Air source heat pumps (ASHPs) might be heralded as the future of home heating, but as of yet, the UK's take-up of the renewable heating technology is one of the very worst in Europe.
In figures provided by the Europe Heat Pump Association to Greenpeace, out of 21 countries in Europe with data available, the UK comes in joint last — with some 32 times lower air source heat pump sales than Norway.
Investing in an air source heat pump as a replacement for a traditional gas boiler is undoubtedly set to become one of the main options for the future of homes in the UK, given a gas boiler ban on the horizon, however, it's not a straight forward swap.
The initial investment is high and it may 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, so if it's something you're considering for your build or renovation, we explore the pros and cons, plus all the key information you need to know in our complete guide to ASHPs.
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Pros and Cons of an Air Source Heat Pump
What are the Advantages of an Air Source Heat Pump?
- 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 for clean electricity
- Air source heat pumps have comparatively low running costs, especially when compared with off-grid fuels such as propane, oil or direct electric heating
- Air source heat pumps also work well with underfloor heating and low temperature radiators
- There are certain grants available the offset the costs of installing an air source heat pump, such as the Renewable Heat Incentive (RHI), along with new grants set to replace it when it ends in 2022.
What are the Disadvantages of an Air Source Heat Pump?
- 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.
- Electricity is still needed to power an air source heat pump. This will likely increase your electricity bills, but decrease other heating costs. Campaigners are currently 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.
- 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.
- 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.
How Does an Air Source Heat Pump Work?
A ground source heat pump may reach 42 decibels, and 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.
Although an air source heat pump replaces a boiler as the home’s heat source, the way it works is very different.
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. High temperatures require more work from the compressor and therefore result in lower system efficiencies.
There are two main types of air source heat pumps:
- Air to water heat pumps are used in wet central heating systems (sometimes, but not always replacing the whole system) to heat radiators, underfloor heating and 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.
Do Air Source Heat Pumps Need Electricity to Work?
Yes, electricity is needed to power the pump, but this could be from solar PV, for example, meaning that it's possible to use an air source heat pump off grid. 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
Do Air Source Heat Pumps Work in Cold Weather?
The 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. 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.
Replacing a Boiler with an Air Source Heat Pump
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.
It's not as simple as it sounds to switch from a gas boiler to an air source heat pump, and deciding to retrofit an ASHP 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. For an ASHP, it's 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 heat pump compatible hot water storage.
The lower temperatures will also mean that an air source heat pump is only compatible with homes that have high levels on insulation and airtightness.
The payback on an air source heat pump will largely depend on the actual cost of the swap, taking into account any incentive payments you may receive as well as also accounting for how much you would have spent on a replacement boiler.
The running costs also need to be evaluated against the fuel that you are replacing. If you currently have a gas boiler running on natural mains gas, then there will currently not realistically be any financial saving. Against heating oil and LPG, the payback could be better but that does depend on the cost of those fuels.
How Do Air Source Heat Pumps Compare to Ground Source Heat Pumps?
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There are a number of key differences to note between air source heat pumps and ground source heat pumps, including:
A typical claimed SCOP figure for an air source heat pump might be 3.2; the comparable figure for ground source heat pumps is more like 4, so for every 1kW of electricity, 4kW is generated.
Thus ground source heat pumps appear to be slightly more efficient but as the compressor and the refrigerant is very similar in both systems you really have to check the real temperature of the heat source, namely the ground and the air.
Towards the end of the heating season (January onwards) the ground could be colder as the heat is extracted. If the air temperature is therefore warmer than the ground temperature then it can be argued that the ASHP could be more efficient. Ground conditions and geographical location are crucial when making this choice.
Ground source heat pumps require a large garden or piece of land to be installed, or are installed in deep boreholes. Both types of installation result in excavation costs. Air source heat pumps are not installed in this way. Instead the external condenser unit sits in a box on the outside wall and so is cheaper to install.
The Renewable Heat Incentive offers a more generous return for owners of domestic ground source heat pumps — 21.16p as compared with 10.85p for domestic air source heat pumps. The maximum claim is 30,000kWh per year for ground source heat pumps and 20,000kWh for air source heat pumps.
Is my Home Suitable for an Air Source Heat Pump?
There are some key criteria you're home will need to meet to make installing an air source heat pump viable.
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 ASHP is generally smaller than a gas boiler, so you should have no issue fitting this in it's place, though if you're continuing to use a gas boiler in tandem, you'll need to consider this internal space too.
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.
How will insulated is your home?
ASHPs 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. When replacing a gas boiler, an ASHP 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?
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. Like fridges, 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 Renewable Heat Incentive, and it's a worthwhile investment in the future of your home, even if you don't see savings compared to your existing gas boiler.
What are the Renewable Alternatives to an Air Source Heat Pump?
If you're looking for an alternative to a gas boiler, but aren't convinced by the offering of an air source heat pump, there are many other systems you could investigate more:
- Produce 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.
How Much Does an Air Source Heat Pump Cost?
A basic air source heat pump costs from £1,600 for a small air source mono block unit to around £14,000 for a top end large capacity fan unit.
More expensive air source heat pumps can have higher quality components and software for monitoring heat pump operation, while more expensive alloys are also often used in the construction of the cases and components.
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How Much Does Installing 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. Additional acoustic housings may also be required, which can also increase your installation bill.
