Air source heat pumps might be heralded as the future of home heating, but as of yet, the UK's take-up of this renewable heating technology flags behind European counterparts.
Investing in an air source heat pump (ASHP) as a replacement for a gas boiler is undoubtedly set to become one of the main options for UK homeowners, given a gas boiler ban on the horizon and the government's newly launched Boiler Upgrade Scheme, which provides a grant of £5,000 towards the installation of this low-carbon heating source. However, it's not a straightforward swap.
The initial investment is high and it will ideally require improvements to the fabric of a home to make it viable — which is often why installing an air source heat pump is more popular for new builds than retrofitting, at present.
However, air source heat pumps are hard to beat when it comes to producing renewable heating for the home. Here we explore the pros and cons, plus provide all the key information you need to know before making a decision, in our complete guide.
What is an Air Source Heat Pump?
An air source heat pump is a low-carbon heating source. It uses a refrigeration cycle to move heat from the air outside and transfers it as useable heat in your home.
Air source heat pumps, like boilers, are a heat source, but they deliver lower temperatures than a boiler. As such, they ideally need to be carefully designed into well-insulated, airtight homes to ensure optimum efficiency.
There are two main types of an air source heat pump:
- Air to water heat pumps are used in wet central heating systems to heat radiators, underfloor heating and sometimes to generate hot water.
- Air to air heat pumps are perhaps better known as air conditioning, and heat the air directly. This will include some level of air movement and noise as you'd expect from an air conditioning unit.
This article will focus on air to water heat pumps.
How Does an Air Source Heat Pump Work?
If you're wondering how do heat pumps work, then it's first important to note that air source heat pumps work in a very different way to boilers. An air source heat pump does not create heat — it simply moves it from one place to another through the vapour compression cycle (or refrigeration process) to make it more useable.
Heat from the air gets absorbed into a fluid, which causes it to ‘boil’ and become a gas. The gas is then compressed, raising its temperature. The higher temperature is then transferred into the heating system (i.e. the radiators or underfloor heating).
Discover how air source heat pumps work with our video…
Do Air Source Heat Pumps Need Electricity?
Yes, like ground source heat pumps, electricity is needed to power an air source heat pump.
The efficiency, or the measure of the heat energy output per kW of electricity, is stated as the COP (Coefficient of Performance) or SCOP (or Seasonal Coefficient of Performance — the SCOP is the average COP over a defined period of time such as a year).
For example, a SCOP of 3.2 means that for every 1kW of electricity, 3.2kW of heat is generated.
However, electricity could be provided solar PV (together with a battery for storage), meaning that it's possible to use an air source heat pump off grid.
What are the Advantages of Air Source Heat Pumps?
It is essential to design and specify the system correctly and ideally the insulation, airtightness and emitters (typically underfloor heating or carefully sized radiators) of the property should be optimised, to allow you to get the most out of your air source heat pump.
There are many benefits to installing an air source heat pump, including:
- Unlike a gas boiler, an air source heat pump does not produce carbon when operating. While they do use electricity, ASHPs can be combined with solar PV panels or a renewable/green energy tariff for clean electricity.
- In the right context, air source heat pumps have comparatively low running costs, especially when compared with off-grid fuels such as propane, oil or direct electric heating.
- The outdoor unit takes up less space as compared to an above-ground oil or LPG tank.
- Air source heat pumps partner well with underfloor heating.
- There are certain grants available the offset the costs of installing an air source heat pump, such as the Boiler Upgrade Scheme.
- Some models can also be used for cooling a home, but you will need to weigh up heat pumps vs air conditioning before opting for this solution.
What are the Disadvantages of Air Source Heat Pumps?
While there are many benefits, there are also some cons too, including:
- They are not a ‘one-size-fits-all’ solution and may not be suitable for every home in the same way as gas boilers are. They're ideally paired with well-insulated, airtight homes.
- Installing an air source heat pump in an existing home is more complex and replacing a gas boiler will inevitably be much more expensive than a like-for-like replacement.
- Electricity is still needed to power an air source heat pump, so they are not entirely 'renewable' unless paired with PV panels or a renewable energy tariff.
- Electricity prices are currently high. Campaigners are asking the government to move environmental levies off of electricity bills with the focus on air source heat pumps, to ensure that it's always cheaper to run a heat pump than a gas boiler.
- The bigger the difference between the outside air and the target temperature (either the indoor room temperature or domestic hot water), the lower the efficiency.
- A separate solution may be required for providing hot water for the home.
- Some models can potentially be noisy. An air source heat pump may reach 40 to 60 decibels, but this depends on manufacturer and installation, according to Quiet Mark, which certifies the quietest heat pumps.
Do Air Source Heat Pumps Work in Cold Weather?
An air source heat pump gets its energy from the surrounding air, so as the ambient air temperature drops, so does the efficiency. It is therefore key to understand the heat load of the property and the performance characteristics of the heat pump.
When it gets closer to zero degrees outside, the air source heat pump will also need to perform a defrost cycle more often and this will also lower the efficiency.
