Reversible heat pumps are being fitted in more and more British homes, often as a winter heating system to replace an old boiler. Then summer arrives and many owners discover the same unit can blow cold air. That raises a question plenty of people ask: when a reversible heat pump is cooling, is it simply an air conditioner? The answer is yes, and that is good news for your bills.
The same machine, the same cycle
An air conditioner and a heat pump rely on the same piece of physics: a refrigeration cycle that moves heat from one place to another using a refrigerant gas and a compressor. In winter a heat pump draws warmth from the outside air, even in cold weather, and brings it indoors; in summer it reverses the cycle and does the exact opposite, pulling heat out of your rooms and dumping it outside. In cooling mode, a reversible heat pump is, to all intents and purposes, an air conditioner. The difference lies not in the cooling but in the fact that the same unit can also heat efficiently through the colder months.
There is a reason almost every split unit on sale is reversible: the part that cools and the part that heats are one and the same, and adding the winter function costs manufacturers very little. Anyone buying an air conditioner today is, in practice, taking home a heat pump.
What it really means for your bills
The appeal is one system covering two seasons, with no need to buy and install two separate machines. When it comes to summer running costs, the same rules apply as for any air conditioner: the efficiency rating, an output matched to the room's floor area and, most of all, the temperature you set. Every degree you knock off the thermostat in cooling mode pushes consumption up by around 5 to 7%, so setting 24C rather than 20C can nearly halve what you spend. Under the Ofgem price cap, with electricity at about 26p per kWh, that difference adds up by the end of the season. A good place to start is our air conditioning guide, which explains how to size and run a unit.
How to get the most from it in summer
The real strength of a reversible heat pump is efficiency: it moves more heat energy than the electricity it uses, whether heating or cooling. To get the best out of it in summer, a few simple habits are all it takes: a thermostat that is not set too low, the dehumidify function on muggy days (it is often the humidity, not the temperature, that makes a room uncomfortable), clean filters so the unit is not working harder than it needs to, and rooms kept shaded during the hottest part of the day. If you already own a reversible heat pump, you effectively have an air conditioner that turns into efficient heating in winter: the trick is to get the settings right, not to run it at its coldest. A few sensible steps from our guide on how to save electricity will help it pay off in both seasons, making a single purchase work twice as hard.