t want to overuse flux and contaminate the system. Many fluxes are corrosive so wipe it all off one the joint cools to prevent leaks.

— Bryan

 

 

 

Dielectric grease is an often misused and misunderstood product that could easily benefit HVAC/R technicians in a variety of ways. From food service to electrical connections, dielectric grease can help lubricate mechanical components and prevent corrosion on electrical connections. But we need to understand what it is to begin with, in order to properly apply it in the field.

Dielectric grease is silicone-based grease with insulating properties. Common uses for dielectric grease include electrical connections, spark plug wires, and mechanical connections. The most common misuse of dielectric grease relates to electrical connections.

I mentioned dielectric grease acts as an insulator, yet many technicians mistake silicone grease as conductive. For conductive grease, Conducto-Lube Silver or any carbon conductive grease will do. Conductive grease is for conducting electricity from one conductor through the grease to another conductor. 

 

To apply silicone dielectric grease properly to electrical connections, make sure the conductor mating surfaces are bonded before applying the grease. In coastal climates, low voltage wiring is particularly in danger of corrosion, especially right on the waterfront. 

         

To prevent corrosion and to protect the connections, make a solid connection with your exposed conductor wire with an appropriately sized wire nut. Then remove the wire nut and dip the exposed conductor into dielectric grease. Next, put the wire nut back on. If you really want to get crazy, you can then wrap the connection with electrical tape. For contactors and other connections, wire up the components as usual, then apply a dollop of Daisy…I mean Dielectric grease to the connection points.

         

Dielectric silicone grease can be used in a variety of mechanical applications, as well. The Refrigeration Technologies Silicone Grease is also food-grade and can be used in many refrigeration applications. 

 

Remember to always double-check your electrical and mechanical connections for the correct torque before applying the grease. If you’re not careful, things can get messy quick!

 

-Kaleb

I had an old-timer tell me that you can never connect two transformers together because they will “fight one another”.

If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.

In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.

The truth is you can connect two transformers together so long as you are careful, but you need to know why you’re doing it and then do it properly.

Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won’t get into here) the transformer can handle on the secondary.

Above we show two 75VA transformers with 24V secondary windings.

75VA÷24V=3.125A

So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.

In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way as we show above… Pretty easy

It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).

In fact, it is so important that I advise that you only do this if you have two identical model transformers.

To be even safer, connect the primary windings first and check the secondary’s against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.

— Bryan


In Florida, there are not many gas furnaces, At least not as many as up North. Sometimes we can look like real dummies compared to techs who work on them every day.

One thing to know about 80% gas furnaces with cased evaporator coils is that you can often check the evaporator coil by removing the high limit and running an inspection camera up through the opening.

You may also be able to use a mirror and flashlight but you usually won’t see much due to the heat exchanger being in the way. Otherwise, you are stuck removing the entire blower assembly… and that’s no fun at all.

Another practice is bench marking the static pressure drop across a new coil when it is dry and wet when installed or during the first service call. You can then easily watch coil loading over time without the need to look at the coil visually.

— Bryan

Let’s get something out of the way right off the top. Saying that we learn best “hands-on” is sorta like saying we prefer to breathe air…

WE ALL NEED TO APPLY THINGS TO LEARN THEM DEEPLY

David Sandler wrote the book “You can’t teach a kid to ride a bike in a seminar” and the same is certainly true for the trades.

But there is a distinction that needs to be made between “learning to ride a bike in a seminar” and “learning more about bikes in a seminar” or “learning about better riding technique in a seminar”  because these both could be valuable once you’ve already been riding a while.

This article is about HVAC training, but it’s also about things I’ve learned running a business, being homeschooled myself and home educating our kids as well as having a large family.

This is the perspective of one man so take it with a grain of salt.

I’m more of a hands-on learner

Inevitably when I teach a class, or give a seminar, or send an article or make a video or podcast or suggest that someone RTFM… there is someone who says some version of “I’m more of a hands-on learner”

Which… to be clear… is totally cool and should never be disregarded especially when learning an entirely new concept.

I went with my kids to the science museum the other day and the “Bernoulli table” with balls floating on high-velocity air streams created the “hands-on” and visual experience to illustrate Bernoulli’s principal.

