Should You Leave a Heat Pump On All the Time?

Almost every heat pump owner asks this in the first fortnight, usually after a well meaning relative insists that leaving any heating on all day must be wasteful. With a gas boiler that instinct is roughly right. With a heat pump it is usually wrong, and acting on it is one of the most common reasons a system costs more to run than it should.

The reason comes down to how the two machines make heat in the first place.

Why boiler habits do not transfer

A gas boiler burns fuel at a fixed efficiency whenever it fires. It does not much care whether it is sending water out at 50°C or 75°C, so blasting the house warm in an hour and letting it cool down again costs you very little in efficiency terms.

A heat pump is different. It moves heat rather than making it, and how much electricity it needs depends on the gap between the outside air and the temperature of the water it is sending to your radiators. Narrow that gap and efficiency climbs. Widen it and efficiency falls.

That is what the SCOP figure describes. A seasonal performance of 2.5 to 4 is the realistic range, and the difference between those two numbers is mostly about flow temperature. At the top of the range you are getting four units of heat for every unit of electricity you buy. At the bottom, two and a half. On grid electricity at around 24p per kWh, that gap is worth a great deal over a winter.

What low and slow actually means

A heat pump asked to recover a cold house in an hour has to run its flow temperature up high, exactly where it is least efficient. A heat pump holding a house at a steady temperature can trickle along at a much lower flow temperature and spend most of the winter in its efficient band.

So the sensible pattern is to leave the heating on, set the rooms to the temperature you actually want, and let the system modulate. It will not run flat out. Modern units turn their compressors down and tick over at a fraction of their rated output when the house only needs a little heat.

This also explains why people sometimes report that the house never feels warm. A heat pump delivers gentle heat continuously rather than bursts of very hot radiators, so the radiators feel lukewarm to the touch while the room sits comfortably at 21°C. That is the system working correctly, not underperforming.

Weather compensation does the thinking

Properly commissioned, a heat pump adjusts its own flow temperature as the outside air changes. On a mild October morning it might send water out at 32°C. In a hard frost it lifts that to 45°C or more. This is weather compensation, and it is set up during commissioning against the heat loss figures for your house.

Once it is configured, the system is already doing the economising for you. Fiddling with the thermostat on top of it usually makes things worse, because every manual knock up forces a recovery the compensation curve was designed to avoid.

If your installer did not explain the compensation curve to you, it is worth asking. Commissioning is where a good specification either delivers its efficiency or quietly throws it away.

Where turning it down still makes sense

Leaving it on does not mean never changing anything. A small setback of one or two degrees overnight, or while the house is empty all day, is reasonable and most controls handle it well. The rule is that the setback should be shallow enough that the system can recover without reaching for a high flow temperature.

What does not work is switching the heating off entirely for twelve hours in January and expecting a cheap, quick recovery. Deep setbacks in a cold snap cost more in the recovery than they save while the system is off.

Hot water is the other exception. Cylinder reheats run at a higher temperature than space heating, so timing them for the cheap window on a heat pump tariff, or for the middle of the day if you have solar, is a straightforward saving.

If yours feels expensive

Before assuming the technology is at fault, check the flow temperature, whether weather compensation is actually enabled, whether the radiators were balanced at handover, and whether your tariff suits a heat pump. In our experience most disappointing running costs trace back to one of those four, not to the heat pump itself.

If you are weighing up a heat pump and want the numbers for your own house, including what the £7,500 Boiler Upgrade Scheme grant leaves you to pay, our survey is free and carries no obligation.