Review of Heat Pump Parts

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A typical heat pump installation consists of two parts; an indoor, which is usually an air handler or a furnace and outdoor unit where the compressor and evaporator coils are located. During heating an outdoor part is where the evaporator is located to extract the heat from the surrounding air, while during the cooling process, the evaporator is located indoor from where it extracts the heat out via the condenser coil. Below are the main heat pump parts:

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The heat pump compressor is the heart of the system because it circulates the refrigerant through the system loop, and can be located indoors or outdoors. It is designed to compress the low pressure to high pressure vapor and move it from the evaporator to condenser where the refrigerant will change its state from vapor into liquid.

Evaporator/Condenser – heat exchanger coil

Both the evaporator and condenser coils are used as the heat exchangers. A coil is a loop of tubing where the heat transfer occurs. And since the heat pump is used for both heating and cooling it is good to know that the only difference is what direction does the refrigerant flow… meaning that either heat exchanger coil can be used as the evaporator or condenser.


The refrigerant is the liquid or gaseous substance that is used to absorb, transport and release the heat, while circulating through the loop of a heat pump. The most common type is the R-410A that is both efficient and environment-friendly. Most of the manufacturers utilize the new ozone-friendly refrigerants.

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The accumulator is used to collect the excess liquid refrigerant which didn’t evaporate in the evaporator. The liquid will then boil off into a vapor and safely return to the compressor. If for any reason the liquid gets into the compressor it could damage it. The accumulator is not present on all brands and models.

Crankcase heater

The crankcase heater is used in air-source heat pumps to raise the temperature of the oil in the compressor, or keep the compressor oil warm. This allows refrigerant to vaporize, preventing it from mixing with the oil. Otherwise, there will be strain on the compressor due to an increased volume of the liquid, which could damage the compressor.


Electronically Commutated Blower Motors, or short ECM are brushless direct current motors which are used in heat pumps due to their higher efficiency, and ability to work in a wider range of speeds and with less power draw than other motors. These can be found on most high efficient two-stage or multi-stage heat pumps. Other types of motors that are commonly found on standard heat pumps are known as permanent split capacitor blower motors or short PSC.

Reversing valve

Reversing valve is used to control direction of the refrigerant flow, automatically. It changes the heat pump from the cooling mode to heating and from heating to cooling. It is controlled by the room thermostat or defrost control during the defrost cycle. During the failure, it is very easy to troubleshoot and replace the reversing valve. When the reversing valve begins or ends defrost, the valve reverses and pressure in the system equalizes resulting in the “whooshing” sound.

Expansion device

Expansion device is used to control the flow of the refrigerant from the condenser to the evaporator. It is used to reduce the pressure and temperature from high (in condenser) to low (evaporator) and extract the heat. Thermostatic expansion valve are used in the advanced heat pumps for the efficiency increase.

Electronic control

Electronic control is the brain of the heat pumps; it monitors and controls the whole operation of the unit; temperature, speed, defrost mode, diagnostics…

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Thermostatic Expansion Valve (TXV)

Thermostatic Expansion Valve is found on all fan coils and furnace coils and is designed to adjust to changing load conditions by maintaining a preset superheat temperature at the exit of the evaporator coil.


Thermostat is used to control and monitor the temperature, providing better comfort with tighter tolerances.

Defrost system

When the outside air temperature is below 32 F (0 C), and when the air-to-air heat pump operates in the heating mode the frost could build-up on the evaporator coil. This results in reduced air flow and lower efficiency since the evaporator cannot extract heat due to the reduced capacity.

This is the reason why defrosting system is needed (only air-sourced HP), allowing the compressor heat to be re-directed to the outdoor coil and melt the ice. This is a cooling mode which runs for a short time without affecting the indoor climate. The defrost system can be time/temperature related or demand related, which is a preferable method.


Fan is consisted of the fan motor and fan blades. Fan motor rotates the fan blade and depending on the orientation of the blade, the system either draws or blows air and through the outdoor coil it exchanges heat between the refrigerant and surrounding air. On new models, bearings are always lubricated so no lubrication is needed as part of the maintenance.


Variable-speed or dual-speed motors on fans and blowers is better than standard type as it provides a better comfort on the air velocity and the noise level is lower.

The above replacement parts are just for your reference. If you are in need of any of the above heat pump parts, the best is to check one of the manufacturer's websites to get an original part for the service or do it yourself repair.

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