Key Takeaways
- The short explanation of how heat pump hot water works is: units draw in air, absorb heat, amplify it and pump it into water. For the longer story, read on….
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‘Heat pumps’ are a broad technology found in a range of applications. For example there are heat pump air conditioners (“split systems”), heat pump pool and spa heaters, heat pump dryers and of course, heat pump hot water.
A heat pump is called a 'heat pump' because it, quite simply, pumps heat from one area to another.
Fun fact: your split system is a ‘heat pump.’ For some reason we call them ‘splitties’ in Australia, while the rest of the world calls them heat pumps.
A heat pump gathers heat from one place and puts it in another place - this can be like a fridge or air conditioner, extracting heat from one place and sending it outside. “Reverse cycle air conditioner” can send the heat both ways, thereby offering heating and cooling. When operating in heating mode, reverse cycle air conditioners draw heat from outside and pump it inside. When operating in cooling mode, they draw heat from inside and pump it outside.
Heat pump hot water works through the following process:
- A fan draws in air. There is heat in the air, always, even at really cold temperatures. For the science nerds among us, there are three temperature scales: Celsius, Fahrenheit and Kelvin. Kelvin is a measure of absolute temperature: 0 degrees Kelvin, when there is a ‘complete absence of thermal energy’, is -273 degrees Celsius. So even though 0 degrees Celsius is chilly and will turn water into ice, there’s still a very long way to go before there is no heat in the air. In practice, heat pump technology around the world operates without issues as low as -20C.
- The heat in the air is absorbed by a refrigerant, the hero of heat pump technology. A refrigerant is a fluid (it’s a liquid early on in the process and a gas later on) that moves around the heat pump. The refrigerant absorbs heat in the air by flowing through a heat exchanger. As it flows through, it starts boiling (converting from a liquid to a gas), effectively collecting heat.
- The refrigerant is then compressed (hence the term “Compressor”) which increases the pressure and heats it up further.
- The refrigerant is now a hot gas. It flows to a heat exchanger, transferring the heat into the water with the help of a fan.
- The refrigerant cools down as the heat is transformed. It is also allowed to expand, which has the opposite effect of compressing it, allowing it to cool down and turn back into a liquid. The process then restarts.
Fun fact: heat pumps can draw their source of heat from multiple heat sources. There are air-sourced, water-sourced and ground-source (or geothermal) heat pumps. Air-sourced heat pumps are most common in Australian heat pump hot water units.
Do they work in the cold?
Yes - many with a political agenda including fossil fuel companies claim they don’t. Heat pumps are prominent throughout cold climates of Europe and North America, far colder than Australia. For example, 60% of buildings in Norway are heated with heat pumps.
Heat pumps work less efficiently in the cold as there is less heat to capture, so they need to work harder. The colder climate may also limit the choice of refrigerant - some will not work so well.
If you live in a cold climate area of Australia like Hobart or Canberra, you can certainly still benefit from heat pumps - you may just need to be more selective.
