How a heat pump actually works

Your fridge takes heat out of the food compartment and dumps it into the kitchen — that is what the warm grille at the back is. It does this by pumping a refrigerant round a loop, compressing it (which makes it hot) and letting it expand (which makes it cold).

A heat pump is that machine aimed at your house. It takes heat out of the outside air or the ground and dumps it inside. The reason this beats an electric heater is that the electricity is not being turned into heat — it is only running the pump that moves heat that was already there.

Hence the awkward question people ask: how can it work when it is cold outside? Because “cold” is relative. There is still plenty of heat energy in air at 0°C — a long way above absolute zero. Efficiency does fall as the outside temperature drops, which is why sizing for your actual climate matters more than the number on the box.

If you only remember one technical distinction, make it this one, because it decides how much grant money is on the table in the UK:

• Air-to-water. Takes heat from outside air and puts it into water — your radiators, underfloor heating and hot water cylinder. This is what most people mean by “getting a heat pump” and it replaces a boiler.
• Air-to-air. Takes heat from outside air and blows it in as warm air. No radiators, no hot water. Closer to reversible air conditioning — and in most cases it can cool in summer too.
• Ground source. Takes heat from the ground through buried pipework rather than from the air. More stable source temperature, considerably more disruptive and expensive to install.

Under the UK’s Boiler Upgrade Scheme that distinction is worth thousands: air-to-water and ground source attract £7,500, air-to-air attracts £2,500 and only for residential properties. The £9,000 off-gas-grid uplift applies to air-to-water and ground source only — not air-to-air.

A gas boiler will happily push water round your radiators very hot. A heat pump is most efficient when it does not have to — the smaller the gap between the source temperature and the output temperature, the less work the pump does.

That is the real reason heat pump installations involve conversations about radiator sizes, pipework and insulation. A well-designed system runs cooler water through bigger emitters for longer. A badly designed one tries to behave like a boiler, works hard, and produces the horror stories.

It is also why the survey matters more than the brand. Two identical units in two identical houses can perform very differently depending on how the system around them was designed.

• It will not fix a leaky house. It will heat one, but the running cost follows the heat loss.
• It is not instant like a combi. Hot water usually comes from a cylinder, which needs space and a bit of planning around usage.
• It is not silent. There is an outdoor fan unit. Modern ones are quiet, but siting still matters — and so do your neighbours.

If you are in the UK, the money question is the grants overview — and if your home is off the gas grid, the £9,000 grant is the one to read first. If you are in the US, start with what is left after the federal credit ended.