Heating with your air conditioner through a European winter

Updated August 28, 2026 · Claude AC · 2 sources cited

Along the southern and eastern edges of Europe, the appliance everyone calls « the air conditioner » is also the thing that heats the flat in January: in Bulgaria, Greece, Portugal and much of southern Italy, a wall-mounted split is the primary heat source in millions of homes. Fifteen hundred kilometres north, an identical machine sits switched off from October to May above a gas boiler that does all the work. The hardware is the same. What differs is one ratio — what a kilowatt-hour of electricity costs next to a kilowatt-hour of gas — and that ratio is now moving.

Every reversible split is a heat pump

A four-way reversing valve swaps the roles of the two coils. The outdoor unit becomes the evaporator, harvesting heat from cold outside air; the indoor coil becomes the condenser that releases it into the room. Nothing is burned, heat is only moved, and that is why the output can be three to four and a half times the electricity consumed — where an oil-filled radiator is stuck at one for one and a gas boiler can never exceed the energy in the gas.

Two numbers on the datasheet describe how well it does this, and only one of them gets advertised. SCOP is the seasonal coefficient of performance declared under the EU labelling rules, published for an « average » heating season and frequently for warmer and colder seasons too — the same framework that produces the SEER figure explained in our guide to the EU energy label. The second number is the heating capacity at low outdoor temperature, normally tabulated at +7 °C, −7 °C and sometimes −15 °C. Shops quote the first. February is decided by the second. Ask for the capacity table before you ask for the price.

The calculation that settles the argument

Compare the cost of a kilowatt-hour of delivered heat, never the cost of a kilowatt-hour of fuel:

• Reversible split: electricity price ÷ SCOP.
• Condensing gas boiler: gas price ÷ seasonal efficiency — 0.85 to 0.95 when return temperatures are low enough for the boiler to actually condense, closer to 0.80 on an old high-temperature radiator circuit that never lets it.
• Electric convector: electricity price ÷ 1.

The tariffs below are indicative bands, not quotes: EU household electricity averaged roughly €0.29 per kWh including taxes in the second half of 2025, with national prices spread from about €0.11 to €0.40, while household gas sat in a €0.09–0.13 band across most member states. Substitute your own two prices and the arithmetic takes ten seconds.

A general rule falls straight out of it: a split heating at a real seasonal SCOP of 3 beats a 90 %-efficient gas boiler wherever electricity costs less than about 2.7 times gas per kilowatt-hour, and at SCOP 4 the break-even stretches to roughly 3.6. Against direct electric heating there is no contest anywhere on the continent — the same euro buys three to four times more heat.

Where Europe actually sits on that ratio

Electricity-to-gas ratios cluster between 2.5 and 4 across the EU, which places different countries on different sides of the same equation:

• Ratios near 2.5–3, common across central and eastern Europe, put the split clearly ahead of gas. That is not a coincidence in Bulgaria, where cheap electricity and cheap split units built an entire national heating habit around a machine the rest of Europe files under « cooling ».
• Ratios near 4 — the historical position of Germany, the Netherlands and Belgium, where electricity carries heavy levies — mean near parity. The decision then turns on installation quality and on how much of the home two or three indoor units can genuinely reach.
• No gas connection at all — rural Ireland and France, the Nordics, the Greek islands — and the comparison is against oil, LPG or direct electric resistance. The split wins comfortably, which is why it spread through those markets first.

The ratio is also drifting. Carbon pricing on heating fuels arrives with the EU’s second emissions trading system, ETS2, whose start was pushed back by a year to 2028; it prices the gas side, not the electricity side, so households sitting close to break-even today are likely to find themselves on the heat pump’s side of it before the decade is out. National subsidy schemes are pulling the same way, although many of them exclude air-to-air equipment on principle — a trap detailed in our guide to grants and subsidies for air conditioning in Europe.

Cold weather, described honestly

Three things happen as the outdoor temperature falls, and a salesperson quoting only SCOP will mention none of them.

Capacity fades. A unit badged « 3.5 kW » delivers that in mild conditions and materially less at −7 °C — often a fifth to a third less on standard ranges, precisely when the building needs the most. Cold-climate models built for the Japanese and Nordic markets hold their rated output down to −15 °C and keep working to −25 °C, and in continental winters that specification is worth more than any efficiency class on the label.

