Heat from the air conditioning

Air conditioners are designed to provide cooling during what are often hot summers these days. However, researchers at TNO have discovered that more and more people are also using them for heating in winter. This has implications for the electricity grid. Did you stay nice and warm this winter thanks to your air conditioning? If so, you’re not the only one. Whilst network operators and policymakers in the energy sector were busy assessing the impact of the expected growth in the use of air-to-water heat pumps and solar panels, some energy consumers took a different approach: they used their air conditioners to heat their homes. This was the conclusion reached by the TNO research institute earlier this year, based on a questionnaire survey conducted in collaboration with market research agency Motivaction.Of the roughly one in five Dutch households that own an air conditioner, 85 per cent also use it to heat their homes, and 32 per cent even rely on it as their sole or main source of heating in the living room. It is a trend that has grown significantly in recent years. On the one hand, this is beneficial, as an air conditioner is more energy-efficient than a gas-fired central heating boiler, but it also places an extra strain on the electricity grid – which is already under pressure. Type verwarmingsinstallatie dat in de woonkamer als hoofd- of bijverwarming wordt gebruikt (n=240). Bron: 'Verwarmen met een airco: resultaten van een enquête naar gebruikersgedrag en -ervaringen in Nederland', TNO, 2026. Return‘An air conditioner is, in fact, a heat pump,’ says Vera Rovers, an energy transition researcher at TNO. An air conditioner extracts heat from the air and, in summer, transfers it from the indoor air to the outdoor air to cool the house, and in winter, if desired, does the reverse to heat the house. The term ‘heat pumps’ usually refers to devices that extract heat from the air or the ground and transfer it to water, which then flows through a heating pipe or out of the tap. As a result, there is a difference in energy consumption between air conditioners and other heat pumps, says Laure Itard, professor of building energy epidemiology at Delft University of Technology and a researcher at TNO. ‘Both in terms of efficiency and peak demand.’The average efficiency of a heat pump during the heating season is often expressed as SCOP, or seasonal coefficient of performance. This is the ratio between the amount of heat produced and the amount of electricity required to produce it. For modern air-to-water heat pumps, the SCOP averages between 3.0 and 5.0. ‘Not much is known about the SCOP achieved when using air conditioners for heating in homes,’ says Rovers. ‘Air conditioners used for heating have so far attracted little attention from researchers.’It is likely that the efficiency of air conditioners when used for heating is generally low compared to air-to-water heat pumps. In such cases, an air conditioner requires more electricity than other types of heat pump. ‘This is mainly because air conditioners are often not designed for heating,’ says Itard. ‘If they are optimised, as is the case with modern reversible air conditioners, the difference is considerable.’In theory, an air-to-air heat pump should actually have a slightly higher efficiency than an air-to-water heat pump, says Itard. The smaller the temperature difference between the heat source (in this case, the outside air) and the medium that absorbs the heat (the indoor air or the heating water), the higher the SCOP and, consequently, the lower the electricity consumption.‘With an air-to-water heat pump, you have to heat water to at least 30 to 40 degrees Celsius – for use in underfloor heating, for example – after which it in turn heats the indoor air,’ says Itard. ‘With an air-conditioning system, you heat the indoor air directly.’ To heat that indoor air to a temperature between 20 and 22 degrees Celsius, an air-conditioning unit typically blows out air at between 25 and 35 degrees. This is to compensate for heat loss as the air circulates.In de zomer haalt de airco warmte uit de binnenlucht en verplaatst die naar de buitenlucht. In de winter kan dat ook andersom. Beeld: DepositphotosPeak demandAnother important difference between air-conditioning units and air-to-water heat pumps is that an air-conditioning unit cannot produce hot tap water. A boiler is still required for this, and it consumes a lot of energy: an electric boiler for an average household uses almost 1,700 kilowatt-hours per year, according to the Milieu Centraal website. By way of comparison, an electric air-to-water heat pump that replaces the central heating boiler consumes, roughly speaking, 3,400 kilowatt-hours of electricity per year.Furthermore, unlike an air-to-water heat pump, an air-conditioning system cannot store heat. As a result, the use of air-conditioning systems leads to greater peaks in electricity demand. ‘In homes with air-conditioning for heating, the heat dissipates quite quickly,’ says Rovers. ‘With a water-based system, which releases heat through radiation from the radiators, the heating materials and the building itself retain the heat for longer. The heat accumulates within the building structure, which causes the peaks to shift and level out.’CoolingIt is estimated that one in five homes in the Netherlands has air conditioning, and that figure is on the rise: 90 per cent of fixed air-conditioning units in homes were installed after 2020, and around 20 per cent of households are considering purchasing one in the coming years. TNO estimates that by 2030, over 40 per cent of households will have at least one air-conditioning unit, bringing the total number of units to around 6.3 million.This means more energy-efficient heating than a central heating boiler in winter, but also an increase in energy consumption for cooling in summer. It is estimated that in 2022, 1.6 petajoules of electricity