Almost every fixed split system sold in Britain is reverse-cycle: it can run the refrigerant circuit in either direction, moving heat out of the room in summer and into it in winter. In heating mode it is a small air source heat pump, delivering several kilowatts of heat for each kilowatt of electricity it consumes.
That does not make it a replacement for a whole-house heating system. It makes it an unusually good answer to a specific and very common British problem: single rooms that need warmth outside the main heating season.
Why the shoulder seasons are where it shines
In September, October, March and April, most households do not want the whole heating system running for one cold room. Firing a boiler and warming a full circuit of radiators to take the edge off a home office is a blunt instrument, and the boiler is operating at its least efficient in short mild-weather bursts.
A reverse-cycle unit heats that one room in minutes, at heat pump efficiency, with no effect on the rest of the house. Heat pump performance is also at its best in mild weather, so this is precisely the condition in which the technology looks strongest.
Where it fits, and where it does not
Strong fit
Rooms and patterns that suit reverse-cycle heating well.
- Home offices used during the day while the house heating is off
- Loft conversions and garden rooms poorly served by the radiator circuit
- Extensions where adding radiator capacity is awkward
- Bedrooms wanting cooling in summer and gentle warmth in autumn
Weak fit
Situations where it will disappoint.
- Replacing whole-house heating with several wall units
- Rooms where warm air blowing at head height feels uncomfortable
- Hot water — a split system does nothing for taps or showers
- Deep cold snaps as the sole heat source in a poorly insulated room
Things to know about heating mode
- Defrost cycles pause heating briefly in cold, damp weather while frost is cleared from the outdoor coil
- The outdoor unit will drip water in heating mode, so its position and any path below matter
- Warm air rises, so floor console units often feel more comfortable than high wall units for heating
- Capacity in heating mode differs from cooling capacity — check both figures at specification
- Filters need the same cleaning attention in winter as in summer
- Efficiency drops as outdoor temperatures fall, which is why it complements a boiler rather than replacing one
Questions people ask
Is heating mode as efficient as people claim?
In mild weather, yes — several units of heat per unit of electricity is normal. Performance falls as it gets colder, so the honest claim is that it is excellent in spring and autumn and decent in winter.
Will it be cheaper than my gas heating?
It depends on the gap between electricity and gas prices when you compare, and on whether you are heating one room instead of the whole house. Heating one room instead of eight is often the bigger saving.
Why does the fan slow down or stop occasionally in winter?
That will normally be a defrost cycle clearing frost from the outdoor coil. It is standard operation and lasts a few minutes.
Can it heat a poorly insulated conservatory?
It will help significantly, but glazing loses heat rapidly. Set expectations around comfort in the shoulder seasons rather than mid-winter, and size the unit for the room's real losses.
If a system is already on the wall, try the heat button in October. It is the cheapest way to discover that the summer purchase you made has a second, quieter job for two-thirds of the year.