Using air conditioning to heat a Cheam home this winter: efficient room-by-room warmth

Winter in Cheam and the rooms people actually use

Housing in and around Cheam is a real mix: 1930s semis off the Ewell Road and around North Cheam, older cottages near the village, post-war bungalows towards Belmont, and plenty of loft conversions, rear extensions and garden rooms added over the years. Those add-on spaces are often the ones that feel cold first when the clocks go back, because they sit at the far end of a central heating system or were never connected to it at all.

Many households here also spend most winter evenings in just two or three rooms: a living room, a home office and perhaps a bedroom. Heating the whole house to keep one room comfortable is where a lot of energy goes. That is the situation where reversible air conditioning tends to be most useful.

Reversible air conditioning is an air-to-air heat pump

Most modern split air conditioning systems can run in reverse. In heating mode, the outdoor unit absorbs heat from the outside air, even on a cold day, and the refrigerant circuit moves that heat indoors, where the wall or ceiling unit releases it as warm air. This type of system is called an air-to-air heat pump.

Because it moves heat rather than turning electricity directly into heat, a heat pump can deliver more heat energy than the electrical energy it uses. A plug-in fan heater, oil-filled radiator or old storage heater converts electricity to heat roughly one-for-one. That is why, for the rooms you occupy, heating mode can be an economical choice compared with direct electric heaters.

The Energy Saving Trust's guide to air-to-air heat pumps (energysavingtrust.org.uk/advice/air-to-air-heat-pumps/) is a good independent overview. One point it makes clearly is that air-to-air systems provide warm air only; they do not produce hot water, so you will still need a separate way of heating water.

What actually decides the running cost

We are often asked whether heating with air conditioning is cheaper than gas. The honest answer is that it depends, and we would not promise either way. Gas and electricity prices move independently, and the comparison changes with your tariff and your home.

The main factors

• Your electricity tariff, including any time-of-use rates you are on
• The heat demand of the room: its size, glazing, aspect and how draughty it is
• Insulation in walls, lofts and floors, which matters a great deal in older Cheam properties
• Outside temperature: efficiency is higher on a mild day than on a frosty morning
• The efficiency of the particular unit, shown on the manufacturer's data as SCOP
• Defrost cycles in cold, damp weather, which use some energy
• How you use it: steady settings in occupied rooms versus heating empty spaces

A hypothetical worked example

This example is for illustration only. The figures are made up to show the arithmetic and are not a current tariff, a quote or a guaranteed running cost.

The calculation is: electrical input in kW multiplied by your tariff per kWh equals the cost per hour. Suppose a unit is drawing 0.8 kW of electricity at a given moment and your tariff were a hypothetical 25p per kWh. That hour would cost 0.8 × 25p = 20p.

The common mistake is to use the rated heat output instead. A unit described as "3.5 kW heating" is telling you how much heat it can deliver, not how much electricity it uses. Its actual electrical draw varies constantly as the inverter compressor speeds up and slows down. The manufacturer's technical data for your exact model is the only reliable source for input power and efficiency figures.

Practical settings for winter

• Select HEAT mode, usually shown with a sun symbol, rather than AUTO, so the unit does not swap between modes
• Choose a sensible, steady target temperature and leave it, rather than turning it right up to warm the room faster
• Use timers so the room is warm when you need it, and close internal doors to keep warmth where you are
• Set the louvres so warm air is directed towards the lower part of the room where you sit
• Clean or check the indoor filters as the manual for your model describes
• Keep the outdoor unit clear of leaves, bins, covers and garden furniture so air can flow freely

Defrost is normal

On cold, damp days, frost can form on the outdoor unit. The system will periodically pause heating to clear it, and you may see steam or water from the outdoor unit and notice the indoor fan stop for a few minutes. That is a normal defrost cycle, not a fault. Make sure meltwater can drain away rather than pooling or freezing on a path.

Heating mode is not a substitute for a dehumidifier. If a room has condensation or damp problems, those need looking at separately.

Servicing and correct sizing

A system that has not been looked after works harder for the same warmth. An autumn service is a sensible time to check filters, coils, drainage and the outdoor unit before the coldest weather; see our air conditioning servicing page for what that involves.

Sizing matters just as much. An undersized unit in a large through-lounge or a poorly insulated extension will struggle on cold days, while an oversized one cycles inefficiently. We survey each room, looking at its dimensions, glazing, insulation and existing heating, before recommending equipment. If you are considering heating with air conditioning, our heating page explains the options, and we can give you a free quotation after a survey.