Scotland Regional Winner, Heating Installer Awards 2026Highly Commended, Scottish Energy Efficiency Awards 2025
🎉 Scotland Regional Winner at the Heating Installer Awards 2026
Scotland Regional Winner, Heating Installer Awards 2026Highly Commended, Scottish Energy Efficiency Awards 2025
🎉 Scotland Regional Winner at the Heating Installer Awards 2026
Home / Blog / How does an air source heat pump work
How does an air source heat pump work? It takes heat out of the outdoor air, concentrates it with an electric compressor and passes it into the water that runs through your radiators and hot water cylinder. On a Glasgow winter night that sounds like a stretch. The Met Office’s 1991 to 2020 averages for Glasgow put the typical January overnight low at 1.3°C, and air at 1.3°C still carries plenty of heat for a refrigerant that boils at around -42°C.
Sean Hogan · Heat Geek engineer · Updated October 2026 · 13 min read
An air source heat pump is an electric heating appliance that collects heat from the outdoor air and moves it into the house. It burns nothing on site, so there is no flue, no gas supply and no oil tank.
Two types share the name.
An air to water heat pump puts its heat into water, and that water flows through a wet central heating circuit of the kind a gas or oil boiler feeds now: radiators or underfloor pipes, plus a coil inside a hot water cylinder.
An air to air heat pump blows warm air into rooms through wall-mounted indoor units, close to an air conditioner running in heating mode, and it cannot heat the water for your taps.
A Scottish house replacing a boiler needs the air to water type.
The fridge in your kitchen pulls heat out of the food compartment and gives it off from the coils at the back, which is why the back of a fridge feels warm. A heat pump runs the same cycle with the job reversed: it pulls heat out of the garden air and gives it off into your heating water. A sealed loop of refrigerant does the carrying, and it passes through four parts in turn.
A fan draws outdoor air across a finned coil holding refrigerant at low pressure. The refrigerant is colder than the air, even on a frosty night, so heat flows into it and it boils into a vapour. Propane (R290), the refrigerant in the Vaillant aroTHERM plus, boils at around -42°C at normal atmospheric pressure, far below any temperature a Scottish winter produces.
The compressor squeezes that vapour into a smaller space, and the pressure rise drives its temperature up, the way a bike pump’s barrel warms as you pump a tyre. The compressor uses most of the electricity the heat pump draws; the fan and the water pump take the rest.
The hot vapour passes through a heat exchanger with your heating water on the other side. It gives its heat to the water and condenses back into a liquid.
The liquid passes through a narrow valve that drops its pressure, and its temperature falls with it, well below the outdoor air. It flows back to the evaporator and the loop starts again.
On a monobloc unit, the design used on the Vaillant aroTHERM plus we installed in Glasgow and Bathgate, the whole refrigerant loop stays sealed inside the outdoor box. Only water pipes cross your wall, so nobody handles refrigerant inside the house.
“Great experience using Aventus Eco to install a Heat pump at our terraced home. Very efficient from start to finish from Sean & co, took 4 days and we’re really happy with how it’s working. Sean also took time to talk us through how it all works and has been on hand to answer any question post-installation.”
David Acheson, ★★★★★ Google Review
The heat leaves the condenser in the heating water, and a valve sends that water one of two ways: round the heating circuit, or through the coil of the hot water cylinder.
On the heating side, the water runs through radiators or underfloor heating pipes at a lower temperature than a boiler sends. Underfloor heating suits a heat pump well, because a whole floor gives out heat at a water temperature a radiator would struggle with. Our Bathgate new build runs underfloor heating on both floors from a 35°C flow.
Hot water is where a combi household sees the biggest change. A combi fires its burner the moment you open a tap and heats the water as it passes. A heat pump works at a steadier, lower output (the units above are 7kW models), so it heats a cylinder of water ahead of time and you draw from the store. Heat pump cylinders carry a larger internal coil than boiler cylinders to pass heat across at lower water temperatures, and Home Energy Scotland’s guidance has the system lift the stored water above 60°C from time to time to kill legionella bacteria. Our guide to heat pump hot water and cylinders covers sizes and where the tank fits in a house that never had one.
COP, the coefficient of performance, is the heat a heat pump delivers divided by the electricity it uses at a given moment. A COP of 3 puts 3kWh of heat into your water for every 1kWh off the meter, which some installers quote as 300% efficiency.
Manufacturers test COP at fixed points named after the outdoor air (A) and the water temperature (W).
Vaillant lists its 7kW aroTHERM plus, the size we fitted in both Mount Florida and Bathgate, at a nominal COP of 4.8 at A7/W35, meaning 7°C outside and water heated to 35°C.
At A-7/W35, with the unit running at its rated 7kW output, the figure is 2.8.
Colder air and hotter water both cost efficiency, and those two test points show the range a Scottish winter moves through. The Met Office’s 1991 to 2020 averages for Glasgow give January a mean daytime high of 6.3°C and an overnight low of 1.3°C, with around ten days of air frost.
SCOP, the seasonal coefficient of performance, averages COP across a modelled heating season, weighting mild days and cold ones by how often they come round. It is the lab figure behind the energy label. Once a heat pump runs in a real house, the measured equivalent is the seasonal performance factor (SPF), which covers a full year of heating and hot water in a real house, including the circulating pumps and any backup heater. The UK government’s Electrification of Heat trial, with Warmworks running its Scottish strand in the south-east of the country, recorded a median SPF of 2.78 across 428 monitored air source heat pumps (Energy Systems Catapult final analysis, December 2024).
Our own monitored figure covers one month. A Bathgate new build on underfloor heating at a 35°C flow logged 440% efficiency through November, in outdoor temperatures between 0°C and -5°C. That is a well-insulated new house, so treat it as one month’s reading from a careful low-temperature design. Our article on air source heat pump efficiency in Scottish winters looks at how performance holds up across a full cold season.
