Tag: charging

 

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

 

Before I start on this one… At HVAC School we focus on a wide range of topics, many of them are very basic. My experience as a trainer for over 16 years has taught me that no matter what I assume others SHOULD know, it doesn’t change that fact that they often do not. This write up is very basic but you may find that some of the content will be useful for you to give apprentices or junior techs or it may give you a new idea of how to explain it to them… or maybe not. Either way, I feel an obligation to cover even the most basic concepts in the trade to help ensure that nothing gets missed. 

Thanks for understanding.


Before reading this you need to understand some of the terms surrounding air conditioning charging and diagnosis, specifically the term saturation

Next, you need to know something of the basic refrigerant circuit, I suggest that you understand these words and concepts before you ever dive into attempting to charge an air conditioner.  Many who start here may ask “what should my pressures be”, this is NOT how you charge a system so if you are reading this to try and find that answer just be aware, it isn’t that simple.

READ THE MANUFACTURER SPECS ON PROPER CHARGING FOR THE MODEL YOU ARE WORKING ON FIRST WHENEVER POSSIBLE

In order to set a proper charge on an A/C system, you must first know the type of metering device.  The piston / fixed orifice type system primarily uses the superheat method and the TXV / EEV primarily uses the subcooling method.  When setting a charge, it is always preferable to set the charge in cool mode.  Whether you set the charge in heat or cool mode, you should always follow the manufacturer’s recommended charging specifications.  In this section, we will discuss manufacturer-recommended charging and some indicators that you have set a proper charge in heat mode.

But first, There are some things that Trump these guidelines and should make you stop and do more diagnosis

A properly running A/C system with indoor and outdoor temperatures above 68 degrees will have a suction saturation above 32 degrees (freezing), don’t leave a system with a below 32˚ saturation suction without doing more diagnosis even if the superheat/subcool looks correct.

If you see a liquid line pressure that is more than 30 degrees saturation above outdoor temperature (like a 440 psi liquid pressure on an R410a system on a 90 degree day), do not proceed until you have further addressed the possible causes of high head, regardless of what the superheat or subcool might be reading.

Always purge your hoses to prevent introducing air into the system and never mix gauges when using low loss fittings and different types of refrigerants.

Charge in the liquid phase (tank upside down) and add the refrigerant slowly and carefully to ensure you do not flood/slug the compressor with liquid refrigerant. You can do this by watching your manifold sight glass or using a special liquid preventing adapter such as the Imperial 535-C Kwik Charge.

These precautions will prevent causing system damage.

Also, at a minimum doing a full visual inspection of the equipment including

  • Inspect filters
  • check blower wheel
  • check evaporator coil
  • check condenser coil cleanliness
  • make sure the system is wired properly and running in the correct mode

Note: This is only a basic guide for charging. There are innumerable conditions that can alter refrigerant pressures, superheat, subcool and saturation that are not related to the refrigerant charge. This is not intended to cover the complete diagnosis of the refrigerant circuit. 


Superheat Charging

To charge a system using superheat, you will need to monitor the actual temperature of the low-pressure suction line, the saturation temperature of the low side suction gauge and the indoor and outdoor temperatures entering the unit(s).

Most if not all manufacturers have a charging chart available with their respective units.  With the information you have gathered on indoor and outdoor temperatures, you can calculate the recommended superheat or in a pinch, you can use a superheat calculator such as the Trane superheat calculator or a free app like our superheat calculator or even better the MeasureQuick app. A good calculator will require that you determine the wet bulb temperature in the return air stream.  Without a sling or digital psychrometer or hygrometer, you will not be able to determine wet bulb temperature.

Once you know the target superheat you can adjust the system charge to hit it. Let’s say, the recommended superheat was 18 degrees, you would add/remove refrigerant to the system until the actual temperature of the suction line was 18 degrees above the indicated saturation temperature from your low-pressure gauge. Adding charge will decrease the superheat and recovering refrigerant will increase the superheat. 


Subcool Charging

To charge a system using subcool, you will need to monitor the actual temperature of the liquid line and indicated saturation temperature on the high-pressure gauge.  Information on the entering temperatures is not necessary to charge the unit by the subcooling method.

Most manufacturers have recommended subcool charging information with the units if it is designed for a TXV (TEV).  If for some reason, there is no information with the unit, or if it has worn off, you can set a typical residential air conditioner charge to 10 to 12 degrees of subcooling which is a relatively safe range to use.

