The fan pulls outside air through hundreds of thin aluminium fins. The fins give the air lots of metal surface to touch, so heat can pass into the pipes behind them.

A typical home unit moves about 2,000 to 4,000 cubic metres of air an hour. That's like emptying a small house of air every few minutes.

If the fins get blocked by leaves or packed snow, less air touches them and the unit collects less heat. That's why outdoor units sit raised on feet, clear of the drifts.

The pipes are sealed and never open. The same few kilograms of liquid, called refrigerant, travel round the loop for years.

One pipe is wrapped in white foam so it doesn't lose heat on the way. The hole in the wall only needs to be about 6 cm wide.

If the loop leaks, the refrigerant escapes and the heat pump slowly stops working. Installers test the pipes under pressure before filling them.

Inside the indoor unit is another set of fins and pipes, plus a quiet fan. Room air blows across the hot pipes and comes out about 35–45 °C.

Some homes use radiators or warm floors instead of a wall unit. The idea is the same: the heat in the pipes is handed to the house.

So the question is: how do you get heat out of air that's already freezing? First, here's why it's worth the trouble.

Engineers call this the coefficient of performance, or COP. A COP of 3 means 3 units of heat for every unit of electricity. An electric heater has a COP of exactly 1.

The COP is lower on very cold days, because the compressor has to squeeze harder. Good cold-climate units still reach about 1.75 or more at −15 °C.

Frost forms when water in the air freezes onto the icy fins. It blocks the airflow, so less heat can be collected.

Sensors spot the frost, then the reversing valve flips the loop for a few minutes. You might see a puff of steam from the outdoor unit; that's the frost melting, not smoke.

In Norway, a very cold country, about two out of three homes have a heat pump. They're built for long, dark, freezing winters.

Because they use far less energy than burning fuel, heat pumps can cut heating bills and pollution, especially when the electricity is clean. So how does the free heat get in? Let's x-ray the wall.

The loop has four main parts: an outdoor coil, a compressor, an indoor coil and an expansion valve. The refrigerant visits them in the same order, over and over.

The colours show temperature: blue is icy, orange is hot, and yellow is warm. Watch how the colour changes at each part.

Many new heat pumps use a refrigerant called R-32. At normal air pressure it boils at about −52 °C, so even freezing air is hot enough to boil it.

Turning a liquid into a gas takes a lot of heat, and turning the gas back into a liquid releases that heat again. The refrigerant uses this to carry heat from one end of the loop to the other.

The loop is sealed for life. A few kilograms of refrigerant go round and round, never used up, unless there's a leak.

The two parts on the outside wall, the compressor and the valve, control pressure. Pressure is what sets how hot or cold the refrigerant is at each moment.

If any one part fails, the loop stops working. A stuck valve or broken compressor means no heat comes in, even though the fans still spin. Let's visit each stop, starting with the compressor, the heart of the loop.

When you push on a gas, the work you do goes into the molecules, making them move faster. Faster jiggling means higher temperature.

Home compressors are usually scroll or rotary types, not pistons, but the job is the same. They spin tens of times a second, and they're the part that uses most of the electricity.

If the compressor stops, nothing gets squeezed, so the indoor coil stays cold. That's the most expensive part to replace.

The pressure in the hot pipe is high, around 30 times normal air pressure. That's why the pipes are thick copper with brazed joints.

The squeeze doesn't create most of the heat. It concentrates heat collected from outside, so the same heat is now at a useful temperature.

Modern inverter compressors can change speed smoothly instead of switching on and off. On mild days they hum along slowly and save energy.

Cold-climate heat pumps use stronger compressors and special tricks to keep squeezing hard when it's −25 °C outside. Up next: the hot gas gives away its heat indoors.

Heat always flows from hot to cold. Now the refrigerant is the hot one, so its heat flows into the 20 °C room air.

Outside, as we'll see soon, the same downhill rule works the other way round: there the refrigerant is colder than the air, so heat flows in.

As the gas cools, it condenses back into a liquid. Turning back into a liquid gives out all the heat the refrigerant soaked up outside by boiling.

Heat pumps work best with a gentle, steady warmth instead of short blasts. Many run almost all day in winter, using little power.

Some systems heat water instead of air. The hot coil warms water for radiators, underfloor pipes or a hot water tank.

If the indoor fan or filter is blocked, heat can't leave the coil easily, so the system works harder and warms the house less. Next stop: the valve.

The valve is a small brass part with a needle that can move in and out. A computer or a sensor adjusts the gap every few seconds.

If the gap is too wide, liquid floods the outdoor coil without boiling fully. Too narrow, and too little refrigerant flows to collect heat.

Lower pressure means a lower boiling point. Right after the valve, the refrigerant's boiling point falls far below its own temperature, so part of it boils instantly and chills the rest.

This is the step that makes the fins colder than the outside air. Without the valve, the loop would never get cold enough to collect heat in winter.

The whole trip round the loop takes about a minute in a typical home system. Each lap carries another load of heat inside.

Next stop: the outdoor coil, where the icy liquid meets the freezing air.

How much colder the refrigerant is than the air decides how fast heat flows. A gap of about 10 °C is typical. On a colder night, the system lowers the pressure to make the refrigerant colder still.

The thin fins are pressed onto these pipes. They give the air about 20 times more metal to touch than bare pipes would.

Turning a liquid into gas takes a lot of energy. Boiling one kilogram of R-32 soaks up roughly 300 kilojoules of heat, enough to warm a large mug of tea by about 70 °C.

By the end of the coil, all the liquid has boiled into a cool gas. It's still below freezing, but now it's holding all the heat it collected.

Chilled air comes out of the back of the unit about 5 °C colder than it went in. Then it mixes with the rest of the outdoors, which has endless heat to spare.

Below freezing, water in the air can freeze onto the cold fins as frost, which is why the unit defrosts now and then. But how can −15 °C air have heat to give at all? Let's zoom between two fins.

Temperature is a measure of how much the molecules are jiggling. In a warm room, an average air molecule zooms at about 500 metres per second, faster than a passenger jet.

We can't see molecules: each one is less than a millionth of a millimetre across. This view is zoomed in millions of times and slowed right down.

The coldest anything can ever be is −273 °C, called absolute zero. Compared with that, −15 °C air is still packed with heat: about 88% of the jiggle energy of a 20 °C room.

That's the secret. Freezing air isn't empty of heat. It just has a bit less than we're used to, and there's an enormous amount of air outside.

Watch the molecules near the fins: they slow down and turn blue as they hand their energy to the colder metal.

That's the whole trick: the valve keeps the fins at about −25 °C, colder than the −15 °C air, so the air's jiggle flows in and the loop carries it home.