How Heat Pumps work

Heat pumps are common

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. That’s why it’s good to have space around your fridge. This space allows the heat extracted from inside the fridge to move away from the fridge.
  • 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 are efficient

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.

Heat pumps for hot water

Here is a description of one sort of heat pump and how you can use it to heat your hot water. With this setup, you run the heat pump when the sun is up, powering the heat pump with electricity from photovoltaic solar panels. After installing the panels and heat pump, you heat your water for free, generating no emissions. 

How a heat pump works to heat water

A heat pump can heat your hot water by moving heat from the air outside your house into your hot water tank. Your heat pump will have:

  • A refrigerant: The heat pump has a refrigerant, i.e., a liquid that boils at a low temperature like minus 26 C.
  • An electric fan: The fan warms the refrigerant by blowing air from outside your house over a heat exchanger, which warms the refrigerant. While the air temperature is above minus 26 C, it will heat and boil the liquid refrigerant in the evaporator.
  • An Evaporator: In the evaporator, the heated refrigerant evaporates, becoming a gas as the external air heats it. It evaporates like water becomes steam in a pot on the stove. As the boiling point is so low, e.g., minus 25 C, it will work even on a cold day when the air temperature is 5 C. Note that the fan works longer to boil the refrigerant on a cold day.
  • A Compressor: A valve closes, shutting the refrigerant gas in the compressor where a pump compresses the air. 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.
  • A Condenser: 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 gas cools and condenses into a moderate temperature liquid, still under high pressure.
  • An Expansion Valve: The expansion valve opens, and the cooled liquid moves from the high-pressure condenser into the evaporator at normal room pressure. The refrigerant is back where it started, a liquid ready for heating and evapourisation, ready to repeat the cycle.

So, you can harvest heat by repeatedly boiling a liquid, collecting the gas given off, and compressing the gas.

Further information


Updated: 24 Jan 2022

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