Let’s say for example the manufacturers recommended subcool is 14, you would add enough refrigerant to the system so the actual temperature of the liquid line was 14 degrees less than the saturation temperature, as indicated on the high-pressure gauge for that particular refrigerant. Adding more refrigerant will increase the subcool reading and recovering refrigerant will decrease the subcool reading. 


Approach Method

 Lennox factory information asks that we charge by the approach method on TXV systems. I suggest charging to at least a 6˚ subcool before even attempting to calculate the approach method. 

The approach method is a calculation based on the relationship of liquid line temperature to outdoor temperature.  To calculate approach, subtract outdoor ambient from actual liquid line temperature.  The outdoor temperature used to calculate approach should always be taken in the shade and away from the hot condenser discharge air. To increase the approach differential you would remove refrigerant to decrease it you would add refrigerant. 

Some Lennox heat pump systems come with a subcool chart next to the approach chart. This subcool chart is for < 65˚.  This means the subcool chart is only valid when the outdoor temperature is below 65˚.  Follow the instructions on the unit carefully when charging in subcooling in <65˚ temperatures.  The method requires that you block sections of the coil to achieve higher head pressures before setting by subcooling.


Heat Mode Charging for Heat Pumps

In most, if not all, cases you will charge a unit in heat mode according to the manufacturer’s recommendations.  In those cases where no information is available, there are other indicators that you may use to set a proper charge in heat mode.

First, make sure you switch your hoses so the suction gauge is reading off of the “common suction” port that taps in between the compressor and reversing valve. You may put your high side gauge on either the discharge or liquid (on most systems) depending on what you are checking.

Before doing any heat mode charging use common sense, if installing a new system the best bet is to calculate line distance and weigh in any additional charge before moving on to the detailed testing phase.

The first one is the 100˚ over ambient discharge temperature rule.  The general rule to this is that a properly charged unit will have a discharge line temperature of 100˚ above the outdoor ambient temperature but this only a rule of thumb and cannot be relied upon.  If the discharge line is too hot. you would add refrigerant which would lower the discharge temperature.  Alternately, if the discharge line were too cool, you would remove refrigerant to raise the discharge temperature.  This rule is to be used only as an indicator and, in some instances, may not be accurate given some other factors such as dirty coils, excessive superheated refrigerant entering the compressor, etc.  

Another common rule of thumb is suction pressure will be close to the outdoor temperature in an R-22 system, this is totally a fluke and has no scientific basis other than it just generally tends to work out that way. this means that on a properly functioning R22 system if it is running in heat mode and its 40 degrees outside the suction pressure tends to be around 40psig. This guideline obviously doesn’t work on an R-410A system or any other refrigerant.

A more applicable guideline is 20˚- 25˚ suction saturation below outdoor ambient temperature. This means if it is 50˚ outside the suction saturation temperature would generally be between 25˚and 30˚on a functioning system.

Remember that in heat mode the colder it gets outside, the lower the suction pressure and the hotter it gets inside, the higher the head pressure.  Since the roles of the coil are reversed in heat mode, if you notice an abnormally high head pressure it may be due to a dirty air filter or evaporator coil.  A dirty condenser coil would cause the suction pressure to drop below normal and also cause superheat problems.

Once heat mode a charge is set, whether by manufacturer specification or an alternative method, you can still verify the subcool and superheat on the unit in some cases.  Do not confuse the superheat or subcool methods recommended by the manufacturer though when running in heat mode.  These are only used for setting the charge in cooling mode and not in heat. Look for heat mode specific or low ambient guidelines. 


 

Finally and most importantly is ALWAYS TEST EVERYTHING. Airflow, Delta T, Superheat, Subcool, Suction Pressure, Head pressure, Amps, Incoming voltage, Filter etc…

Read manufacturers specs, understand the units the units you are working on, only then will guidelines and rules of thumb help instead of hinder you.

— Bryan

 

 

 

This is the article you read BEFORE you call and ask a senior tech what your subcool should be, or the one you send to a junior tech when the call and ask you.

So what is subcooling? (or subcool as many call it)

Subcooling is a measurement of temperature DECREASE of a liquid below its saturation (mixed liquid/vapor) temperature at a given pressure.

