No engineering degree required — here's a plain-language explanation of how a heat pump turns ambient air into hot water.
Written by the March Technology Technical Team · Last Updated: July 2026 · 5 min read
The core idea behind a heat pump is simple: rather than generating heat directly from electricity like an immersion element, it moves existing heat from one place to another — in this case, from the outdoor air into your water tank. This is the same underlying principle as an air conditioner or refrigerator, just running to produce heat instead of cold.
In practice: the outdoor unit draws in air and passes it over a coil containing refrigerant, which absorbs the ambient heat and evaporates into a gas. A compressor then squeezes that gas, which raises its temperature significantly. This hot refrigerant gas passes through a coil inside the water tank, transferring its heat to the water before cycling back outside, cooled, to repeat the process. Because moving heat takes far less energy than generating it, the whole cycle uses a fraction of the electricity a direct immersion heater would need — see our benefits of heat pumps guide for what that means in practice.
Core principle: Moves heat rather than generating it
Typical efficiency: 3-4x more efficient than direct electric heating
Operating hours: Works day or night, in most ambient temperatures
Main components: Outdoor unit, compressor, refrigerant coil, water tank
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Yes, exactly — it uses the same refrigerant cycle as an air conditioner or refrigerator, just configured to move heat into your water rather than pulling heat out of a room.
Yes, they extract heat from ambient air regardless of whether it's day or night, unlike solar water heaters which depend on direct sunlight.
Yes — heat pumps don't rely on sunlight at all, only on ambient air temperature, so cloudy or rainy weather has minimal effect on performance.
A fan draws air across a refrigerant-filled coil, and a compressor raises the temperature of the absorbed heat before it's transferred to your water tank.