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!