Air Conditioners with Heating Function

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Air conditioners with a heating function - when they can replace heating and when they can't

The question "can an air conditioner heat too?" comes up in a heating equipment shop almost every other day, especially in spring and autumn when people are dealing with a renovation or a house extension. The answer is a clear yes - the vast majority of today's wall-mounted split air conditioners (including ordinary home models) have what's called a reverse cycle, meaning they can also work as an air-to-air heat pump. In practice this means that the same outdoor and indoor unit you buy for summer cooling can, during the transitional season (spring, autumn, mild winter), lower your heating bills, add heat to a studio flat, an attic room, an office, or any space where it pays to have a separate, fast-reacting heat source alongside the main heating.

This article explains how the heating function in an air conditioner technically works, when it actually pays off to use it as a supplementary or even primary heat source, where its limits are during frost, and what to watch out for when choosing a specific model. We draw on the common practice of installation companies and on the real parameters of the units we offer.

How an air conditioner with a heating function works

An ordinary air conditioner in summer removes heat from the interior and releases it outside via the outdoor unit - that's exactly why you feel a warm stream of air from the fan outside during cooling. A unit with a heating function additionally has a so-called four-way valve (reversing valve) in the refrigerant circuit, which can reverse the direction of refrigerant flow. The result is that the compressor and the whole circuit work exactly the other way round - heat is removed from the outdoor air (yes, even when it's only 5 or 7 °C outside, the air still contains usable heat) and delivered indoors via the indoor unit.

This is exactly the same principle on which air-to-water heat pumps work, except that instead of heating water for radiators or underfloor heating, the heat is delivered directly into the room's air via the indoor unit's fan. That's why this technology is also called an air-to-air heat pump. The key difference compared to classic electric heating (direct electric heaters, infrared panels, an electric boiler) is that the air conditioner does not produce heat from electricity through a direct 1:1 conversion, but instead "pumps" heat from the outdoor air - from 1 kWh of electricity it can actually deliver 2 to 4 kWh of heat into the room, depending on the outdoor temperature and the specific model. This ratio is called the COP (coefficient of performance).

When heating with an air conditioner is genuinely worth it

In practice we see three typical situations where customers use the heating function with good results:

Transitional season (spring, autumn, mild winter) - while it's only worth switching on the main heat source (boiler, air-to-water heat pump) once the weather turns colder, an air conditioner can handle heating at outdoor temperatures roughly from +15 °C downward without needlessly starting the whole heating system for a few cold days.
Standalone spaces not connected to central heating - studio flats, attic rooms added later, offices, studios, weekend cottages. Here a split air conditioner is often cheaper and faster to install than running new piping or electric direct heaters, and in summer you get to use the same unit for cooling.
Supplementary heating alongside an existing system - in a living room or a room with large glazed areas, where you need to quickly warm the space (for example in the morning before the family gets up), while underfloor heating reacts slowly.

On the other hand, an air conditioner as the sole heat source doesn't make sense for a year-round family house in a region with long, freezing winters and no backup source - we cover the limits at low temperatures in the next section.

Comparing the cost of 1 kWh of heat

To understand why heating with an air conditioner pays off specifically during the transitional season, let's look at a simple illustrative calculation. At an electricity price of €0.20/kWh and an average air conditioner COP in mild weather of around 3.0, 1 kWh of heat delivered into the room works out to approximately €0.067. An electric direct heater or infrared panel (COP = 1, i.e. 1 kWh of electricity = 1 kWh of heat) works out to the full €0.20 for the same 1 kWh of heat. A condensing gas boiler with an efficiency of around 92 % and a gas price of €0.09/kWh works out to approximately €0.098 for 1 kWh of heat. This is an illustrative example at specific assumed energy prices - your real tariffs may differ, but the principle (an air conditioner, thanks to a COP above 1, works out cheaper than direct electric heating at mild temperatures) holds true in practically every case.

