Air Source Heat Pump

An air source heat pump (ASHP) is a renewable type of central heating system. It does the same job as a boiler but uses the outside air instead of oil or gas. Depending on the kind of heat pump you choose, it can also take care of a household’s hot water needs.

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An air source heat pump unit outside a house.

Since heating and hot water make up around 70% of our home energy usage, air source heat pumps have a big part to play in Scotland’s net zero mission.   

Although heat pumps have been around for a while, they’re still quite a misunderstood technology. Our Top five ASHP myths article fact-checks some of the common criticisms out there. With the truth separated from the fiction, this guide can help you decide if an air source heat pump is right for your home.

Already have an ASHP but not sure how to use it efficiently? Check out our How to use an air source heat pump guide.


An air source heat pump explained – how an air source heat pump works

Ask an expert how an air source heat pump works, and they’ll probably tell you: “It operates like a fridge, but in reverse.” This is very helpful if you already know how a fridge works. For the rest of us, the information below should make things a little clearer.

Put simply, an air source heat pump takes warmth from the air outdoors, heats it up some more, and lets us use that heat indoors. Even if it drops to -15°C outside, the heat pump can still comfortably heat your home. 

If the heat pump is an air-to-air type, then hot air blows into your rooms through fan units. These look and act like air-conditioners, except they blow out hot air, not cool air.  

If the heat pump is an air-to-water type, then the hot air heats up water. The water then passes through the pipes in your radiators. It can also heat a hot water tank to provide hot water for your kitchen and bathroom.   

The vast majority of air source heat pumps in Scotland are air-to-water types.

A heat pump makes use of the fact that when a gas turns into a liquid, heat energy is released. This heat energy is then compressed (squashed) to raise its temperature further.   

First, we need to create a gas. The air source heat pump contains a refrigerant. This refrigerant has a very low boiling point. When fans in the heat pump blow the outside air over the refrigerant, the refrigerant boils, changing from a liquid to a gas.   

Second, now we have our gas, we need to raise its temperature. To do this, the gas is passed through a compressor. Since the gas is now in a tighter space, its particles knock into each other more often. This creates more heat energy.    

Third, we need to make the gas give up its heat energy so we can use it in our home. To do this, the refrigerant gas passes through a condenser, which changes it back to a liquid. As this happens, heat is released.  

Finally, that heat moves to where we need it in our home:   

If the heat pump is an air-to-air type, then heat blows into your rooms through the fan units  

If the heat pump is an air-to-water type, the energy heats up water, which passes through your radiators. It also passes through the coil in your hot water tank to provide hot water.  

As the refrigerant is now a liquid, the cycle can start again.


A graphic of an air source heat pump. It shows how the heat pump supplies hot water and heating throughout the home.

1. Heat from outside enters air source heat pump

2. Compressor and condenser heat up water in pipes

3. Heated water flows to radiators, underfloor heating, and hot water cylinder

4. Water from hot water cylinder can be used for bath, shower and taps

Air source heat pump advantages

  • Very energy efficient. At their best, air source heat pumps work at 350% efficiency.  
  • Low carbon. Most of the energy used comes from the air. 
  • Potential to save money on your heating bills, depending on your current system.

Air source heat pump disadvantages

  • You’ll probably need to upgrade your radiators.
  • Not efficient in poorly insulated homes.
  • Outdoor space needed for the unit.

Is air source heat pump heating right for my home?  

When considering a heat pump for your home, it’s worth asking:

  • Is my home properly insulated?
  • What will the heat pump connect to in my home?
  • Will it save me money on my energy bills?

Is my home properly insulated?

In winter, most of our energy costs go towards heating our homes. If our homes aren’t insulated properly, a lot of this heat simply disappears through our walls, roof, windows and floor. We don’t get to feel its benefit at all.   

Insulating our homes is the number one thing we can do to keep warm whilst reducing our bills and carbon footprint. This is true regardless of what heating system we have, but it’s especially important if you’re thinking of installing an ASHP.   

Air source heat pumps have a lower flow temperature than gas and oil boilers. This means that the water a heat pump sends to your radiators is cooler (around 45 degrees) than the water a gas or oil boiler sends to your radiators (around 60 degrees).   

A lower flow temperature means it will take slightly longer for your home to heat up. In a poorly insulated home, the heat isn’t trapped in the room for long enough to get up to the temperature you want. It’s already started leaking out of your home.  

To find out more about insulation, see our insulation guides.

What will the heat pump be connecting to?

Most air source heat pumps in Scotland are the air-to-water type. This means they will be part of a ‘wet’ central heating system – one that sends hot water around your radiators. The efficiency of an air source heat pump depends on your heating having a large surface area. In practice, this means large radiators and underfloor heating.

