How a heat pump heats a house

A gas boiler makes heat. It takes a fuel with chemical energy in it, burns it, and hands the result to the water in your radiators. However well it is built, it can never give out more heat than was in the gas, and in practice it gives out rather less.

A heat pump does something different. It does not make heat at all. It collects heat that already exists in the air or the ground outside, concentrates it, and releases it indoors. The electricity it uses is not the source of the heat; it is what powers the moving.

This is why a heat pump can deliver three or four kilowatts of heat for every kilowatt of electricity it draws, a figure that sounds like a conjuring trick until you notice that the other two or three kilowatts came from the garden.

The cycle

Inside the machine is a sealed loop of refrigerant, a fluid chosen because it boils at a very low temperature. Four things happen to it, over and over.

1. It evaporates. Cold liquid refrigerant, colder than the outside air, passes through a heat exchanger with outside air blown across it. Heat flows from the air into the refrigerant, as heat always does, from warmer to colder. The refrigerant boils and becomes a gas. The air leaves a few degrees colder than it arrived, which is why the air coming off a running heat pump feels chilly.
2. It is compressed. A compressor squeezes the gas. Squeezing a gas raises its temperature, which is the same reason a bicycle pump warms up. The refrigerant leaves the compressor hot, considerably hotter than the water in your heating system. This is the step the electricity pays for.
3. It condenses. The hot gas passes through a second heat exchanger, with your heating water on the other side. Heat flows out of the refrigerant into the water. The refrigerant cools and turns back into a liquid, and the water goes off to the radiators.
4. It expands. The liquid passes through an expansion valve, its pressure drops, and its temperature falls back below the outside air. It is ready to collect more heat, and the cycle begins again.

A refrigerator does exactly this, with the cold end inside the cabinet and the hot end at the back. A heat pump is a refrigerator arranged so that the useful end is the hot one.

The one thing that matters: the lift

The work the compressor has to do depends on the gap it has to bridge, between the temperature of the heat it collected and the temperature of the water it must produce. That gap is called the lift, and it governs almost everything about how a heat pump behaves.

Collecting heat from air at 7 °C and producing water at 35 °C is a lift of 28 degrees, and a modern machine will return four units of heat for one of electricity. Collecting from the same air and producing water at 55 °C is a lift of 48 degrees, and the same machine will return perhaps 2.8. The machine is the same in both cases, and only the size of the lift has changed.

This is why so much of the design work in a heat pump installation is really about the temperature of the water, and why the standards devote so much attention to radiator sizes. A bigger radiator delivers the same heat from cooler water, and cooler water is a smaller lift, which is why the emitters matter to the running cost as much as the machine does.

It also explains the weather. A heat pump works harder in February than in October, because the air it collects from is colder while the water it must produce is hotter. Efficiency quoted at one temperature says little; the seasonal figure, which averages a year, says a great deal.

Air and ground

An air source heat pump takes heat from outside air. It is a box outside the house with a fan in it, connected to the heating system by two insulated pipes. It is cheaper to install because there is nothing to dig, and the great majority of British installations are of this kind.

Its weakness is that outside air is coldest exactly when the house wants most heat. It also has to defrost: when the outside heat exchanger falls below freezing, moisture from the air condenses on it and turns to ice, and the machine must periodically run backwards for a few minutes to melt it. That costs a little energy and a little output, and it is why you sometimes see steam rising from a heat pump on a cold morning.

A ground source heat pump takes its heat from pipes buried in the garden, either in long horizontal trenches or in vertical boreholes. Two or three metres down, the ground stays close to the local annual average air temperature all year, which in Britain is somewhere between 8 and 11 °C. The heat pump therefore faces a much smaller lift in January, and returns a higher seasonal efficiency for it.

The cost of that is the groundworks, which are substantial, and the need for either land or a drilling rig. It also introduces a design problem that air source does not have: a ground loop that is too small for the load will gradually cool the ground around it over a season, so the standards set limits on how cold the loop may be allowed to run.

How one behaves in a house

People used to boilers are often surprised by how a heat pump runs, and a good deal of dissatisfaction traces back to running one as though it were a boiler.

A boiler is powerful and intermittent. It blasts heat into the house for twenty minutes, stops, and lets the house cool. A heat pump is the opposite: modest and continuous. It is at its most efficient producing the lowest water temperature that will keep up, and it keeps up by running most of the time, not in bursts.

Modern units modulate, meaning the compressor can run at a fraction of full speed. A well set up system spends a January day ticking over at perhaps a third of its capacity, producing water at 38 degrees, and the house simply stays where it was put. Weather compensation does this automatically: the controller reads the outside temperature and produces exactly as much water temperature as the day requires, no more.

Two consequences follow. First, turning the heating off during the day usually costs money instead of saving it, because recovering the lost temperature demands hotter water than steady running ever needed. Second, closing radiators off in unused rooms works against the system, since it reduces the surface the heat pump has to give its heat away through and pushes the flow temperature up.

What a house needs to have

A heat pump does not require a new house, but it does require that a few things be true, and finding out whether they are is what a survey is for.

Emitters large enough to deliver each room's heat loss at a low flow temperature. Sometimes the existing radiators are already generous enough, because they were sized for a fabric the house no longer has, but often some of them need replacing.

Pipework that can carry the flow. A heat pump moves more water than a boiler did, because it moves it at a smaller temperature drop. Microbore pipework in particular can become the limit on the whole system.

Somewhere for a hot water cylinder, if the house has a combination boiler and therefore has not needed one, because a heat pump cannot heat water on demand.

An outside position for the unit that meets the noise limits and the siting rules, and a network connection the local operator is content with.

None of these is exotic. All of them are cheaper to establish before signing a contract than to discover afterwards.