Efficiency at low temperatures - where the limits are

The colder it is outside, the less "free" heat there is in the air to extract, and the harder the compressor has to work - so the COP drops as the outdoor temperature falls. As a rough guide, typical values for mid-range split units look like this: at an outdoor temperature of around +7 °C the COP is typically around 3.5 to 4.0, at 0 °C it drops to approximately 2.5 to 2.8, and at -7 °C it falls to just 1.8 to 2.0. Below this threshold (usually somewhere between -10 and -15 °C, depending on the specific model), most ordinary air conditioners either stop offering the heating mode altogether or switch into an emergency electric backup heating mode, where the COP is practically equal to 1 - meaning you lose the main advantage of this technology.

At very low temperatures there is also a risk of frost forming on the outdoor heat exchanger, which the unit must periodically defrost (by briefly switching back into cooling mode) - during defrosting it stops delivering heat for a few minutes, which is normal behaviour, not a fault. This is precisely why, in regions with freezing winters, we recommend thinking of an air conditioner with a heating function as a supplementary rather than sole heat source, and having a backup solution on hand (boiler, air-to-water heat pump, or an electric direct heater) for the coldest days of the year.

A practical installation example

A common scenario installation companies encounter: a family finishes an attic as a child's room, but the main heating (a boiler with radiators) isn't extended there for budget or structural reasons. Instead of electric direct heaters or an expensive extension of the piping, a single wall-mounted split unit is installed - in summer it cools the attic (which, without air conditioning, is typically the hottest room in the house during summer), and in spring and autumn it heats the room without needing to fire up the boiler for the whole house because of one room. During the hardest frosts, the room is either heated with a portable oil-filled radiator for a handful of days a year, or simply covered with a warmer blanket - in practice this works as a fully adequate solution for the large majority of the year, and represents a significant saving compared to structural work to extend central heating.

Choosing the right model - what to look at

When choosing an air conditioner you actually intend to use for heating too (not just cooling), it's worth checking three things: the rated heating output relative to the room area, the guaranteed operating temperature (down to what frost the manufacturer allows the heating mode to be used at all), and the energy class/SCOP for heating mode. The common recommendation from installation companies is to plan for an output of approximately 100 W per m² for a standard insulated room with a standard ceiling height - for poorly insulated spaces or large glazed areas, allow a reserve.

From our current range, we recommend the Vaillant climaVAIR pro series - a proven, reliable brand you'll also know from boilers and heat pumps, available in several output classes depending on room size:

VAIB1-025WN outdoor and indoor unit with remote control - the smallest output step in the series, suitable for smaller rooms, offices, or attic rooms up to roughly 20-25 m² with normal insulation. An ideal choice for the scenario in the previous section (adding heat to a single completed room). Price from €527.00.

VAIB1-035WN outdoor and indoor unit with remote control - a mid output step, suitable for an ordinary living room or larger room, a good choice as supplementary heating alongside existing central heating in spaces with larger windows. Price from €561.00.

VAIB1-050WN outdoor and indoor unit with remote control - the most powerful step of the three, suitable for larger open-plan spaces (living room combined with kitchen, larger office, studio). Recommended wherever the air conditioner needs to genuinely carry a larger share of the space's heating load. Price from €833.00.

Comparing the output series

Below is a summary of the three models mentioned in one place, so you can quickly compare price and approximate output class by model designation (025/035/050 - the higher the number, the higher the output and the larger the recommended room area).

Installation and operating costs

Installing a split air conditioner with a heating function is technically no different from installing an ordinary "summer" air conditioner - it requires connecting piping (refrigerant lines) between the outdoor and indoor unit, condensate drainage, and an electrical connection. The difference compared to cooling-only units is that the outdoor unit tends to be somewhat more robust (for operation at sub-zero temperatures), and the installation company should also consider drainage of defrost water in winter when positioning the outdoor unit - so we do not place the outdoor unit directly above a footpath or entrance, where it could cause ice in winter.

We covered operating costs above using the example of the price of 1 kWh of heat - in practice this means that in milder weather (autumn, spring, mild winter) heating with an air conditioner is generally cheaper than a direct electric heater and comparable to or cheaper than a gas boiler, but during hard frosts this advantage shrinks or disappears entirely due to the drop in COP. We therefore recommend genuinely using the air conditioner as a "smart" supplementary source - switching it on instead of the main heating precisely during periods when it's still relatively warm outside, and switching back to the main heat source during heavier frosts.

