Heat pump water heaters operate using a refrigeration cycle that moves heat from a high temperature source (the outside air) to a low temperature destination (the hot water in the tank).

In nature heat flow tends to go from higher to lower temperatures. Heat pumps mimic this natural flow by using a refrigerant that boils at a temperature less than the coldest expected outside air temperature. This is the same principle at the core of the operation of reverse cycle air conditioners and refrigerators.

What is the process?

A heat pump is a closed circuit consisting of a compressor, a condenser, an evaporator and an expansion valve. A refrigerant flows through this circuit, taking on a liquid or gaseous state in response to pressure and temperature.

1. In the heat pump, the evaporator coil is exposed to the outside air.
2. The refrigerant enters the evaporator as a cold liquid/gas mixture at low pressure.
3. Heat from the outside air causes the refrigerant to boil into gas.
4. This gas is pumped through to the condenser coiI by a compressor which also increases the refrigerant’s temperature and pressure.
5. The refrigerant is now hotter than the temperature of the water in the tank, so heat flows out, heating up the water in the tank and condensing the refrigerant back into a liquid.
6. The refrigerant then flows through an expansion valve that allows the pressure and temperature to drop again by the time it reaches the evaporator coil.
7. Once the refrigerant reaches the evaporator coil the cycle is repeated.

 

How much heat will a heat pump produce?

For heat pumps operating within their intended temperature range, the amount of heat moved from the evaporator to the condenser can be between two and five times the amount of energy required to operate the compressor.

The ratio of the heating produced over the electrical energy consumed is referred to as the “Coefficient of Performance” or COP. Where the COP is greater than one it means the heat pump is more energy efficient than an electric storage water heater where the heat delivered to the tank is equal to the electrical energy consumed.

The advantage of using a heat pump is that it produces more heat (energy) than the energy it uses to achieve this.The COP ranging from 2 to 5 allows a heat pump to achieve a huge energy saving compared with an electric resistance (also known as electric storage) water heater where the heat delivered to the tank is equal to the electrical energy consumed. The higher the COP the better.

What factors impact a heat pump’s Coefficient of Performance?

• A heat pump’s COP generally increases with increasing outside air temperature 
• COP generally decreases as the water in the tank heats up. 
• High levels of moisture in the air on frosty days may inhibit the evaporator’s heat transfer efficiency.

Design Features

• Some heat pumps have sub systems that detect or predict the formation of frost and periodically heat the evaporator coil to remove frost by reversing refrigerant flow.
• Other heat pumps use an electric resistive heating element in the tank which heats the water instead of the heat pump when the outside temperature is low.

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