Radiators that currently run off an oil or gas boiler will usually be too small to use with a heat pump. This is because these radiators are designed for a system with a high flow temperature (as explained above).   

For the heat pump to work efficiently, you will likely need to have your current radiators replaced with larger ones. If you have electric storage heaters, these will also need to be switched for ‘wet’ radiators.

More and more people are choosing to combine a heat pump with underfloor heating. This is like having a very large radiator under your floor. The heat pump can warm the room more efficiently as it has a large surface area to work with.

Unlike combi gas or oil boilers, heat pumps can’t produce hot water on demand. If you want your air source heat pump to provide your hot water, you’ll need a hot water cylinder. It’s likely that your current hot water cylinder can be adapted, but you’ll need to install one if there’s no cylinder in place already.

Air source heat pump location

The air source heat pump unit will need to be installed outdoors. It should either be fitted on a wall or the ground, avoiding the corners of the building. If possible, try to site it away from a bedroom window.

Wherever you install your heat pump, you’ll need to leave enough space for air to circulate round the unit.

With a monobloc air source heat pump, everything is contained in the outdoor unit. Split system air source heat pumps have both an indoor and outdoor unit. A typical indoor air source heat pump unit is the size of a small fridge.

Will air source heat pump heating save me money?

Most of the heat energy produced by an air source heat pump comes from the air, which is free. However, there are still some running costs to consider.

The cost of air source heat pumps varies as they run on electricity. They use this electricity incredibly efficiently: at their best, air source heat pumps are 350% efficient. This means that for every kilowatt of electricity you put in, you get around 3.5 times that amount of energy out. As a comparison, traditional boilers run at about 90% efficiency.

If the outdoor temperature dips below zero, heat pumps drop to about 200% efficiency. Whilst this still makes them at least twice as efficient as a gas boiler, unfortunately it doesn’t make them twice as cheap to run. That’s because, at the time of writing (October 2023), gas is still significantly cheaper per unit than electricity.

All of this means that whether a heat pump can save you money depends on:

  • What type of fuel you’re switching from (e.g. gas, oil, electricity, wood)
  • What energy tariff you are on (i.e. how much you’re paying per unit for electricity)

As a general rule:

  • If you have electric storage heaters, switching to an air source heat pump could save you a significant amount of money. If you have old storage heaters, then the saving could be as much as £1,500 a year.
  • If you have an oil, gas, or LPG boiler that’s E-rated, or if you heat your home with coal, the savings could be significant.
  • If you have an oil, gas, or LPG boiler that’s A-rated, switching to an air source heat pump will save you little and could end up costing you more.

Planning permission for air source heat pumps

Most homes don’t need planning permission for an air source heat pump. However, if you live in a conservation area or listed building, you’ll need to get in touch with your local authority to check the rules.

Air source heat pump noise

The outdoor unit of an air source heat pump uses fans to make the whole system work. The fans are the source of the noise.

Air source heat pump noise typically ranges from 40 – 60 decibels. 40 decibels is slightly quieter than the hum of a fridge. 60 decibels is the level of a conversation between two people.

The higher levels of noise (60 decibels) are usually only reached when the heat pump is working really hard. To reduce any potential annoyance to yourself or your neighbours, you’ll need to consider the positioning of the external unit before installation.

Servicing air source heat pumps

A full servicing of an air source heat pump should be carried out every 3-5 years. Some warranties specify more frequent servicing than this, so make sure to check.

As well as a professional servicing of an air source heat pump, it’s good practice to carry out a yearly check yourself. This would include making sure:

  • The inlet grill and evaporator aren’t blocked. The unit in general needs to be clear of any leaves, plants or debris.
  • The pressure of the central heating gauge is in the right range. The correct pressure will depend on the size of your home. This is something the installer will be able to advise on. 

Air source heat pump grants

Air source heat pump grants of £7,500 are available through Home Energy Scotland. Householders who live in remote or rural areas of Scotland may be eligible for £9,000 towards an air source heat pump.

For the most up-to-date information about heat pump grants, visit the Home Energy Scotland grant and loan pageAlternatively, you can call Home Energy Scotland free on 0808 808 2282.

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Changeworks delivers Home Energy Scotland in the south east and Highlands and Islands on behalf of the Scottish Government and Energy Saving Trust.

As well as providing free, impartial expert advice to thousands of people every month to help them to keep warm in their homes for less, they identify funding opportunities for households seeking to install energy efficiency measures.

For more information, give Home Energy Scotland a call on 0808 808 2282 or email and the team will be happy to help you.