For example, water boils at 212° Fahrenheit at sea level (atmospheric pressure of 14.7 PSIA). If water is at 212°f and at atmospheric pressure at sea level you can be sure it is at saturation, which means it is either in the process of boiling or condensing. If you measure that same water and it is at 202° you can be sure that it is fully liquid and that it is no longer in the process of either boiling (changing from liquid to vapor) or condensing (Changing from vapor to liquid). Because the water is at 202°  instead of 212° we know it is liquid and we can also say it is subcooled by 10°. This 10° of subcooling PROVES that not only is it fully liquid but that it has given up more sensible heat energy enough to drop 10° below the boiling temperature at that pressure.

With refrigerant, we measure the subcooling between the condenser and the metering device and it gives us a lot of information. It not only tells us whether or not the line is full of liquid it gives us indications of refrigerant charge as well as condenser efficiency when viewed in conjunction with the condensing temperature (high side saturation temperature). Now be careful, like with all measurements, it is only as accurate as your tools, it must be taken using liquid line pressure and temperature (Line between the condenser and metering device) NOT discharge line pressure and temperature (line between the compressor and the condenser) AND you must have a good connection to the port. I can’t tell you how many times green techs have called me with “crazy” readings only to find out their hose was not depressing the Schrader core fully.

So what should it be?

Generally speaking 10° – 12° of subcooling at the outlet of the condenser coil is most common but you must look for the proper design subcooling for the particular system you are working on. Some systems will require subcooling readings of up to 16° for maximum efficiency and capacity.

Many techs will say that subcooling  is how you “set a charge” on a TXV / TEV / EEV metering device system

Subcooling is one of many factors you consider when setting a charge but you first need to make sure that your equipment is properly matched with the correct metering device. The air flow is set in properly, the blower, air filter, condensing coil and evaporator coils are clean and WHENEVER adding or removing charge use a scale so you can monitor your progress.

While it is true that subcooling is the primary charging measurement on a TXV /TEV / EEV system, subcooling is important to check on every system, every time you connect (whenever possible).

Negative Subcooling isn’t possible if the liquid line temperature and pressure are taken at the same point. What is possible is to have a miscalibration of your tools that make a zero subcooling look like a negative subcooling.

Zero Subcooling means that the refrigerant in the liquid line is a mix of liquid and vapor, this is not an acceptable condition except in cases where the system is designed to inject discharge gas into the liquid line on purpose to increase liquid pressure (headmaster).

Low Subcooling is an indication that not enough refrigerant is contained or “packed” in the condenser. This can be due to undercharge, poor compression, or a metering device oversized or failing open (overfeeding).

High Subcooling is an indication that more than the designed amount of refrigerant is “Backing up” or “packed” into the condenser.  This can be caused by overcharge, restriction (such as a contaminated line drier or kinked liquid line) or an undersized or failing closed metering device.

Keep in mind, the subcooling can often read in range on a system that still has issues. Many times this is because the previous tech simply “set the charge” by subcooling without fully testing all aspects of the equipment such as airflow.

— Bryan

When we say that there is “flash gas” at a particular point in the system it can either be a bad thing or a good thing depending on where it is occurring.

Flash gas is just another term for boiling.

It is perfectly normal (and required) that refrigerant “flashes” or begins boiling directly after the metering device and as it moves through the evaporator coil. In order for the evaporator to transfer heat from the air into the refrigerant in large quantities, we leverage the “latent heat transfer of vaporization”. In other words, we transfer heat into the boiling refrigerant, or “flash gas”.

In a boiling pot of water, we create flash gas by increasing the temperature of the water until it hits the boiling temperature. At atmospheric pressure that occurs at 212°F which is the boiling or flashing point, we are most familiar with.

Inside of a refrigeration circuit we get flash gas when the pressure on the liquid refrigerant drops below the temperature/pressure saturation point or if the temperature of the refrigerant increases above the same point. In other words, either a drop-in pressure, an increase in temperature or both can result in flashing or boiling.

This “flashing” can occur in the liquid line when the liquid line is long or too small and also in cases with line kinks and clogged filter/driers. All of these instances result in a pressure drop and a drop in the saturation temperature.

This flashing can be prevented by keeping line lengths and tight bends to a minimum, insulating the liquid line where it runs through very hot spaces and keeping the refrigerant dry and clean with one properly sized filter/drier.

It can also be prevented in most cases by maintaining the proper levels of subcooling. A typical system that has 10°+ of subcooling will not experience flashing in the liquid line under normal conditions. Setting the proper level of subcooling acts as headroom against pressure drop in the liquid line due to long line lengths.