It was a lot of fun and very interesting to see the balls suspended in the air, but imagine if I started to explain the principals of pressure and velocity and mass to the kids using words and they just look blankly and say “I’m more of a hands-on learner”.

Do you see the issue?

This is hands-on learning… it just isn’t ONLY hands-on learning.. almost nothing is ONLY hands-on learning if you want to understand what is going on.

Language needs to be used to explain the “why” behind something we can experience hands-on and if you refuse to listen or read the manual or plaque then you are left with experiences and observations that have no context or meaning.

To some degree, we are all hands-on learners but to really understand we would be well served to become, attentive readers and listeners as well.

You must be so patient 

It’s no secret that Leilani and I have 10 kids. When Leilani goes to the grocery store she gets three comments from people most often

  1. I don’t know how you do it
  2. You must be a saint
  3. You must have so much patience

We laugh because we ARE NOT naturally patient people AT ALL and we have no secret magical powers or heavenly bestowed holiness. People imagine that to have 10 kids and remain (mostly) sane you must have some special gift.

The truth is much more boring and mundane.

You don’t need a huge dose of natural patience, but you do need to work at being patient. You don’t need to be a saint but you do need to work to control your emotions when life gets crazy.

In the same way, you don’t need to be naturally gifted at listening or reading to learn, but it sure helps if you work at it

Obviously, some people are more academically gifted naturally than others and some people have learning challenges and disabilities. This isn’t to downplay that reality but I do think you would be better served to stop using it as an excuse.

Becoming a Visual Problem Solver

A visual problem solver is much like a hands-on learner in that they prefer to have a problem in front of them to find the solution rather than using words to describe it.

Some of the BEST problem solvers I’ve ever met weren’t big talkers, instead, they create images in their mind of a problem, structure or machine and work over the problem using the visual centers of their brain.

If you think about it, converting ideas and mental pictures to language is actually pretty inefficient if there is no reason to do so.

The challenge comes in when you need to communicate those ideas to another human.

If you start describing a problem to a visual problem solver they may request to take a look, or have a photo or screenshot sent to them, these are ways the visual problem solver has found to get around the challenge of translating things to language all the time.

They will often draw diagrams or ask you to draw diagrams and they may stare at them a bit as they build the visual model or “cartoon” in their head.

The visual problem solver doesn’t make excuses about how they prefer to learn. They don’t make it someone else’s fault that they aren’t getting a concept.

The visual problem solver finds workarounds to get things out of words and into their head where they work on it and ultimately SOLVE THE PROBLEM.

Take responsibility for the translation gap 

Good teachers find ways to meet their students where they are and teach to their learning style. The BEST teachers do the same and then ALSO teaches their students how to translate a world that doesn’t always cater to their learning style.

As a learner, it is our responsibility to take what we can from all sources and methods we can to learn how to problem-solve. I would argue that visual problem solvers actually have a HUGE advantage in our trade because by it’s very nature it is visual and hands-on.

It doesn’t change the fact that reading manuals is often the only way to get certain information and may continue to be a bit of a struggle. Where the person who always repeats that they are a “hands-on learner” may wait for someone to translate words on the page for them, the visual problem solver may build a cartoon in their head or doodle a diagram… whatever it takes to the solve the problem.

— Bryan

 

 

 

There are many examples of teaching using metaphor to help someone get a grasp of how something works without being EXACTLY correct.

Some examples are how we often use water flow to explain electrical flow or refrigerant circuit dynamics. It’s enough like the way it works to get our heads wrapped around it but there are many differences and the metaphors eventually break down.

This is definitely the case with air and nitrogen “absorbing” water

I’ve done podcasts and videos about how air can “hold” less moisture when it is cooler and more when it is hotter. You have likely heard old school techs talk about triple evacuation and sweeping with nitrogen to “absorb” the moisture from the system.

News Flash, Air and Nitrogen DO NOT absorb or hold moisture… They ignore one another at parties and they certainly don’t shake hands.

Water vapor in the air behaves much like all the other gasses contained in the air with the notable exception that water exists in both vapor and liquid states at atmospheric pressure and temperature.

When the temperature of water vapor is higher, a higher percentage of the air by volume can CONTAIN water vapor, but the air itself isn’t what is holding it. It does interact with it as the molecules move and bounce around and the percentage of water vapor in the air does impact the mass/weight of the air by volume (water vapor weighs less than dry air) so there are certainly impacts to the makeup of the air based on moisture content.

