Using air conditioning to heat in winter

An air conditioner that heats is working as an air-to-air heat pump, moving heat into your home from the air outside. It behaves differently from a radiator. The warmth arrives as moving air and builds up gradually, and in cold, damp weather the system has a few habits that are easy to mistake for faults.

Settings for winter

Because the unit measures the air it draws in near the ceiling, you may need to set it a degree or two higher than you would a central heating thermostat. Adjust a degree at a time until the room feels right where you sit.

How well it heats in the cold

There's still heat to collect from outdoor air below freezing. The refrigerant in the outdoor unit is colder than the air around it, so heat flows into it even on a frosty day. Most systems are designed to keep heating well below freezing, and the manual gives the lowest outdoor temperature yours is rated for.

Efficiency falls as it gets colder. In mild weather, small air-to-air systems commonly deliver 3 to 5 units of heat for each unit of electricity they use (a coefficient of performance, or COP, of 3–5), and less than that on a frosty day. On many models the maximum heat output drops in very cold weather too, so a room that warms quickly in autumn may take longer during a cold snap.

Warm-up pauses and defrosting

When you switch on, the indoor fan may wait a few minutes before blowing. The unit is warming its coil first so it doesn't send cold air into the room.

In cold, damp weather, frost builds up on the outdoor unit's coil. It builds up fastest when the air is damp and around freezing or a few degrees above, which is common in a UK winter. To clear it, the system reverses for a few minutes and sends warmth to the outdoor coil. While it does:

• the indoor fan stops and a light on the unit may flash
• the outdoor unit may give off a cloud of vapour and release a lot of water
• you may hear a hiss or whoosh as it switches over and back

This is normal, and why the outdoor unit defrosts in cold weather explains it in more detail. Defrosting happens more often in damp, near-freezing weather, which slows the warm-up on those days. Thick ice that stays on the unit, or ice building up on the pipes, isn't normal; see ice on the pipes or outdoor unit.

The outdoor unit in winter

• Let the water drain. The outdoor unit produces water while heating, especially when defrosting. If it drips onto a path or patio it can freeze, so watch for ice where people walk and raise it with your installer if it's a problem.
• Keep it clear. Clear snow, fallen leaves and plant growth from around and underneath it so air can flow through and water can get away.
• Don't cover it while the system is in use.
• Don't chip at ice. A sharp tool can bend the fins or puncture a pipe. Let the defrost cycle do its work, and call an engineer if ice keeps building up.

Using it as your heating

Many homes use air conditioning to heat some rooms while a boiler and radiators heat the rest. A few points help the two work together:

• If your central heating thermostat is in a room the air conditioning heats, that room can reach temperature and switch the boiler off while the rest of the house stays cold. Moving the thermostat, or controlling the boiler from another room, avoids this.
• In a room with both, decide which does the heating. If the radiator valve is turned up, the air conditioning may barely run, and the other way round.
• Warming air lowers its relative humidity, as with any heating, so a heated room can feel drier even though heat mode doesn't take moisture out.

Whether heating with air conditioning costs more or less than gas depends on your tariffs and on how efficiently each system runs; see heating with air conditioning compared with gas.

In England, one of the conditions for fitting an air-to-air system without planning permission is that it isn't used solely for cooling. Do you need planning permission? covers the other conditions. Scotland, Wales and Northern Ireland have their own rules.