When you walk up to a liquid line near the evaporator and you hear that hissing/surging noise or when you look in a sight glass and see bubbles you are seeing refrigerant that is at saturation, meaning it is a mix of vapor and liquid. This doesn’t necessarily mean it is “flash gas”in the truest sense, it could very well be that the refrigerant was never fully condensed to liquid in the condenser in the first place. This can be due to low refrigerant charge and in these cases, the subcool will be at 0° Even when taken at the condenser.

The true liquid line “flash gas” issues are cases where you have measurable subcooling at the condenser coil outlet but still see, hear or measure boiling/flashing refrigerant in the liquid line before the metering device or see it in a sight glass.

— Bryan

When you ask many people nowadays how to check the charge on a heat pump during low outdoor temps they will say that you need to “weigh in and weigh out” the charge. While this may be an effective method it isn’t always practical.

Now… If you are making a refrigerant circuit repair, weighing out and weighing in makes perfect sense, especially since microchannel condensers and scroll compressors make pumping down less viable anyway. But there are many cases where you just need to check the charge to make sure the system is working properly and in these cases, weighing in and out would be plain silly.

I originally wrote this guideline back in 2003 and truthfully, not much has changed since then in regards to checking a heat mode charge on a heat pump.

Step #1 – If there is any frost on the outside unit get it completely defrosted first.

Step #2 – Check all the obvious things first, filter, coils, blower wheel etc… If the unit isn’t clean it will be really hard to check.

When charging in heat mode Read manufacturer specifications first. Lennox gives specific instructions for charging their units in below 65˚ outdoor ambient conditions. It involves blocking off the condenser coil with cardboard (or even better using a charging jacket) while continuing to run the system in cool mode. Lennox gives specific instruction for how high to raise the head pressure, and what level of subcooling you should expect.

Remember that in heat mode on a heat pump the evaporator is outside, and the condenser is inside. This is important because in cool mode a dirty air filter caused low airflow on the evaporator. This would typically cause a low suction pressure, and a low superheat. In heat mode, a dirty air filter causes low airflow across the condenser. This can cause Extremely high head pressure. In heat mode, a dirty outdoor coil can cause a low suction pressure.

As an example, Trane includes a pressure curve chart with many heat pump condensing units. Be sure to use the scale all the way to the right that says heat mode. Indoor and outdoor dry bulb temperatures are necessary to use the Trane pressure curve. Carrier supplies many heat pump condensing units with a pressure guideline chart. Carrier only wants the heat mode pressure chart used as a guideline, not as a charging tool. Always reference manufacturer guidelines before setting any charge.

100˚ Over Ambient Rule of Thumb

Even though manufacturer specifications should be followed, there are some basic guidelines that will aid in charging and diagnosis in a pinch. The most widely quoted rule of thumb is the 100˚ – 110˚ over ambient discharge rule. This guideline states that a properly charged unit will have a discharge line temperature of 100˚ – 110˚ above the outdoor temperature. If the discharge line is too hot add refrigerant (If the charge is the issue and not another problem). If the discharge line is too cool remove refrigerant (again only if the charge is diagnosed as the issue).

Keep in mind that this rule only works if you are close to being in the correct zone. For example, an extremely overcharged system with an outdoor TXV can actually show a high discharge temperature. It’s just a rule of thumb and you shouldn’t reply too heavily on it.

First off, the photo above was taken in 2003 so give me some slack on my gauges. Nowadays I would be using my Testo 550’s.

To give a simple example using the 100˚ – 110˚ over ambient rule. If it were 60˚ outside you could say by the 100˚ – 110˚over ambient rule, the charge is about correct. If it were 30˚ outside the 100˚ – 110˚ over ambient rule would show undercharge (or other conditions that can cause high discharge line temp see this article) . If for example the discharge temperature were 210˚ with a 150 P.S.I. head pressure and a 10 P.S.I. suction with a 50˚ outdoor temperature; this would show an extreme undercharge. Subcool and superheat can still be checked in heat mode, the problem is since there are rarely any set guidelines it is difficult to tell when the charge is set correctly by simply checking subcool or superheat alone. Generally, you will see normal superheat (8-14) on a system with  heat mode TXV and the subcooling will generally be a bit higher than usual, especially when measured outside.

