The most efficient way to run an air-to-air heat pump in winter

The most efficient way to run an air source heat pump, when the heat pump in question is the wall-mounted split in your living room, is to set 19 to 21°C, leave the fan on auto and the vanes pointing down, run it steadily with a small overnight set-back rather than switching it off and blasting the room back up, and keep the filters clean. Do those four things and you are most of the way there. This guide is written for air-to-air units specifically (the reverse-cycle splits that heat and cool a room directly, which is every unit on our site), not for the air-to-water heat pumps that feed radiators, which is what nearly all the advice online is about.

Below are the ten things that genuinely move the bill, ranked by impact, then what not to do, a worked month for a badly run unit against a well run one, and a settings table. If you are new to heating with a split, our guide to heating with air conditioning covers cost per hour and defrost; the air-to-air heat pumps guide covers the bigger picture.

The ten things that change the bill, ranked

1. Run it low and steady, not off and blasting

This is the one that catches people out, because the answer is different from gas. An air-to-air unit has an inverter compressor that varies its speed to match how fast the room is losing heat. Danfoss, which makes these compressors, points out that "only a few percentages of operations are at full load in a building", and the design is built around part load. A unit ticking over at a third of its output to hold 20°C is in its most efficient territory. A unit running flat out to lift a cold room by six degrees is not.

So "is it cheaper to leave the heating on all day" has two answers. For a boiler, the Energy Saving Trust is clear: "keeping your heating on all the time will use more energy and cost you more money." For a heat pump the same page says the opposite, "if you have a heat pump, then you can save money by leaving the heating on all day", and its heat pump guidance adds "keep your heat pump on all the time, and let your 'set-back' maintain a lower temperature." Nesta agrees: turning a heat pump off at night "forces the system to work harder to recover the lost heat", and a slightly lower set-back "can save energy without reducing comfort."

Two honest caveats. That advice was written for radiator systems with a lot of thermal mass; an air-to-air unit heats the air, so the room cools faster once it is off but warms faster once it is on. And "leave it on" does not mean heating an empty house. For short absences (a couple of hours, and overnight) a set-back of no more than about 3°C below your comfort temperature beats switching off; Daikin's own testing found a unit left running through a half-hour absence used less energy than one switched off and on. For a full working day in a leaky room, or a weekend away, switching off wins, because the heat you put in leaks out. Good insulation strengthens the continuous case; a draughty Victorian bay weakens it.

Turning a heat pump off at night can be inefficient, as it forces the system to work harder to recover the lost heat.

2. Set 19 to 21°C, and know what each degree costs

The Energy Saving Trust recommends "somewhere between 18°C and 21°C", and says turning down from 22 to 21°C "could save you £120 a year in Great Britain". As a rule of thumb, each degree on the setpoint changes heating energy by roughly 6 to 10% (British Gas, citing the Energy Saving Trust, quotes "up to 10%"). Daikin UK suggests 21°C in winter, and notes that a unit high on the wall may need a degree more than a floor-standing one because warm air rises. A unit set to 24°C is not "a bit warmer" than one at 20°C; it is a quarter to a third more heat lost through the walls and glass, all season.

3. Fan speed on auto

Daikin's engineers put it plainly: "After initially increasing the airflow to quickly collect heat, the unit will then automatically switch to steady operation." Max fan pushes air over the coil faster than the coil can warm it, so the air arrives tepid and draughty and you turn the setpoint up to compensate. Quiet-only starves the coil of air, so the unit cannot deliver its rated output and runs harder for longer. Use Quiet for an hour at bedtime if you like; do not leave it there all winter.

4. Vanes down for heating

The indoor unit sits at ceiling height and warm air rises, so it has to push heat down the wall to where you are. Mitsubishi Electric's operating instructions give the recommended position as "HEAT: downward airflow position"; Daikin's guide says the flaps "spread the air along the floor" in heating. If you fixed the vanes horizontal for summer, drop them, and keep the path clear: Daikin UK's line is "if furniture is in the way, it won't circulate as effectively."

5. Do not use the timer to heat a cold room in ten minutes

Timers are useful; Daikin describes them as ideal "for automatically switching the air conditioner on/off at night or in the morning." The mistake is expecting a 20-minute blast from cold to feel like a warm room. It will not, and it forces the unit through its least efficient full-load minutes plus a defrost or two. Start the timer 30 to 60 minutes before you need the room at your normal setpoint, or better, use an overnight set-back so the room never gets cold in the first place.

