How a heat pump works

A gas furnace or an electric radiator makes heat — by burning fuel or forcing current through a wire. A heat pump makes almost none. Instead it collects heat that already exists in the outside air, or the ground, and moves it indoors.

That sounds impossible in winter, because cold air does not feel like a source of warmth. But “cold” is relative. Even at −15 °C the air is far above absolute zero (−273 °C), so it still holds an enormous amount of thermal energy. A heat pump’s job is to gather that low-grade heat and concentrate it.

It does this with a sealed loop of fluid, a pump, and one everyday piece of physics: compress a gas and it gets hot; let it expand and it gets cold. Everything below follows from that.

01Heat is not the same as temperature

Temperature tells you how hot something feels. Heat is the total thermal energy inside it. A lake at 4 °C feels cold, yet it contains far more heat than a red-hot nail — simply because there is so much more of it.

So there is plenty of heat in winter air. The catch is that it sits at a low temperature — lower than your living room — and heat only flows on its own from warm to cold. To move it the “wrong” way, from the cold outdoors into a warmer house, you need a device that pushes it uphill. That device is the refrigeration cycle.

A heat pump does not create heat. It relocates it — and spends a little electricity on the move.

02The four-part loop

Seal a special fluid — the refrigerant — inside a closed loop of pipe, and choose one that boils at a very low temperature, well below freezing. Now push it around the loop through four components, in order.

Play the animation and select each component to read the refrigerant’s pressure, temperature and state at that point. Follow one drop the whole way round and the trick becomes obvious: it is colder than the outside air when it meets it, and hotter than your rooms when it meets them — because the compressor and the valve change its pressure in between.

03Not 100 % efficient — more like 300 %

A resistance heater turns 1 kWh of electricity into 1 kWh of heat. That is its ceiling; it cannot do better. A heat pump is not really an “efficiency” device at all, so it is not bound by that ceiling. For every unit of electricity the compressor uses, several more units of heat are dragged in from outside for free.

That ratio — heat delivered ÷ electricity used — is the coefficient of performance, or COP. A COP of 3 means 1 kWh in, 3 kWh of heat out. COP falls as the outside air gets colder, because the compressor must bridge a bigger temperature gap. Move the slider.

04Air source vs ground source

The loop is identical; only the outdoor half changes. An air-source heat pump pulls heat from outside air with a fan and a coil — cheap to install, but its COP sags on the coldest days, exactly when you need the most heat. A ground-source (geothermal) heat pump runs its cold side through pipes buried in soil or a borehole, where the temperature holds near 8–12 °C all winter, so its COP stays high and steady. The trade-off is a much larger installation cost.

05Where heat pumps struggle

A heat pump is not magic, and a few honest limitations shape how it is installed.

• Defrost. In damp weather near 0 °C, moisture freezes onto the outdoor coil. The unit briefly reverses to melt it, spending energy instead of collecting it.
• Emitter temperature. Heat pumps deliver warmth at a lower water temperature than a gas boiler. They work best with underfloor heating or large radiators; small old radiators may need upgrading.
• The coldest hours. Air-source COP can fall toward 1 in extreme cold, when a small backup heater takes over. Good design keeps those hours rare.
• Noise. The outdoor fan and compressor make sound. Placement away from bedrooms and neighbours matters.
• Refrigerant. Many refrigerants are potent greenhouse gases if they leak. Newer units use low-impact refrigerants such as propane (R290); keeping the loop sealed matters.

06The short version

• A heat pump moves heat; it does not burn anything to make it.
• Compressing the refrigerant makes it hot enough to heat your home; expanding it makes it cold enough to absorb heat from outside.
• COP is heat out ÷ electricity in — typically 2.5 to 4.5, versus exactly 1 for any resistance heater.
• Cold weather lowers COP; ground-source loops and good insulation keep it high.
• The same loop run in reverse is an air conditioner — and most heat pumps can do both.

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