Suction Pressure / EVAP DTD Rule of Thumb 

Another common old school rule of thumb is suction pressure should be close to the outdoor temperature in a R22 system. However, this rule of thumb (obviously) does not work on an R-410A system. A more applicable guideline is 20˚-25˚ suction saturation below outdoor ambient. This means if it is 50˚ outside the suction saturation temperature should be between 25˚and 30˚ (on most systems).

Head Pressure / CTOA Rule of Thumb

Because the evaporator coil is substantially smaller than the condenser you will usually see higher head pressure (condensing temperature) in relationship to the condensing air, in this case, the indoor air. This can vary a lot depending on the age / SEER of the unit, the size of the coil and how the indoor airflow is setup but generally will be 30˚ – 40˚ condensing temperature over the indoor dry bulb.

Checking Without Gauges 

Here are some quick tests you can do on a heat pump to confirm it is operating close to specs without using gauges when the coil is frost-free and the outdoor temps are 65˚ – 15˚.

  • Check the discharge (vapor) line, it should be 100˚ – 110˚ over the outdoor ambient temperature
  • Suction line Temp should be 5˚ – 15˚ cooler than the outdoor temperature
  • Liquid Line should be 3˚ – 15˚ warmer than the indoor temperature
  •  Delta T indoors will vary greatly depending on the outdoor temperature.

If anything looks off, go ahead and connect gauges to verify further…. and like I said several times already, follow manufacturers guidelines.

The best way is to verify total system capacity (with heat strips off) using dual in duct thermometers and manufacturer specs but I understand how challenging it can be to ACCURATELY verify system airflow so it likely won’t always be your first move. We are a big fan of MeasureQuick around our business so I would suggest checking it out for this.

— Bryan

This article is written by RSES CM and excellent market refrigeration tech – Jeremy Smith

————————————-

I frequently see techs online struggling with charging or troubleshooting refrigeration equipment and using subcooling as a diagnostic or charging method. Please don’t do this unless you understand it fully.

Many times, trying to charge a refrigeration system to a specific subcooling value is going to result in a serious overcharge.

Why?

Glad you asked.. First, let’s take a look at a simple system and focus on the condenser, liquid line and metering device.

As we condense the refrigerant and fill the liquid line and condenser, the metering device restricts flow somewhat, causing liquid to back up into the condenser.

This ‘stacking’ effect as it’s commonly called, allows more time for the liquid to be in the condenser and to reject heat. That heat rejection is what results in additional subcooling. Adding more gas to this system will simply result in more liquid being stored in the condenser, more heat rejection from that liquid and, consequently an increasing subcooling value. That’s the system that you understand and that subcooling can be effectively used as a diagnostic and charging metric.

 

Now, let’s put a receiver in the system between the condenser and the metering device.

We’ve got liquid in the condenser and it enters the receiver before the metering device. As the liquid line fills and the metering device starts to restrict as before, where does the liquid wind up? The receiver. It doesn’t wind up in the condenser where heat can be rejected, but rather in a tank to be stored. Now, if you’re measuring subcooling, before OR AFTER the receiver, you’re not going to see a significant change in that value before or after we reach a proper charge.

 

If you continue to add gas to the system it’s going to continue to fill the receiver until that liquid backs up to the inlet port of the receiver. Now, you’re seriously overcharged because a receiver shouldn’t be more than 80% full, but the system can now back liquid up into the condenser and allow for the subcooling to increase.

This is why, when you have a receiver, you need to use either a sight glass or some form of receiver level monitoring to determine if you’re charge is correct and not just use subcooling.

 

— Jeremy Smith

Suction pressure, head pressure, subcooling, superheat, Delta T

Taking all five of these calculations into account on every service call is critical. Even if further diagnostic tests must be done to pinpoint the problem, these five factors are the groundwork before more effective diagnosis can be done. I would also add static pressure as an important reading that should be checked regularly (Keep TESP between .3″wc and .7″ wc on most systems) but I would still place it slightly below these five as far as fundamental HVAC technician measurements.

Some of these are “rules of thumb” and obviously are for reference only. Refer to manufacturer recommendations when setting a charge.

Suction Pressure / Low Side
Suction pressure tells us several things. The first thing it tells us is what the boiling temperature of the refrigerant in the evaporator is. If the suction pressure is below 32° saturation temperature, the evaporator coil will eventually freeze.

