How Heat Pumps work

Heat pumps are common & efficient

Electric heat pumps are the essential part of standard household equipment:

  • In fridges, a heat pump moves heat from inside the fridge into your kitchen.
  • In reverse cycle air-conditioners, the heat pump cools a room by moving heat from the room to outside the house. When you reverse the cycle, the heat pump heats the room by transferring heat from outside your home to inside the room.

Heat pumps use electricity to run a fan and compressor – and can be amazingly efficient. A high-performance residential heat pump can use one unit of electrical energy to move six times as much heat energy into your house, an efficiency of 600%.

By comparison:

  • Electrical heaters have an efficiency of 100% as they can convert all the electrical energy into heat, and
  • Gas heaters have an efficiency of between 50% and 95%, as some of the heat generated heats the house exterior.

A Heat pump hot water service

One use of a heat pump is to heat your hot water. You can run the heat pump when the sun is up and use electricity from photovoltaic solar panels to power the heat pump. After installing the panels and heat pump, you heat your water for free, generating no emissions. 

The heat pump heats your hot water by moving heat from the air outside your house into your hot water tank. It uses a refrigerant, a liquid that boils at a low temperature like minus 26 C and works in a cycle as follows.

(The numbered points in the following text further explain the below diagram.)

  1. In the heat pump evaporator, the refrigerant is intially a liquid at low pressure.
  2. Boiling the refrigerant: An electric fan warms the refrigerant by blowing air from outside the house over a heat exchanger, which transfers the heat to the refrigerant. While the air temperature is above minus 26 C, it will heat and boil the liquid refrigerant. The refrigerant evaporates like water becomes steam in a pot on the stove. The cold air of a freezing day will still boil the refrigerant because boiling point of the refrigerant is so low. However, on a cold day, the system is less efficient as the fan must work longer to boil the refrigerant.
  3. In the compressor, the refrigerant is initially a cool gas at low pressure.
  4. Compressing the refrigerant gas: A valve closes, shutting the refrigerant gas in the compressor where an electric pump compresses the gas. As the pressure builds, the gas gets hotter, just as a bicycle pump gets hot as you pump up the tyres. The gas can heat to 95 C.
  5. In the condensor, the refrigerant gas is initially hot and at high pressure.
  6. Producing hot water & cooling the refrigerant: The hot, pressurised gas then passes its heat, via another heat exchanger, to where you want it, to the water in your hot water tank. In doing this, the refrigerant gas cools and condenses. The refrigerant is under pressue so the condensing / boiling point is higher than it was at room pressure in the evaporator.
  7. Pressurised Liquid: Now, this refrigerant is a warm liquid, still under high presssure.
  8. Dropping the pressure: The expansion valve opens and the pressure drops, causing the liquid refrigerant to vapourise. It mixes with the liquid refrigerant at room pressure in the evapourator.
  9. Liquid refrigerant at room pressure: The refrigerant is back where it started in the evaporator, ready to repeat the heat harvesting cycle shown below.

The Heat pump cycle

Refrigerant liquid under low pressure.
(1) & (9)
Evaporator: Fan blown air boils the refrigerant (2)
Expansion valve: Release the pressure (8)Refrigerant gas, cool & at low-pressure (3)
Refrigerant warm liquid at high pressure (7)Compressor: Pump pressurises the refrigerant gas (4)
Condenser: Producing hot water cools the refrigerant (6) Refrigerant gas at high temperature & high-pressure (5)

Further information

Updated: 29 Jan 2022

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