were used by air conditioning units in homes. The best estimate for 2030 is four petajoules. This is equivalent to 2 per cent of residential electricity consumption in 2022 and 5 per cent in 2030.Cooling using a reversible air-to-water heat pump is also possible, but is considerably less straightforward than with an air conditioner. This is because the cooling does not take place directly, but via the water in the radiators and any underfloor heating. If these cool down too much, condensation forms, resulting in a wet floor and wet radiators. Some companies do, however, already offer solutions for this. One option is to add a special (mechanical) ventilation unit to the system, where supply air is cooled via a few extra pipes. This is already common practice in non-residential buildings, but is still rarely seen in homes.Bron: CBSAir conditioning versus heat pumpCompared to other heat pumps, air conditioning units are inexpensive to buy and easy to install yourself. According to the Vereniging Eigen Huis on its website, a fixed air-conditioning unit is an efficient and sustainable way to heat specific rooms in the home. An air-conditioning unit heats a room quickly and is much more energy-efficient than the infrared panels or electric heaters often used for this purpose.However, heating with an air conditioner requires an indoor unit for each room. Furthermore, some people find the airflow uncomfortable and air conditioners do make a noise – the indoor unit sounds a bit like a fan. In cold weather, below 7 degrees Celsius, the air conditioner sometimes switches off because the condenser needs to defrost, the Vereniging Eigen Huis also notes. The colder it gets, the more often this happens and the longer the defrosting takes.‘For people with allergies, an air conditioner can be a nuisance because it causes a lot of air movement,’ says Itard. ‘That results in more dust particles in the air.’ On the other hand, filters can also be fitted to air conditioners, which purify the indoor air to some extent.‘And sometimes people think they don’t need to ventilate because the air conditioning already does that,’ says Itard. However, that is not the case: an air conditioning system does not refresh the air, but merely removes the heat – or, conversely, adds it. Ventilation therefore remains necessary.Energy flexibilityJust as ‘smart charging’ is now the norm for electric cars – whereby cars are plugged in at times when there is a high supply of electricity and low demand – an air-to-water heat pump can ‘heat smartly’. ‘That’s the kind of energy flexibility we’re aiming for,’ says Rovers. ‘With a standard heat pump, this doesn’t happen very often yet, as far as I know, but the possibility is certainly there. And with larger water buffers, it’s easier to do, including for domestic hot water. A great deal of research is currently being carried out into the energy management system and how to smooth out peak demand.’Consumer interest in this could well increase rapidly. This is because a new system for electricity charges is in the pipeline. Network operators want the currently fixed network tariff for electricity to become partly dependent on the time at which electricity is consumed, from 2029 onwards. The tariff will then be higher for people who use a lot of electricity, particularly in winter and during peak hours on the grid.It is still difficult to say what the most sustainable way to use an air conditioner is, says Rovers. ‘More research needs to be done on this first. How sustainable is heating with an air-conditioner when used optimally? And what exactly constitutes optimal use? How, for example, do you combine the air-conditioner smartly with the central heating and the hot water supply? We don’t know, so we can’t yet say whether it’s a good idea to use air conditioning for heating.’Rovers: ‘Our message isn’t that people should stop using air conditioners for heating. Our message to policymakers, network operators and model developers is, above all: be aware that this is happening. Include air conditioners in the models and projections for climate and energy forecasts. Look at how they affect peak demand. Because air conditioners are still flying under the radar, but their influence is growing.’ Research: don’t wait for the best possible insulation Earlier this year, TNO put forward a plan to speed up the installation of air-to-water heat pumps. This was because research among housing associations in the social housing sector had shown that the annual number of heat pumps installed would need to quadruple by 2024 in order to make the housing stock gas-free andcarbon-neutral by 2050. Whereas the current approach involves a phased process – first optimally insulating homes and then installing all-electric heat pumps – TNO advocates installing heat pumps even if the home is not yet optimally insulated.Even if the insulation is only moderate, research carried out by TNO shows that combining all-electric heat pumps with insulation is the most efficient investment. Insulating a property without immediately installing a heat pump is the least efficient investment. Scenarios involving hybrid heat pumps – heat pumps combined with gas-fired heating – fall between these two extremes.‘For the same investment, more homes gain access to lower energy bills more quickly if a heat pump is installed than if they are simply well insulated,’ says expert Laure Itard of Delft University of Technology and TNO, who was involved in the research. The difference is 30 per cent. At present, it is not yet entirely clear what this means for comfort levels in these homes. ‘But pilot projects are underway,’ says Itard. ‘Such as the one by the Parteon housing association in the Zaanstreek, which is installing all-electric heat pumps in homes that are not fully insulated and monitoring the results.’Opening image: Depositphotos

Heat from the air conditioning

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