Heating with an air conditioner in winter: how cold it works and what it costs

An air conditioner is not only a cooling appliance — in heating mode it is the cheapest form of electric heating most homes can install. That surprises people who assume it works like a fan heater. This guide covers how the heating actually works, down to what outdoor temperature you can rely on it, what it costs to run, and what to look for if heating is part of why you are buying.

In heating mode, an AC is a heat pump

A resistive heater — a fan heater, an oil radiator, an electric panel — turns electricity into heat directly. One kilowatt in, one kilowatt of heat out. That is the ceiling, and nothing can beat it.

An air conditioner does something different: it moves heat instead of making it. In heating mode the cycle runs in reverse, pulling heat out of the outdoor air and releasing it indoors. Because it is transporting energy rather than converting it, the heat delivered indoors is several times the electricity consumed. Even at 0 °C outside there is plenty of heat energy in the air to move — "cold" is relative.

The number that describes this is the COP (coefficient of performance). A COP of 3.5 means 3.5 kW of heat for every 1 kW of electricity. On the EU energy label you will more often see SCOP, the seasonal version, which averages the whole heating season including the cold days.

How low it actually works

Two separate things drop as the outdoor temperature falls, and mixing them up causes most of the disappointment:

• The operating limit. Most current inverter models are rated to run in heating mode down to roughly −15 °C, and low-temperature models down to about −25 °C. Below the rated limit the unit stops or falls back to a protective mode.
• The output. Well before that limit, heating capacity declines. A unit rated 5 kW of heat at +7 °C outside may deliver noticeably less at −10 °C, because there is less heat available in the air to move — and the COP falls with it.

The practical consequence: a unit sized only for its cooling job may cool a room perfectly in August and struggle to hold temperature on the coldest January nights. If winter heating is a main reason for the purchase, size with the heating figures in mind rather than the cooling ones, on top of the normal room-size calculation.

What it costs to run

The COP is the whole story. At a COP of 3.5, every unit of electricity buys three and a half units of heat — so heating the same room costs roughly a third of what the equivalent resistive heater costs, on the same electricity tariff. That gap is why an AC is usually the cheapest electric heating available, and it holds even as the COP declines in cold weather; the advantage narrows on the coldest days rather than disappearing.

Two things protect that number in practice. First, an inverter compressor: fixed-speed units lose more to stop-start cycling, which is the same reason they cost more to run in summer. Second, a clean filter and coil — a restricted airflow forces the unit to work harder for the same heat, which is one of the reasons regular servicing pays for itself.

Defrost cycles and what is normal

In cold, damp weather, moisture in the air freezes onto the outdoor coil. Every heat pump handles this the same way: it periodically reverses for a few minutes to melt the frost, then resumes heating. During that pause indoor air blows cool and the outdoor unit may visibly steam and drip. It is normal, most common near 0 °C, and worth knowing about before you assume something is broken.

Two things make it worse than it needs to be: an outdoor unit that sits in a spot where meltwater cannot drain away, and one that is boxed in without airflow — which is one more reason where the units go matters.

Getting the most out of it

A few habits make a measurable difference over a winter:

• Set a steady moderate temperature rather than cranking it to 30 °C. A heat pump is most efficient running gently and continuously; a high setpoint does not heat the room faster, it just runs the compressor harder.
• Point the louvres down. Warm air rises, so heating mode needs the opposite airflow direction from cooling.
• Keep the outdoor unit clear of snow, leaves, and stacked balcony furniture.
• Clean the filters — monthly through the heating season is not excessive.

Buying with heating in mind

If the unit will heat as well as cool, three specs matter more than the marketing: the SCOP and heating energy class on the EU label, the rated operating range in heating mode (the −15 °C or −25 °C figure), and the heating capacity at low outdoor temperature, if the manufacturer publishes it. An inverter is effectively a requirement — the efficiency advantage is larger in heating than in cooling.

Whether the unit heats or only cools does not change the installation itself; installation is priced by size class either way, so the installation cost guide applies. Compare units from local stores on their heating specs, not just the cooling kW on the front of the box.