“You would have to look far and wide to find a Company as helpful as Aventus Eco . Sean from Aventus was able to face time my dad and talk him through concerns about how his air source heat pump was performing during cold weather . We managed to resolve the issues with Sean’s patient help and expert knowledge.”
Sid Ambrose, ★★★★★ Google Review
A heat pump keeps heating below freezing because the evaporator coil only has to be colder than the air around it, and the expansion valve drops the refrigerant below the outdoor temperature whatever that temperature is. Colder air means the compressor has a bigger temperature gap to close, so COP falls as the weather turns.
The Electrification of Heat trial measured how far: on each home’s coldest day of the monitoring period, with outdoor temperatures averaging about -4°C, the median air source heat pump still delivered 2.27 units of heat per unit of electricity.
Frost is the part you can see. When the air sits in low single figures and carries a lot of moisture, ice forms on the fins of the outdoor coil and chokes the airflow. Mitsubishi Electric UK points out that this happens above zero, and that in humid air near freezing a unit may defrost several times an hour. During a defrost the heat pump sends warm refrigerant through the outdoor coil, the ice melts and runs off as water, and a plume of vapour lifts from the fins. That cloud is the steam homeowners notice on winter mornings. It is a normal part of the cycle, and it clears within a couple of minutes.
The same Mitsubishi guidance notes that drier air at lower temperatures means fewer defrosts, so a damp 2°C morning in the west of Scotland can keep a unit busier than a dry, still night several degrees colder. The meltwater matters for siting too: a unit beside a back path needs somewhere for that water to drain, or it freezes into a sheet of ice where you walk.
The flow temperature, the temperature of the water leaving the heat pump, moves its efficiency more than anything else a designer controls, because every extra degree is lift the compressor has to supply. Home Energy Scotland notes that gas and oil boilers tend to be designed to heat water to 60°C or hotter, and a heat pump pushed to boiler temperatures loses efficiency.
The flow temperature a house can run at comes from two figures for each room: the heat that room loses on a cold Scottish day, and the heat its radiator or underfloor loop gives out at a low water temperature. Weather compensation then lowers the flow on mild days and raises it in a frost (Home Energy Scotland’s example has a heat pump circulating at 50°C at freezing to hold 20°C indoors).
We designed the 1920s sandstone end-terrace in Mount Florida around a 45°C flow and the Bathgate new build around 35°C, and our heat pump radiator sizing guide shows how we check each room’s radiator against its heat loss.
“The level of detail and care he put into both the installation and the advice was impressive. It’s clear that Aventus Eco is dedicated to not just installing systems, but making sure they perform at their best for the customer.”
Tony McCole, ★★★★★ Google Review
Outside, the unit stands against a side or rear wall on a base, feet or brackets, with open space in front of the fan so nothing blocks the air it draws in. Two insulated pipes carry the heating water through the wall.
Inside, the cylinder takes an airing cupboard, a loft or a basement, and a small set of valves, a pump and an expansion vessel sit beside it. The controls replace the old boiler programmer.
The Mount Florida job shows how that fits in an older Glasgow house. The 7kW Vaillant aroTHERM plus went outside, and the 250-litre Vaillant cylinder went into a basement 1.2 metres high together with the rest of the system kit, which kept the upstairs rooms clear. Penny and Monique run the heating from wireless Vaillant controls and an app. If your home is in or around the city, our page on heat pump installation in Glasgow is the place to start.
Noise comes from the fan pulling air across the coil and, at a lower level, from the compressor. A defrost starts with a short hiss of released air, and the sound rises during a defrost and when the unit works hard on the coldest days, and the position of the unit decides most of what you and your neighbours hear: which way the fan faces, and how far it sits from bedroom windows and the boundary. Our article on whether heat pumps are noisy covers the sound levels and siting in detail.
The cycle is the same in every air source heat pump.
The result in your house depends on
A £0 survey measures each of those, room by room, and gives you the figures before you choose any equipment.
It works like a fridge in reverse. A fan pulls outdoor air over a coil of refrigerant colder than that air, which absorbs the air’s heat and boils; a compressor squeezes the vapour to raise its temperature, and that heat passes into your heating and hot water before the refrigerant cools and goes round again.
Yes. The refrigerant in the outdoor coil is kept colder than the air even in a frost, so the unit keeps collecting heat. In the Electrification of Heat trial, which included homes in south-east Scotland, the median air source heat pump delivered 2.27 units of heat per unit of electricity on its coldest day, when outdoor temperatures averaged about -4°C.
The unit is running a defrost cycle. In damp air near freezing, ice builds on the outdoor coil; the heat pump warms the coil to melt it, and the moisture lifts off as vapour. It clears within a couple of minutes and is normal operation.
COP, the coefficient of performance, is the heat delivered divided by the electricity used at one test condition: a COP of 3 means 3kWh of heat per 1kWh of electricity. SCOP averages that across a modelled heating season, and the seasonal performance factor (SPF) is the same idea measured in a real house.
Yes, if the heat pump is to make your hot water, which it must for the Home Energy Scotland grant: Home Energy Scotland funds only heat pumps providing 100% of a home’s heating and hot water. A heat pump heats water ahead of time into a cylinder instead of on demand like a combi.
In many houses a good share of them stay. The designer checks each radiator against the heat its room loses at the design flow temperature and replaces the ones that fall short, which tend to be in the coldest rooms, with larger or higher-output units.
An air to water heat pump heats water for radiators, underfloor heating and a hot water cylinder, so it replaces a boiler. An air to air heat pump blows warm air into rooms through indoor units and does not heat tap water.
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