Air Conditioner to Heat?

When most people think of air conditioning, the first thought is cooling. That's because we commonly use it in our cars, and originally air conditioning was just for cooling. Times have changed — people much more knowledgeable than I am found ways to reverse the cooling process to create heating.

When many hear the term heat pump they have one of two reactions: they're brilliant and save a fortune, or they're an absolute waste of time. The truth is that a heat pump, installed correctly in the right building with the appropriate solution, delivers the first part of that statement. The association we often have with heat pumps is heating homes through a traditional 'wet' system of radiators and pipes, which is why correct design is critical: heat pumps run at much lower flow temperatures than gas boilers, and run far more efficiently there.

Air to air is a different game

With an air conditioner — formally an air to air heat pump — we extract heat from outside and transfer it indoors for heating in winter, or reverse the direction to remove heat from indoors and reject it outside for cooling. Because we aren't heating water, we can run far more economically. A wet system may need a 40°C flow temperature to distribute meaningful heat; with air to air you might target a 21°C room temperature, and the unit never needs to run that hot to achieve it.

Over 300% efficiency, explained

Most modern units can take 1 kW of input power and deliver over 3 kW of heat into the space. A refrigerant circulates through a series of components, extracting heat in one place and rejecting it in another. In a fridge, heat is removed from the insulated box and rejected into the room — that's why the back of a fridge is warm. Heat cannot be destroyed, only moved. The same principle applies to a heat pump.

The outdoor unit draws air across its coils and absorbs heat, even in the middle of winter. Technically there is energy in the air right down to -273°C, so the units keep extracting. As temperatures approach -15°C the unit works very hard and efficiency drops — but in the last decade I don't recall a UK winter colder than -8°C, and at that level a heat pump still performs with good efficiency.

Coefficient of Performance

Heat pump efficiency is measured by its Coefficient of Performance (COP): how much heat is produced per unit of electricity consumed. A COP of 3 means three units of heat for every unit of energy used. The refrigerant's phase changes — liquid to gas and back — enable that heat transfer, with the compressor raising pressure and temperature to facilitate the exchange.

In summary, heat pumps are a smart choice for energy-efficient heating, even in colder climates. And because air conditioning units are classed as heat pumps, you can benefit from 0% VAT.