However, while the efficiency may drop, that doesn't mean that air source heat pumps don't work in cold weather. It's been found that they can extract heat from the air in temperatures as low as -20°C, and are used in cold climates around the world.
Is my Home Suitable for an Air Source Heat Pump?
There are some key criteria your home will need to meet to make installing an air source heat pump viable, including:
Do you have enough space for one to be installed?
You'll need to have space for a unit to be attached to a wall or fitted on the ground, with space around it to ensure a good airflow. For an air source heat pump to be installed under Permitted Development, it also needs to be located at least one metre from the boundary of the house.
The internal unit for an air source heat pump is generally smaller than a gas boiler. However, you will need space for a hot cylinder, if you do not have one already, and controls.
What heating system will you use?
The big question: do air source heat pumps work with radiators? For space heating, air source heat pumps work best with underfloor heating, but low-flow temperature radiators, such as oversized radiators, multi-finned aluminium radiators or fan convectors, will work as well.
In existing homes, this might require you to upgrade the radiators you already have.
How will insulated is your home?
Air source heat pumps work more efficiently when producing heat at a lower temperature, so a home that requires less space heating because it is better insulated is key to getting the most from this technology.
What system are you replacing?
The initial expenditure of an air source heat pump is high, even still with a government-funded grant helping claw back some of the outlay.
Air source heat pumps are much more cost-effective to install in a new home, as the pipework and emitters will be designed around it from the outset. In an existing home, the installation may necessitate work to the existing heating system.
A recent government report found that installation of an air source heat pump in an existing property can run upwards of £27,000 as a result.
Can I Install an Air Source Heat Pump in an Existing Property?
Hybrid heat pumps are also available, which combine an air source heat pump and gas boiler together in one box, designed to work in partnership and get the best from each heating system.
Should I swap my boiler for an air source heat pump? is a popular question at present. It's not as simple as it sounds to switch, and deciding to retrofit an air source heat pump in an existing home will mean certain changes will need to be made.
A boiler works at a high flow temperature of around 70°C, but an air source heat pump operates at a lower temperature — around 35°C to 45°C for the radiator system and around 55°C for hot water.
Unless your home's current radiators are over-specified, it's likely you'll need to replace radiators with new ones that have a higher output, alongside room by room controls and a heat pump-compatible hot water storage.
The lower temperatures will also mean that an air source heat pump is ideally compatible with homes that have high levels on insulation and airtightness.
The levels of insulation and airtightness are often not optimised in older properties. This would cause the heat pump to operate at increased flow temperatures, resulting in lower efficiency and higher running costs. This will mean added cost in remedying these immediate fabric issues before installing an air source heat pump.
"The problem homebuilders and government bodies are facing is the way we have historically built our homes. The 1970s era of homebuilding ignored fabric heat loss, as gas was cheap, which led to the installation of gas central heating," says Keith Bastian, CEO of renewable home heating provider Fischer Future Heat.
"This building approach has put us in a big disadvantage to install heat pumps," continues Keith. "A house that is poorly insulated will require a larger heat pump to make up for heat loss, and this could cost more money to install and to run."
Finally, when weighing up a heat pump vs a gas boiler in an existing home, an air source heat pump may actually cost more to run (though much more sustainable), so when it comes to retrofitting a heat pump, it's often more attractive when replacing a more expensive heating system such as electric heating.
Is an Air Source Heat Pump Worth it?
The efficiency of an air source heat pump is dependent on a number of factors, including the fabric of your home. In a new build, especially one that has been built with fabric first principles at the forefront, equipping your home with an air source heat pump is definitely worth considering. Not only do they offer low carbon heating for your home, they're reliable and last much longer than a gas boiler.
Air source heat pumps do not tend to go wrong too often, and a good quality air source heat pump could last up to 20 years.
Pair that with the potential to regain some of the initial expense through the Boiler Upgrade Scheme, and it's a worthwhile investment in the future of your home.
How Much Does an Air Source Heat Pump Cost?
The cost of installing an air source heat pump will depend on the complexity of the install, alongside factors such as the distance from the heat pump unit.
While a fully installed air source heat pump cost is hard to estimate for these reasons, this basic formula offers a best guess calculation:
- Use £6,000 as a base cost.
- Add £750 per kW to the price (or £1000 per kW for some top end European heat pumps.)
As an example, a 10kW heat pump will cost £6,000 plus £7,500 (10kW x £750), which gives an overall installation cost of £13,500.
However, installation costs will be higher in an existing home for the aforementioned reasons.
How Much Does an Air Source Heat Pump Cost to Run?
The cost of a unit of electricity is currently 28p per kWh and the average efficiency of an air source heat pump is around 220% to 320%.
In a property that has an annual heat load of 15,000kWh, you would expect the electric use to be somewhere between 4,700kWh and 6,850kWh, which is an annual running cost of between £1,320 and £1,920.
Are There Any Air Source Heat Pumps Grants Available?
