It sounds impossible: how can cold air outside heat your house? The trick is that a heat pump does not make heat by burning anything. It moves heat that is already there. Even cold winter air still holds some heat in it, and a heat pump is very good at scooping that heat up and carrying it indoors. It works like a fridge running in reverse. A fridge takes heat out of the food inside and dumps it out the back, which is why the back of a fridge feels warm. A heat pump does the same thing to the whole outdoors, pulling heat from the air and releasing it into your rooms. Because moving heat is much cheaper than making it, one unit of electricity can deliver around three units of warmth. In summer you flip it round and it pumps heat the other way to cool you down. Slide the outdoor temperature in the simulator and watch how much heat each unit of electricity can move.
Most people think a heat pump makes heat the way a boiler or electric fire does. In fact it makes no heat at all; it moves existing heat from the cold outdoor air into the home, which is why it can deliver several times more warmth than the electricity it uses.
What's actually happening
The first thing to unlearn is that heating means burning. A gas boiler or an electric bar fire makes heat by converting fuel or electricity directly into warmth, and at best one unit of energy in gives you one unit of heat out. A heat pump plays a completely different game. It does not make heat at all. It moves heat that already exists in the outside world and carries it indoors. That is why it can seem to break the rules, delivering several units of warmth for every unit of electricity it uses. It is not creating energy from nothing; it is being paid a small amount to do the work of relocation.
To see how, picture your refrigerator. A fridge keeps its inside cold by pumping heat out of the food compartment and dumping it into the kitchen, which is exactly why the coils on the back feel warm. A heat pump is the same machine pointed at the whole house. A special fluid called a refrigerant runs round a loop through four parts. Outside, in the evaporator, the refrigerant is kept so cold and at such low pressure that even frigid winter air feels warm to it, and that air boils it into a gas, handing over its heat. The compressor then squeezes that gas hard, and squeezing a gas heats it, so it comes out hotter than your living room. Inside, in the condenser, it gives up that heat to your rooms and turns back to liquid. Finally an expansion valve lets the pressure drop, the refrigerant goes cold again, and the loop starts over. Heat has been ferried from the cold outside to the warm inside, uphill, which is the part that takes a little electricity.
The number that captures all this is the coefficient of performance, or COP. A COP of three means one unit of electricity moves about three units of heat into your home, three times better than a simple electric heater. But there is a catch the simulator makes plain. The colder it gets outside, the bigger the temperature gap the pump has to bridge, and the harder it has to work, so the COP falls. On a mild day a heat pump is wonderfully efficient; in a deep freeze it is still useful but less so. And because the loop can be run in either direction, the very same box that warms you in winter can pump heat out of the house in summer and act as an air conditioner. One machine, moving heat whichever way you need it, instead of burning anything at all.
A heat pump moves heat from cold outdoor air into your home rather than burning fuel, so one unit of electricity delivers around three units of warmth, less when it is very cold.
- 1Reach round to the back or underneath your fridge while it is running and feel the warm coils there.
- 2That warmth is heat the fridge has just pumped out of the cold food compartment and dumped into your kitchen, the same trick a heat pump uses.
- 3Now imagine pointing those warm coils into your living room and the cold part at the outdoors: that is exactly what a heat pump does to warm a home from cold air.
Common questions
Even cold air is far above absolute zero, so it still holds heat. The refrigerant inside the outdoor coil is kept colder still, so heat naturally flows from the air into it, and the pump then carries that heat indoors.
COP is the coefficient of performance: the units of heat delivered per unit of electricity used. A COP of 3 means one unit of electricity moves about three units of heat, far better than a simple electric heater, which can only reach 1.
The colder it is outside, the larger the temperature gap the pump must bridge between outdoor air and your warm home. Bridging a bigger gap takes more work, so the COP drops, though the pump still moves more heat than electricity.