As a general rule, the higher the temperature of the air passing over the evaporator, the higher your suction pressure will be. A good rule of thumb for suction pressure is 35°  saturation below indoor ambient +/- 5° (Return temperature measured at the evaporator coil). This temperature differential is often called an evaporator split or design temperature difference (DTD). When calculating DTD a “Higher” DTD means lower suction pressure in comparison to the return temperature, a lower DTD means higher suction pressure.

This means that when the temperature of the air passing over the evaporator is 80°, the low side saturation temperature should be 45° when the system is set for 400 CFM per ton output. Remember the temperature scale next to the pressure scale on the gauge represents saturation or if you don’t have the correct sale on (or in your gauge if you have a Digital manifold) you would need to use a PT chart.

This 35° rule only works at 400 CFM per ton, when a system is designed for 350 CFM per ton the DTD will be closer to 38° – 40° +/- 5° 

Make sure you know the actual CFM output of the system before you calculate DTD. It can vary significantly based on the setup of the particular blower. Also, keep in mind that oversized evaporator coils that some manufacturers specify for efficiency can also result in slightly lower DTD (higher suction). If you don’t know all the details it is my experience that using 35° is the best bet.

Head Pressure / High Side
When used in conjunction with liquid line temperature, we can know what state the refrigerant in the liquid line and that the compressor is pumping/operating in the required compression ratio. We can also know something about the state of the metering device as to whether or not refrigerant is “backing up” against the metering device. A good rule of thumb for head pressure is a 15° – 20° saturation above outdoor ambient +/- 3° for most modern systems. These saturation / ambient calculations are only indicators; they are not set in stone. Keep in mind, when I say ambient; I am talking about the air entering the evaporator for suction pressure and the condenser for head pressure.

Jim Bergmann points out that different equipment efficiencies will have different target Condensing Temperature Over Ambient (CTOA) readings. Keep in mind that these date ranges don’t guarantee the SEER but rather give the date ranges that these efficiencies will be most likely. The larger the condenser coil in relation to the volume of refrigerant being moved the lower the CTOA will be.

6 – 10 SEER Equipment (Older than 1991) = 30° CTOA

10 -12 SEER Equipment (1992 – 2005) = 25° CTOA

13 – 15 SEER Equipment (2006 – Present) = 20° CTOA

16 SEER+ Equipment (2006 – Present) = 15° CTOA

Superheat
Superheat is important for two reasons. It tells us whether or not we could be damaging the compressor and whether we are fully feeding the evaporator with boiling, flashing refrigerant. If the system has a 0° superheat, a mixture of liquid and vapor is entering the compressor. This is called liquid slugging and it can damage a compressor. A superheat that is higher than the manufacturer’s specification can both starve the evaporator, causing capacity loss, as well as cause the compressor to overheat. So how do we know what superheat we should have? First, we must find out what type of metering device the system is using. If it is using a piston or other fixed metering device, you must refer to the manufacturers superheat requirements or a superheat chart like the one below.

If it is a TXV type metering device, the TXV will generally attempt to maintain between a 5° to 15° superheat on the suction line exiting the evaporator coil (10° +/- 5°) 

TXV target superheat setting may vary slightly based on equipment type.

Subcooling
Subcooling tells us whether or not the liquid line is full of liquid. A 0° subcool reading tells us that the refrigerant in the liquid line is part liquid and part vapor. An abnormally high subcool reading tells us that the refrigerant is moving through the condenser too slowly, causing it to give up a large amount of sensible heat past saturation temperature. A high subcool is often accompanied by high head pressure and, conversely, a low subcool by low head pressure. Subcool is always a very important calculation to take because it lets you know whether or not the metering device is receiving a full line of liquid. Typical ranges for subcooling are between 8 and 14 degrees on a TXV system, but always check the manufacturer’s information to confirm. in general, on a TXV system using 10° +/- 3° at the condenser outlet is an acceptable “rule of thumb” in the absence of manufacturer’s data.

On a fixed orifice/piston system the subcooling will vary even more based on load conditions and you will see a range of 5° to 23° making subcooling less valuable on a fixed orifice system. In my experience during normal operating conditions the subcooling on a fixed orifice system will still usually be in the 10° +/- 3° range.

Evaporator Air Temperature Split (Delta T)
The evaporator air temperature split (Delta T) is a nice calculation because it gives you a good look at system performance and airflow. The air temperature split during typical conditions will be between 16 and 22 degrees difference from the return to the supply. Keep in mind, when you are doing a new system start-up, high humidity will cause your air temperature split to be on the low side. Refer to the air temperature split and comfort considerations sheets for further information.