Maintenance for year-round operation

A unit that runs both for cooling in summer and heating in winter is in operation practically all year round, which places somewhat higher demands on regular maintenance than a unit used only for the 3-4 summer months. We recommend checking and cleaning the indoor unit's filters at least once every 4-6 weeks during the active season (both summer and winter), and regularly checking the outdoor unit for build-up of leaves, dust, or (in winter) snow and ice that block airflow through the heat exchanger. A professional service check once a year is worthwhile - checking the tightness of the refrigerant circuit, cleaning the heat exchangers, checking the condensate drainage, and the functioning of the defrost cycle. Neglected maintenance shows up faster and more noticeably in heating mode than in cooling mode - a clogged heat exchanger means worse heat transfer, longer compressor operation, and therefore higher electricity consumption for the same result.

The most common mistakes when using an air conditioner for heating

From the experience of installation companies and service call-outs, the same mistakes keep recurring:

Undersized output - a customer buys the cheapest, smallest model to cool a small room, then expects it to also heat the adjoining room with the door left open. Output needs to be calculated for the actual heated area, not just the room where the unit is hanging.
Operating below the guaranteed temperature - trying to heat with an air conditioner even at -15 °C and below, where most units in this range either don't work at all or operate at a COP close to 1 (no saving compared to a direct heater).
Incorrectly set airflow direction - in winter, warm air (which is lighter) tends to rise towards the ceiling, so in heating mode the indoor unit's louvres are set to point downward, the opposite of summer cooling, where the air is directed more horizontally or slightly upward.
Underestimating defrost cycles - in freezing weather it may seem the unit has "briefly stopped heating and is blowing cold air" - this is a normal automatic defrost cycle for the outdoor heat exchanger, not a fault.
Neglected regular maintenance - see the section above; year-round operation requires more frequent care than seasonal cooling alone.

Frequently asked questions

Can every air conditioner also be used for heating?

No, only models with a reverse (reversible) cycle, which manufacturers explicitly label as "heat pump" or with a heating function. Cooling-only units (without a reversing valve) technically do not have a heating mode and it cannot be added afterwards. When choosing a model, always check the stated heating output and the guaranteed operating temperature in the product's technical parameters.

Down to what outdoor temperature does an air conditioner still genuinely heat?

Most ordinary split units for households guarantee heating mode down to approximately -10 to -15 °C; below this threshold efficiency drops sharply or the unit switches to emergency electric backup heating. You'll always find the exact threshold in the technical data sheet of the specific model - if you're choosing for year-round use, it's worth reaching for a model with a lower guaranteed temperature.

Does heating with an air conditioner pay off financially, or is it just a bonus?

At mild outdoor temperatures (spring, autumn, mild winter) it is a real, measurable saving compared to a direct electric heater thanks to a COP above 1 - see the calculation in the article above. During hard frosts the advantage shrinks. As a supplementary source alongside your main heating, it is therefore worth it in practically all cases; as the sole heat source, only in milder regions or spaces that aren't occupied year-round.

Is heating with an air conditioner noisier or harder on the unit than cooling?

Not significantly; the compressor works on a similar principle in both modes. The only difference is a regular short defrost cycle at low outdoor temperatures, during which the outdoor unit may be briefly noisier. In terms of service life, year-round operation is somewhat more demanding than just the summer season, which is why the regular maintenance mentioned above is important.

Can I use an air conditioner to heat a bathroom or a humid room?

Most manufacturers do not recommend, or limit, installation in directly humid areas (for example right above a bathtub or shower enclosure) due to electrical safety and the unit's protection rating - so for a bathroom, other solutions (underfloor heating, an electric radiator with the appropriate protection rating) are recommended for supplementary heating instead. In ordinary living rooms this is not an issue.

How much output do I need for my room?

As a rough guide, plan for approximately 100 W of heating/cooling output per m² for a standard insulated room with a standard ceiling height - with worse insulation, large windows, or higher ceilings, allow a reserve. We recommend having the exact calculation for your specific room assessed by an installation company on site.

Do you have a question about this topic?

Can't decide, or dealing with a specific situation in your household? Write to us - we're happy to help.