How does a heat pump work and when to choose it?


Heat pumps work like a reverse refrigerator by using the differential between the ambient air temperature and the refrigerant in the heat pump, which is at low temperatures. When this heat is absorbed into the refrigerant it evaporates and becomes a gas which is then compressed to superheat it – this high temperature is then transferred to the water via a heat exchanger.

Unlike the refrigerator, which gives off heat after it has cooled the contents of the fridge, a heat pump gives off cold air so it is important to site the heat pump in an area with good air circulation otherwise it will be sucking in cold air instead of ambient temperature air. This would force it to work a lot harder and use more electricity. The ideal siting is against a North or West facing wall where there is good air circulation. The warmer your climate the more efficient will be your heat pump – having said that, heat pumps will still work in sub-zero conditions.

The differences between a heat pump and a solar hot water heater are as follows:

  • Heat pumps have a number of moving parts – a fan, pump and compressor as compared to a thermosiphon solar hot water heater which has none, or a ground mounted tank which has a pump.
  • A heat pump uses electricity all year round, whereas an electrically boosted thermosiphon system only uses electricity to boost in the winter and on cloudy days at other times or days of high hot water demand. Additionally, a ground tank solar hot water system uses small amounts of electricity to run the solar pump and controller.
  • Most heat pumps do not have anything on the roof which means they are much easier and cheaper to install.
  • Heat pumps produce some noise – some more than others. If this noise is during the night it may be noticeable.


  • When you do not have a North facing roof
  • Or you have a North facing roof at the front of your home in an area with a heritage overlay
  • Or your North facing roof is covered in shade
  • Or you have PV panels on your roof that only earn a small feed-in tariff – so you have excess “very low cost” electricity during the day.


If you have a recently installed a PV array for making solar electricity and are only receiving 8 – 10 cents per kW/h, it makes a lot of sense to use a heat pump to heat your water, using a timer to turn on the heat pump between say 11 am and 3 pm when the ambient air temperature is highest and when your PV system is at maximum production.

NEW PRODUCT (Coming Soon)

This makes so much sense that Best Solar Hot Water has worked with a PV specialist to design and supply an add-on package of a 1.5 kW PV system that is grid interactive and will provide enough electricity to provide almost all your hot water for 9 months of the year and up to 60% of your hot water for the remaining 3 months of the year for a correctly sized system.

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