The percentage of the air around us that is moisture can vary from almost zero In cold arctic & Antarctic climates to nearly 4% in hot, tropical climates.

When teaching it we speak as though the air is a sponge and the hotter the air the bigger the sponge. This certainly helps us remember but it isn’t really how it works. In reality water in the air is all about the saturation temperature and pressure of the water and the air has little to do with it.

By Greg Benson

This is the same sort of thinking when a tech is having a hard time pulling a vacuum and they add dry nitrogen to the system to “absorb” the moisture. First off, you will want to sweep the nitrogen through the system, not just pressurize. Secondly, the nitrogen has no special properties that allow it to “grab” moisture. It can entrain the water vapor using Bernoulli’s principle, it will warm up the system a bit, it will certainly add in a bit of turbulence which can help move the oil around and potentially release some trapped moisture… but nothing more than that.

Don’t get me wrong, there is nothing wrong with sweeping with dry nitrogen, even better to use a heat gun and warm the compressor crankcase, receivers and accumulator and coils during a deep vacuum on a large system to help speed up the vaporization of moisture.

It doesn’t change the fact that air and nitrogen don’t “hold” moisture.

— Bryan

 

 

The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?

Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.

In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.


So what should the superheat be?

First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it’s boiling (saturation) temperature at a given pressure.

The air around us is all superheated! Head for the Hills!

How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°

But why do we care?

We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.

#1 – It helps ensure we are not flooding the compressor

First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.

Image courtesy of Parker / Sporlan

#2 – It gives us an indication as to how well the evaporator coil is being fed

When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.

The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.

This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.

This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn’t used to “set the charge”.

#3 – We can ensure our compressor stays cool by measuring superheat

Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.

#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)

Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.

#5 – Superheat is an indication of load on the evaporator 

On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.

When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.

When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for or you can use our special superheat and delta t calculator HERE

Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE

Remember, this chart ONLY applies to fixed orifice systems.

So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect

High temp / A/C systems to run 6 – 14 degrees of superheat

Medium Temp  – 5-10 

Low Temp – 4-10

Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat

When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer’s design specs.

— Bryan

 

Sensors, Measurements, and Physics

As HVAC/R Technicians, we use tools and instruments to make measurements every day. In fact, 90% of our job could not be done efficiently without some kind of measurement. 

“How do we measure?”

“With what instruments?”

“How accurate are these measurements?”

These are all questions a thoughtful technician should ask before spending money on a tool or implementing solutions to solve a problem. 

 

Tools are our primary resource for measurement. Measuring tapes, scales, pressure transducers, thermocouples; the list goes on. But how exact are these measurements? How precise must our measurements be for us to use them to make decisions regarding the mechanical operation, occupant comfort, and occupant health? To answer these questions, we need a crash course in a little bit of physics. 

Don’t worry; we aren’t going into the rabbit hole too deep. I simply want to introduce you to a concept called the Heisenberg Uncertainty Principle in quantum mechanics. The Uncertainty Principle states that the more precisely you determine a particle’s position, the less precisely you can determine that particle’s momentum. Basically, the Uncertainty Principle limits our ability to measure things exactly. In modern physics, there simply is no such thing. There is only the agreed-upon accuracy and precision everyone is satisfied with to make practical decisions. For example, if you asked me how tall I was, my answer might be 5’11”. But is that 5ft. 11in. exactly? The line on the measuring tape with which I measured my height has a thickness, doesn’t it? Where within the thickness of that line do I fall? This principle is, of course, much more noticeable in the quantum (atomic) scale than on the macro scale. However, when measuring airflow and trying to solve occupant health concerns by measuring indoor air quality, accuracy, and precision matters. Our margin of uncertainty matters.