For systems that are set to 400 CFM per ton, you can use a target Delta T sheet like the one shown below

 

If the leaving temperature/delta T split is high it is an indication of low airflow. If it is low it is an indication of poor system performance/capacity.

Again, this only applies to 400 CFM ton. Systems set at 350 CFM per ton or less are more common today than ever, especially in humid climates and in those cases the above chart won’t apply and the delta T will be higher.

Diagnosing With The Five Pillars
The way this list must be utilized is by taking all five calculations and matching up the potential problems until you find the most likely ones. A very critical thing to remember is that a TXV system will maintain a constant superheat, and fairly constant suction pressure. The exceptions to this rule are when the TXV fails, is not receiving a full line of liquid or does not have the required liquid pressure/pressure drop to operate. This situation would show 0° subcooling and in this case, will no longer be able to maintain the correct superheat. Before using this list, you must also know what type of metering device is being utilized, then adjust thinking accordingly. Also remember, in heat mode, the condenser is inside and the evaporator is outside.

Low Suction Pressure
• Low on charge
• Low airflow /load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower, etc.
• Metering device restricting flow too much – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Low ambient (Low evaporator load)
• Extremely Kinked suction line (after the kink)
• Internal evaporator restriction

High Suction Pressure
• Overcharge
• High return temperature (Evaporator Load)
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Too much airflow over the evaporator (Blower tapped or set too high)
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, other compression issues
• Reversing valve bypassing
• Discharge line restriction

Low Head Pressure
• Low on charge
• Low ambient temperature / low load
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Wet condenser coil
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, other compression issues
• Reversing valve bypassing (heat pump units)
• Kinked suction line
• Restricted discharge line
• Severe Liquid Line Restriction
• Wet Condensing Coil

High Head Pressure
• Overcharge
• Low condenser airflow – condensing fan not operating, dirty condenser, fins bent on the condenser, bushes too close to the condenser, wrong blade, wrong motor, blade set wrong
• High outdoor ambient temperature
• Mixed / incorrect refrigerant/retrofit without proper markings
• Non-condensables in the system
• Liquid line restriction + overcharge (someone added charge when they saw low suction) – piston too small, piston or TXV restricted, TXV failing closed, restricted line drier

Low Superheat
• Overcharge
• Low air flow / load – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Low return air temperature
• Abnormally low humidity
• Internal evaporator restriction
• Very Poor Compression (Compressor, reversing Valve Issues) but will also be combined with VERY HIGH suction

High Superheat
• Low on charge
• Metering device restricting flow / underfeeding / overmetering – piston too small, piston or TXV restricted, TXV failing closed
• High return air temperature
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper

 

Low Subcooling
• Low on charge
• Metering device allowing too much refrigerant flow – piston too large, TXV failing open, piston seating improperly
• Compressor not pumping properly – leaking suction valve, leaking discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge Line Restriction
• Compressor not pumping

High Subcooling
• Overcharge
• Metering device restricting too much flow – piston too small, piston or TXV restricted, TXV failing closed
• Liquid line restriction – clogged filter/drier, clogged screen, kinked copper
• Dirty Condenser Coil on New High-Efficiency Condensers (Increased Condensing Temp Can Actually Result in Higher Subcooling)
• Having an H.R.U. in the discharge line (old school I know)
• Internal evaporator restriction

High Evaporator Air Temperature Split
• Low air flow – dirty filter, dirty evaporator, kinked return, return too small, not enough supply ducts, blower wheel dirty, blower not running correct speed, insulation pulling up against the blower etc.
• Abnormally low humidity (WB Temp)
• Blower not running the correct speed or running backward

Low Evaporator Air Temperature Split
• Undercharge
• Severe Overcharge with fixed orifice metering device – because saturation temperature is increased with overcharge
• Metering device not functioning properly – restricting too much flow or allowing too much flow
• Too much airflow through the evaporator – blower not running correct speed
• Heat strips running with air
• Abnormally high humidity
• Liquid line restriction
• Compressor not pumping properly – bad suction valve, bad discharge valve, bad or broken crank
• Reversing valve bypassing
• Discharge line restriction

 

This is an incomplete list designed to help you. Always keep your eyes and ears open for other possibilities. Diagnosis is an art as well as a science.

