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.