A technician doesn’t have to lose sleep over the fact nothing can be measured exactly. In our trade, and most of life, it’s not necessary. We can use this knowledge, however, to be more critical about the types of tools to choose to use. A duct traverse with a rotating vane anemometer can provide a quick and dirty idea for system airflow. Still, it’s not accurate nor precise enough to make complex troubleshooting decisions based on its results. The margin of uncertainty is too high. A flow hood or The Energy Conservatory’s TrueFlow Grid would be required for more accurate measurements upon which to base airflow balancing solutions. So how do we quality check for accuracy and precision? First, here’s a diagram showing the difference between the two and various combinations of accuracy and precision:

It’s important to understand the differences between these different combinations of accuracy and precision. Take any three brands of micron gauges and do a vacuum pump test. It helps if you have a digital gauge on the pump itself. Pull only on the pump first, and record the level of vacuum achieved (a good vacuum with fresh oil should be able to pull below 50 microns). Next, add the gauges one at a time and pull a vacuum in three more separate tests. You will very likely record three different micron levels for each gauge. In this experiment, the pump was our reference. In reality, the pump gauge itself would also need to be scrutinized for accuracy and precision, but as a demonstration, this test works well. Looking at the recorded micron levels from the gauges relative to the pump, where on the graph would your test results lie? Most are either accurate, but not precise, or precise, but not accurate. 

When thinking about buying a measurement tool, first consider what you are trying to accomplish with that measurement. If you aren’t planning on making accurate and precise capacity calculations, then you don’t really need the more accurate and precise hygrometers and airflow measurement tools. You may not need a digital manifold to charge a system to proper superheat and subcooling, because the margin of uncertainty is forgiving in that context. But a more accurate and precise sensor for a digital manifold or probe sure makes a pin-hole leak during a standing pressure test a lot easier notice.

Sensors are another factor to consider when choosing a measurement tool. Sensors are responsible for most of what makes a tool more expensive (not always, but most of the time). The product data for any instrument can be found either in the manuals or through a quick phone call to the manufacturer. In a recent podcast with Aeroqual’s Bernadette Shahin, we discuss some points to look for when researching the quality of a particular sensor.

  • Selectivity
    • A sensor that can measure the target parameter only (humidity, pressure, CO, microns, VOCs, etc.)
  • Sensitivity
    • A quality sensor should be able to have low cross-sensitivity to other parameters and should be stable in its ability to measure the target parameter over time.
  • Speed
    • A sensor has a published response time to change, but a manufacturer can confirm how long a sensor takes to reach an acceptable range of accuracy and precision expected from the sensor

Home automation is becoming very popular in the HVAC trade, and many are using Indoor Air Quality monitors to determine when and for long a mechanical system will operate, in order to maintain a comfortable and healthy home. To do this effectively, the accuracy and precision of the sensors in that monitor should be scrutinized. AQ-Spec is an excellent resource for this specific type of evaluation. Other manufacturers may have done their own third-party testing, and results can be released upon request. 

Keep in mind we aren’t referring to this concept as the “margin of error,” as many often call it. There is hardly ever any ill-intent when it comes to making a measurement, and manufacturers are not trying to create poor quality products. There are simply varying levels of uncertainty when we are using the tools at our disposal, and picking the right tool for the job is essential for quality solution implementation.

Another principle in physics, which all technicians should be aware of, is the Observer Effect. This theory states that the very act of observing a property’s state of being will inevitably change that property’s state of being altogether. In other words, by the time you have made a measurement, the state of whatever property you are measuring has changed, and no longer holds the same state of being it held before you started to measure it. A good example of this would be connecting hoses to a system. The mere act of attaching your hose has let out a little bit of pressure from the system; therefore, the pressure you will read is different from the pressure the system held the moment before you connected. This example might incentivize some to make the switch to probes, and ditch the hoses! Another example of the Observer Effect is checking electrical current on an indoor blower motor with the cabinet door removed.

One last point to make…accuracy and precision ≠ resolution. The resolution simply specifies to what decimal point the measurement is going to display. The resolution does not provide any direct information about accuracy, or precision. So next time you are comparing tools to see which is the best quality for the price, you may find a more accurate tool at a lower relative resolution. It is important to note, however, that precision and resolution are related. The higher the precision, the higher the resolution necessary to interpret the readings. It’s all about what you need the measurement to do for the application in which you use it.

 

Here are some links, in case you want to gain more insight into sensors, the Observer Effect, and the Heisenberg Uncertainty Principle:

https://podcasts.apple.com/us/podcast/going-deep-on-iaq-sensors-and-instruments/id1155660740?i=1000481776701 

https://www.sciencedaily.com/releases/1998/02/980227055013.htm

https://www.khanacademy.org/science/physics/quantum-physics/quantum-numbers-and-orbitals/v/heisenberg-uncertainty-principle

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