6. Close doors, and do not ask one unit to heat the house

A 2.5kW or 3.5kW split is sized for one room. Leave the hall door open and it is trying to heat the hall, landing, and stairs at a permanently higher compressor speed. Daikin UK's advice is to keep doors and windows closed to "maintain a stable temperature and avoid your units trying to heat or cool the outside!" Two warm rooms means a multi-split or a second unit, not a wedged-open door.

7. Keep the filters clean

The cheapest win on the list. The US Department of Energy says replacing a clogged filter with a clean one "can lower your air conditioner's energy consumption by 5% to 15%." Daikin's estimate is starker: a year without cleaning adds "approximately 25%" to electricity use, and three years of dust plus a blocked outdoor unit means consumption "can more than double." Daikin recommends cleaning the mesh filters every two weeks in regular use. It takes five minutes with a vacuum and a rinse; our air conditioning maintenance guide shows how.

8. Keep the outdoor unit clear, and let it drain

The outdoor unit is where the heat comes from, and it needs air. Keep leaves, bins, and bikes away, cut plants back (the US Department of Energy suggests at least 0.6m of clearance), and check the coil is not matted with debris. In heating it will frost up and defrost itself, which is normal, but the meltwater has to go somewhere: if it pools and refreezes under the unit, the fan can end up in ice. In snow, brush the top off. Never box it in or throw a cover over it "to protect it", which is the quickest way to strangle its airflow. Our guide to air-to-air heat pumps in cold weather covers defrost in more detail.

9. Use the eco modes, presence sensors, and Wi-Fi schedules you already have

Most units have features nobody switches on. Daikin's Econo mode "enables efficient operation by limiting the maximum power consumption value", and its Intelligent Eye drops to energy-saving operation "if no movement is detected for 20 minutes." Mitsubishi Electric's i-see sensor switches to "No occupancy energy-saving mode or No occupancy Auto-OFF mode when nobody is in the room." Panasonic's ECONAVI nudges the setpoint by 2°C on activity and stops the unit or drops to a preset when the room is empty. The Wi-Fi apps (Onecta, MELCloud, Comfort Cloud) let you set a weekly schedule with a proper overnight set-back; our smart air conditioning guide covers what each one does.

10. Pair it with a time-of-use tariff if you can shift load

Last because it only pays if you can move demand. Cosy Octopus, for example, offers three off-peak windows (04:00 to 07:00, 13:00 to 16:00, and 22:00 to midnight) at 51% below the day rate, with a 16:00 to 19:00 peak at 50% above, and is open to homes with an air source heat pump. Pre-warm the room in the afternoon window and ride through the peak on a set-back and it works; if your unit has to run flat out at 5pm every day, it can cost more than a standard tariff. Octopus's own figure is around £82 a year for a typical heat pump. Read our heat pump electricity tariffs guide before switching.

What not to do

• Cover the outdoor unit. It needs airflow to pull heat from the air. A cover or a tight enclosure cuts output and traps defrost water.

• Leave Powerful or Turbo on all evening. On Daikin units it times out after 20 minutes anyway. It is a boost, not a mode: full compressor speed and an exclusive high fan setting, the unit's least efficient state.

• Run a plug-in heater alongside it. A 2kW fan heater costs 52p an hour at 26.11p/kWh, roughly two and a half times what a 3.5kW split costs to make more heat. If the room is not warm enough, the split is undersized or badly set, and a fan heater is the dearest fix.

• Set 28°C to "heat faster". An inverter unit already runs flat out when the room is well below setpoint. 28°C does not speed it up; it means it overshoots and keeps running. Daikin UK's advice: "try to keep it at a constant, comfortable temperature."

A badly run unit against a well run one over a month

Same living room, same 3.5kW wall split, 30 days in December, Ofgem price cap for 1 July to 30 September 2026 at 26.11p/kWh. Every input is an assumption, so treat this as an illustration of the ratio, not a prediction for your room.

The maths: electricity used is heat delivered divided by COP, and cost is electricity multiplied by the unit rate. The well run unit spends most of its time at part load with a clean coil, so a COP of 4.0 is reasonable for a unit with a SCOP around 4.5. The badly run one does half as much work again (higher setpoint, open door), does it at full speed after every switch-off, and does it through a clogged filter, which is where the 10% penalty comes from. Around £60 a month, or roughly £250 over a four-month heating season, for the same room and the same equipment.

Rates correct as of September 2026 (Ofgem cap, 1 July to 30 September 2026, 26.11p/kWh). October's price cap moves electricity to 26.32p/kWh, which changes these figures by well under £1 a month.

The settings, in one table

More on running and looking after your system in our running and maintenance guides.