Tag: heat pump

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

Note: My brother Nathan wrote this a few years back and I only did some minor editing

A pool heat pump is essentially a water-cooled air conditioner in reverse with a large air evaporator on the outside that looks like a condenser coil and a heat exchanger (usually tube in tube) on the inside.

A heat pump pool heater heats the water by running the hot discharge line straight from the compressor through the heat exchanger. The heat exchanger works as the condenser for the refrigeration circuit of the pool heater and the refrigerant flows ion the opposite direction as the water.

Leaving the heat exchanger, the copper line should now be a subcooled liquid line that feeds over to some form of metering device. This metering device will then feed the evaporator coil, which is the large coil around the outside of the heater (this can cause confusion because this coil looks like what we would call a condenser coil in an air conditioning application ). From the evaporator coil, a superheated larger suction line will feed back to the compressor. Keep in mind when checking the charge on a heat pump pool heater, that it will be subject to a wider range of temperatures and loads than a normal air conditioner. Higher water/air temperatures tend to produce higher pressure readings, while colder water will tend to lower pressures.

Remember, outdoor air temperature on a pool heater has the same effect on readings, as indoor air temperature would on an AC in cool mode. Often, when the outdoor air is below 60 degrees you may have a suction saturation below freezing, and if the pool is ice cold this will further lower your readings.

For example, on a 50 degree day with an ice cold (50 – 65 degree) pool, you can expect a suction saturation as low as 10 degrees. You can still check superheat and if possible, subcool, If both of these readings appear normal, then your charge is most likely correct (check manufacturer specs first if there are any). If your pressure / saturation readings are high, always suspect water flow, as low water flow is difficult to gauge in these systems. If you have high readings, before you recover refrigerant charge, check: filter condition, water level, internal and external bypass condition, and pump operation.

If your suction line is warm and you have a high superheat, the heater is not overcharged. The electrical circuit and components on a heat pump will vary in style. However, the function of the components remains constant. There will be some form of user controls which report back to a board that takes this input, as well as a reading from a water temperature sensor to determine if the heater should run or not.

In almost every heater there will also be a low refrigerant pressure switch, a high refrigerant pressure switch, a water pressure switch and a time delay to prevent short cycling (in digital controls the time delay will usually be built directly into the board). There are occasionally, defrost controls, or low ambient controls built into these heaters that will allow the fan to continue running while keeping the compressor off.  The best way to reduce freezing on a pool heater coil (evaporator) is to not run them at night when it’s coldest and nobody is swimming anyway. Keep in mind that most pool heaters do not have any form of true defrost, so effectively these controls are just leaving the heater off until the air temperature has defrosted the heater… assuming the air temperature is over 32… and if it isn’t WHY IS ANYONE SWIMMING!

— 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.

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 manufacturers 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.

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.  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

 

 

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