The MeasureQuick app is a great free app that can help you in making a complete diagnosis using these 5 pillars and more.

— Bryan


We have been discussing a lot of methods for checking a refrigerant charge without connecting gauges over the last few months. This got me thinking about the “approach” method of charging that many Lennox systems require.

Approach is simply how many degrees warmer the liquid line leaving the condenser is than the air entering the condenser. The approach method does not require gauges connected to the system but it does require a good temperature reading on the liquid line and suction line (Shown using the Testo 115i clamp and 605i thermo-hygrometer smart probes).

When taking an approach reading make sure to take the air temperature in the shade entering the coil and ensure you have good contact between your other sensor and the liquid line.

The difference in temperature between the liquid line and the outdoor temperature can help illustrate the amount of refrigerant in a system as well as the efficiency of the condenser coil. A coil that rejects more heat will have a leaving temperature that is lower and therefore closer to the outdoor temperature. The liquid line exiting condenser should never be colder than the outdoor air, nor can it be without a refrigerant restriction before the measurement point.

Here is an approach method chart for an older 11 SEER Lennox system showing the designed approach levels.

While most manufacturers don’t publish an approach value, you can estimate the approach by finding the CTOA (Condensing Temperature Over Ambient) for the system you are servicing and subtracting the design subcooling.

6 – 10 SEER Equipment (Older than 1991) = 30°F CTOA

10 -12 SEER Equipment (1992 – 2005) = 25°F CTOA

13 – 15 SEER Equipment (2006 – Present) = 20°F CTOA

16 SEER+ Equipment (2006 – Present) = 15°F CTOA

I did this test on a Carrier 14 SEER system at my office so the CTOA would be approximately 20°

Then Find the design subcooling. in this case, it is 13°F

Subtract 13°F from 20°F and my estimated approach is 7°F +/- 3°F. I used the Testo 115i to take the liquid line temperature and the 605i to take the outdoor temperature using the Testo Smart Probes app and I got an approach of 4.1°F as shown below.

More than anything else, the approach method can be used in conjunction with other readings to show the effectiveness of the condenser at rejecting heat.

If the system superheat and subcooling are in range but the approach is high (liquid line temperature high in relation to the outdoor air), it is an indication that the condenser should be looked at for condition, cleanliness, condenser fan size and operation and fan blade positioning. If the approach is low it can be an indication of refrigerant restriction when combined with low suction, high superheat and normal to high subcooling.

If the approach value is low with normal to low superheat and normal to high suction pressure and high subcooling it is an indication of overcharge.

The approach method is only highly useful by itself (without gauges) on a system that has been previously benchmarked or commissioned and the CTOA and subcooling or the approach previously marked, or on systems (like Lennox) that provide a target approach specific to the model.

— Bryan

I am consistently surprised by how much false information still circulates out in the field and one of the ones I hear often is the idea that you cannot or should not “top-off” or recharge R410a systems on top of an existing charge of R410a when the system is low.

So to be clear before we move on, it is 100% OK to add to an R410a charge without fear of any significant fractionation. If you doubt me, you can read THIS from Dupont/Chemours.

R410a is a near-azeotropic blend of 50% R32 and 50% R125. This means that while it has a tiny amount of temperature glide you can still work with it like a zero glide (azeotropic) refrigerant for all practical purposes.

The fear that some have is that if the refrigerant leaks out in vapor phase, one refrigerant will leak at a higher rate than another which could change the blend as it leaks.

While this can (and does) occur with high glide refrigerants, it has been proven that this is most likely to occur in very slow leaks during long periods of storage when the refrigerant is not moving. An example would be a high glide blend in a tank with a slow leak at the valve on top. This is the worst case scenario and an example of where fractionation can be a real issue.

In a running system or a system that runs most of the time, it is unlikely that fractionation would pose an issue because the movement of the refrigerant in the circuit mixes the refrigerant and prevents one part from leaking significantly faster than another. This study by Purdue covers this as it relates to flammability risks.

The practice of charging blends in liquid phase still makes good sense because fractionation, to the extent it occurs is still most likely to pose an issue in a static vessel like a tank and charging in the liquid state is just cheap insurance against fractionation.

But once again… It does no harm to top off an R410a system with R410a. This is NOT to say I’m advocating recharging systems without finding and repairing leaks where possible, just that fractionation isn’t a reason not to do so.

— Bryan

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