Efficiency falls with it. A machine achieving COP 4.5 at +7 °C is typically nearer 2.5 to 3 at −7 °C. Seasonal averages hide this because most European heating hours happen between +2 °C and +12 °C, where performance is excellent — but your worst week is not your average week.

The outdoor coil frosts. Below roughly +7 °C in damp air, moisture freezes onto the outdoor coil and the unit reverses itself for a few minutes to melt it: steam, a rush of water, a brief pause in heating, then normal service. This is designed behaviour, not a fault, and it is why mild, saturated Atlantic climates can see more defrost cycles than colder, drier inland ones. Two consequences follow: the outdoor unit must stand clear of the ground so meltwater drains away, and that water must not be left to freeze across a shared path. The same drainage logic indoors is covered in our guide to a split unit leaking water.

Put together, these are the reason serious installations are specified bivalent: the split covers the great majority of heating hours, and an existing electric convector, wood stove or boiler is kept for the handful of days a year below the point where the machine can no longer carry the load alone.

Getting real heat out of a machine sold on its cooling

• Point the vanes down. Hot air rises; a heating split blowing horizontally warms the ceiling and leaves your feet cold. Most units switch the louvre angle automatically between modes — check that yours does.
• Leave the fan on automatic. Running permanently on the quietest speed forces a high supply temperature and drags the COP down. Silence is bought with electricity.
• Hold a steady setpoint. Inverters are at their best modulating gently at part load; a deep night setback followed by a full-power morning recovery usually costs more than holding 19–20 °C with a 2–3 °C reduction.
• Wash the filters in winter too. Restricted airflow in heating mode raises the condensing temperature and cuts output exactly as it cuts capacity in summer.
• Accept that a split heats a room, not a house. Heat crosses hallways badly and closed doors not at all. One indoor unit per heated room is the design rule, and it is what a multi-split system exists to deliver from a single outdoor unit.
• Check the night noise question before winter. A unit that defrosts at 3 a.m. is audible in a way a summer cooling cycle is not; the thresholds and setback rules are in our guide to air conditioning noise rules in Europe.

Frequently asked questions

Can an air conditioner really heat a whole house in winter?

It can heat the rooms it serves, very efficiently, and that is a different claim. Millions of homes in southern and eastern Europe are heated exclusively by splits — but usually with one indoor unit per living space, or a multi-split arrangement, rather than a single unit expected to warm a corridor and three closed bedrooms. Judge the proposal by the number of indoor units, not by the kilowatts on the outdoor one.

At what outdoor temperature does it stop working?

Standard ranges keep running well below zero but with reduced output, and most are specified down to around −10 to −15 °C. Cold-climate models operate to −25 °C while holding close to their rated capacity. The operating limit and the useful limit are not the same thing: what matters is the temperature at which the delivered capacity still covers the building’s heat loss, which is the point a competent installer calculates rather than guesses.

Is heating with a split cheaper than my gas boiler?

Divide your electricity price by a realistic SCOP of 3 and your gas price by 0.9, then compare. In most of central and eastern Europe the split wins; in high-electricity-tax countries the two land close together; against oil, LPG or electric convectors the split wins everywhere. The full method for the cooling side, with country tariffs, is in our guide to what air conditioning costs to run in Europe.

Does winter use wear the machine out faster?

No. The compressor’s duty is symmetric and moderate year-round use keeps seals and lubrication exercised. Heating hours simply outnumber cooling hours in most of Europe, so a heated-and-cooled unit accumulates hours faster than a summer-only one — which argues for annual servicing rather than for leaving it off.

Should I buy a different unit if I plan to heat with it?

Yes, and this is the most useful takeaway. A unit chosen for cooling is chosen on SEER and the room’s summer load; a unit chosen for heating is chosen on capacity at −7 °C and on its low-temperature COP. Comparing the two on price alone produces a machine that is excellent in August and disappointing in February. Country-by-country context is on the Europe hub.

Read next

• Cooling a European home without air conditioning: the order that works
• Multi-split systems: one outdoor unit, several rooms
• Cooling a server room or comms cabinet: why a comfort split is the wrong machine
• Putting a European air conditioner on Wi-Fi: modules, apps and what they really save
• Too hot to work? Workplace temperature rules across Europe

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