The short answer
To heat, the system runs its refrigerant loop in reverse. The outdoor coil turns cold and collects heat from outside air, even below freezing, while the indoor coil turns hot and warms the air blown into the room.
On this page
Most wall-mounted systems sold for UK homes can heat as well as cool, using exactly the same parts. It seems unlikely that air on a frosty morning could warm a living room, but the system only needs its refrigerant to be colder than the outdoor air to draw heat from it. The compressor then does the work of lifting that heat to a useful temperature.
Running the loop in reverse
When you choose heat, a reversing valve in the outdoor unit switches over, and you may hear a click or a brief whoosh as it changes. The hot gas from the compressor is now sent to the indoor unit instead of the outdoor one, and the two halves of the system swap roles.
| Cooling | Heating | |
|---|---|---|
| Indoor coil | Cold, takes heat from the room | Hot, gives heat to the room |
| Outdoor coil | Hot, releases heat outside | Cold, collects heat from outside |
| Air leaving the outdoor unit | Warmer than outside | Colder than outside |
| Water forms on | Indoor coil | Outdoor coil |
The stages refrigerant goes through are the same in both modes; only the direction changes. They are set out in the refrigeration cycle, step by step.
Collecting heat from cold air
Air at 0°C still holds a great deal of heat energy. It only feels cold because it is colder than you are. In the outdoor coil the refrigerant is held at a low pressure, which keeps it several degrees colder than the air, so heat flows into it even on a freezing day.
The compressor then squeezes the refrigerant until it is hot enough to warm the room, and the indoor fan blows room air across the hot indoor coil. The heat that reaches your room is the heat collected outside plus the energy the compressor used.
As the weather gets colder, the refrigerant has to run colder to collect heat and the compressor has to lift that heat further, so both the heat output and the efficiency fall. Manufacturers publish an outdoor temperature range for heating, and many wall-mounted models are designed to keep working well below freezing. The headline heating capacity is measured in mild conditions, so if heating matters to you, the output at colder temperatures is the more useful figure.
How much heat for each unit of electricity
Heating efficiency is expressed as COP, the coefficient of performance: the heat delivered divided by the electricity used, at one set of conditions. In mild weather small air-to-air systems commonly reach a COP of 3 to 5, meaning about 3 to 5 kWh of heat for every kWh of electricity. A plug-in electric heater, by comparison, turns each kWh of electricity into one kWh of heat.
COP is not fixed for a given system. It drops as the outdoor temperature falls, and defrosting takes a little heat back in cold, damp weather. SCOP, the seasonal figure on the energy label, is worked out over a range of heating conditions rather than one. There is more detail in COP and EER: what efficiency means.
What heating mode feels like
- A short wait at the start. The indoor fan stays off or runs very slowly until the indoor coil has warmed, usually for a few minutes, so the unit doesn't blow cold air at you.
- Warm air from high on the wall. Warm air rises, so a unit near the ceiling tends to warm the top of the room first. Angling the louvres down sends the air towards the floor, and some units do this automatically in heating mode.
- A reading taken near the ceiling. The temperature sensor is usually inside the indoor unit, where the air is warmest, so the room can feel cooler at chair height than the setting suggests. Some systems can take the reading from a sensor in the remote control instead.
- Occasional pauses. In damp weather around freezing, ice builds up on the outdoor coil and the system stops heating for a few minutes to clear it. The indoor fan goes quiet, steam may rise from the outdoor unit and water runs from its base. This is explained in why the outdoor unit defrosts in cold weather.
Getting good results in winter
A wall-mounted unit warms the air in its own room quickly, because there is no water circuit to heat up first. It is much less effective at warming other rooms, so keep doors closed on the rooms you want it to heat.
- A steady, moderate setting suits the way the system works better than large swings that keep it at full output.
- Keep the outdoor unit clear of snow, leaves and anything else that blocks its airflow, and never cover it while it is running.
- Make sure water from the outdoor unit can drain away and won't freeze across a path or step.
Settings, timers and running it alongside other heating are covered in using air conditioning to heat in winter.
Related guides
- Air-to-air heat pumps explainedAn air-to-air heat pump collects heat from outdoor air and delivers it to your rooms as warm air, usually through wall-mounted units. It is the same equipment as air conditioning that can heat, so it also cools in summer, but it can't heat water.
- Why the outdoor unit defrosts in cold weatherIn heating mode the outdoor coil runs colder than outdoor air, so in damp weather near or below freezing, moisture freezes onto it. Every so often the system briefly reverses to send heat to the outdoor unit and melt the ice, which is normal.
- Using air conditioning to heat in winterChoose heat mode, point the airflow down and give the room time to warm. Most systems keep heating well below freezing, though less efficiently, and pause now and then to defrost the outdoor unit, which is normal.
- COP and EER: what efficiency meansCOP (heating) and EER (cooling) compare the heat a unit moves with the electricity it uses, under one set of test conditions. A COP of 3 means 3 kW of heat for every 1 kW of electricity; higher is more efficient.