There are different heat pump grants available across the UK. These include:
Boiler Upgrade Scheme
The main scheme in England was the Renewable Heat Incentive, but this was replaced with the Boiler Upgrade Scheme this year.
This grant will offer homeowners who successfully apply an upfront payment of £5,000 towards the cost of installing an air source heat pump.
Warmer Homes Scotland
Low income households and tenants of private landlords in Scotland may also be eligible for funding for an air source heat pump through the Warmer Homes Scotland scheme.
Requirements include an energy rating of 67 or lower at a property at which you've lived for over a year, and which is not more than 230 square metres in floor size.
There are more requirement details via the Home Energy Scotland website. Those who do not qualify may still be eligible for a Home Energy Scotland Loan to fund an air source heat pump.
Nest Scheme Wales
Wales' Nest scheme is also available for people who live in Wales with an energy inefficient home who are in receipt of means tested benefit, or a combination of chronic respiratory, circulatory or mental health condition and low income.
Can an Air Source Heat Pump Provide Both Heating and Hot Water?
One of the key decisions to make when buying an air source heat pump is whether it will provide space heating or domestic hot water — or both.
The ‘flow temperature’ (the temperature of the water in the heating system) is lower in a heat pump than it is in a boiler. This means:
- Space heating will usually require a flow temperature of around 35°C to 45°C for underfloor heating or low-temperature radiators
- Domestic hot water will, however, require a minimum flow temperature of 55°C.
On new homes that meet recent and current Building Regulations most air source heat pumps can do both, but this is not always the case.
One solution is to use two heat pumps: one that is optimised for the space heating and another for domestic hot water.
The advantages of using the two heat pumps is that each unit is optimised for the required flow temperature and there is no priority system that causes the space heating circuit to ‘cool’ while the domestic hot water is being reheated.
The domestic hot water heat pump typically uses a different refrigerant that can produce higher flow temperatures but also requires a higher source temperature (10°C) to be efficient.
It tends to be lot smaller than a space heating heat pump and is usually built into the hot water cylinder. It draws its air from the room it is in, or the kitchen or bathroom (or all of them) or from the exhaust waste heat of a ducted mechanical ventilation system — and is known as an ‘exhaust air heat pump’ or a ‘micro heat pump’.
It is crucial that this system is designed properly so as to not over ventilate the property and only uses heat from the air that would normally have been exhausted to atmosphere.
A further option is a high temperature heat pump. There are different variants available, but these could offer temperatures of 60-80°C.
In order to achieve higher temperatures, some manufacturers have built the two different refrigerant systems into one heat pump in a ‘cascade’ system that can create flow temperatures of up to 80°C.
These systems are designed for hot water and should not really be used as a high temperature boiler replacement unless the lower efficiency has been carefully calculated to ensure that it is the best option for the property. In other words, they're best suited to well-insulated, airtight homes.
How Long Does it Take to Install an Air Source Heat Pump?
Air source heat pump installation is a relatively low-disruption process which could take as little time as a single day to undertake.
A solid base for the air source heat pump should be constructed and, when retrofitting, all alterations to the fabric of the house and radiator systems should be completed before the installation date.
An air source heat pump should be installed and commissioned by an accredited Microgeneration Certification scheme (MCS) installer. An incorrectly commissioned air source heat pump may use a lot more electricity.
Do Air Source Heat Pumps Need Much Maintenance?
No, although an annual inspection by qualified engineer is recommended. Most maintenance by the homeowner is visual — checking that the outdoor unit is free from leaves and debris and the pipework is intact.
It's worth noting that some manufacturers have developed controls that can be monitored and maintained remotely. In the event of a breakdown, the system can be checked, faults diagnosed and the correct spares sourced before incurring the expense of going to site.
The engineering accessibility is often an after-sales add-on product, so check costs and requirements before installing your air source heat pump.
In the absence of full remote control and monitoring, it is worth finding a controller that at least stores the operating data on a memory card so that it can be accessed for analysis and perhaps new settings and updates emailed to you for upload.
What are the Renewable Alternatives to an Air Source Heat Pump?
If you're looking for an alternative to a gas boiler, but don't think an air source heat pump is right for your home, there are many other systems you could investigate, including:
- Ground source heat pumps.
- Producing hot water and heating with a biomass boiler.
- Solar thermal panels can also produce hot water, but is weather dependent, making it less reliable in the winter months when most needed.
- Hydrogen heating is currently also being tested as a potential replacement to gas boilers.
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David is a renewables and ventilation installer, with over 35 years experience, and is a long-standing contributor to Homebuilding and Renovating magazine. He is a member of the Gas Safe Register, has a Masters degree in Sustainable Architecture, and is an authority in sustainable building and energy efficiency, with extensive knowledge in building fabrics, heat recovery ventilation, renewables, and also conventional heating systems. He is also a speaker at the Homebuilding & Renovating Show.
Passionate about healthy, efficient homes, he is director of Heat and Energy Ltd. He works with architects, builders, self builders and renovators, and designs and project manages the installation of ventilation and heating systems to achieve the most energy efficient and cost effective outcome for every home.