Larger properties may also need a bigger heat pump, or even more than one unit, which can in turn require a hydraulic design which is more complex.
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.
Putting the costs into real context is not easy but the following costs (made up of the system cost and installation) can be used for guidance:
- Combi-boiler: £2,000 - £4,000
- System boiler with cylinder: £5,000 - £9,000
- Ground source heat pump: £16,000 - £25,000
- Biomass boiler: £14,000 - £19,000
What Other Costs Should I Consider When Installing an Air Source Heat Pump?
A recent government report found that installation of an air source heat pump in an existing property can run upwards of £27,000.
Air source heat pumps are at their most efficient at low flow temperatures, which are slow to react in heating systems. This means that high insulation levels are also required for these systems to heat your home efficiently.
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 cost of installing an air source heat pump system will also be complicated by the difficulty and disruption involved in running new pipework.
This pipework will need to run from the outdoor fan unit to the hot water cylinder, which may be in a cupboard upstairs, as well as to the central heating distribution, which may be located near the existing boiler rather than conveniently placed near the proposed location of the heat pump unit.
If you are switching over from an electrical off-peak storage heater system then you will need to consider the cost of a complete wet radiator system.
Even if you do have an existing wet radiator system, if the emitters (radiators or underfloor heating) are too small, you may need to upgrade the size to accommodate the low flow temperatures.
There may also be a cost in removing the existing boiler. In the case of oil or LPG, the removal of fuel storage tanks, concrete base and fuel pipework will need to be factored in.
How Much Does an Air Source Heat Pump Cost to Run?
The cost of a unit of electricity is around 15p per kWh and the average efficiency of an air source heat pump is 220% to 320%.
In a property that has an annual heat load of 15000kWh, you would expect the electric use to be somewhere between 6850kWh and 4700kWh, which is an annual running cost of between £700 and £1,050.
These are the average costs of using different heating fuels in order to compare with the running costs of air source heat pumps:
- Oil: between £500 and £1,250 per year.
- LPG: between £825 and £1,320 per year.
- Natural gas: between £660 and £825 per year.
An air source heat pump may not be cheaper to run than a gas boiler in many instances, but campaigners are asking the government to reduce the environmental levy on electricity with a looming ban on new gas boilers, to help ensure that an air source heat pump is cheaper to run in the future.
A ground source heat pump costs between £540 and £700 per year to run.
The efficiency of an air source heat pump is also dependent on factors beyond the fabric of your home. The renewable heat produced comes from the ambient air, so as the air around the unit cools down the efficiency will drop.
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.
Also, if not commissioned properly, an air source heat pump could turn direct electric heaters on when they are not actually required, resulting in high electricity bills.
Are There Any Air Source Heat Pumps Grants Available?
There are several heat pump grants available which offer financial assistance for those looking to install an air source heat pump.
Renewable Heat Incentive
The running costs above do not take into account payments you will receive under the Renewable Heat Incentive (RHI) when investing in an air source heat pump.
Under the RHI scheme, those with renewable heating technologies are paid back for the heat they generate for seven years — a real bonus for those considering investing in an air source heat pump.
The domestic Renewable Heat Incentive scheme is still accepting applications, but will conclude in March 2022. It covers England, Wales and Scotland.
The current payment (as set out in 1 July 2020) is 10.85p/kWh, but only air to water heat pumps are eligible.
Clean Heat Grant
The Clean Heat Grant is a new scheme set to replace the RHI when it closes in 2022. This grant will offer homeowners who successfully apply a flat-rate payment of £4,000 for help installing heat pumps, including air source, ground source and water source heat pumps.
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.
How to Choose an Air Source Heat Pump
Can an Air Source Heat Pump Provide Both Heating and Hot Water?
One of the first decisions to make when buying an air source heat pump is whether it will provide space heating or domestic hot water — or both.
The key here is that the ‘flow temperature’ (the temperature of the water in the heating system) is lower in a heat pump than it is in a boiler:
- 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.
Another 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.
If you do not have a ducted ventilation system and don’t want to draw heat from inside the property, you could consider a different type of ‘micro’ heat pump such as a thermodynamic system. It's important to make sure that it is designed and specified properly though and is not being asked to do more than its design capability.
It is in effect an air source heat pump with an outdoor panel evaporator. The outdoor panel contains refrigerant and relies on air temperature and sunlight as a heat source. The panel is often mounted on a roof but can be wall-mounted. Bear in mind that it needs good exposure to sunlight and moving air, so it should ideally not be tucked away behind the garage or shed.
The micro heat pump only draws between 400W (watts) and 800W of electricity, and produces around 1,200W to 2,400W of heat (depending on the compressor and fan size and the air intake temperature), so if you have photovoltaic panels (PV) fitted to the property, the micro heat pump will also be optimised to use the on-house generation and possibly heat your water for free for much of the year.
Higher Temperature Air Source Heat Pumps
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 (such as the Daikin Altherma) 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.
There are also other new technological advances that are worth noting such as compressors that allow the compressed vapour to be re-injected into the compressor to enhance the temperature. These systems can get flow temperatures of around 65°C.
The advantage of this system is that it reduces the complexity of the heat pump and therefore the cost. The operating pressures put a larger load on the compressor and push the tolerance of the refrigerant — examples include:
- Mitsubishi Electric Ecodan
- Stiebel Eltron, amongst others.
Choosing Smart Controls for Your Air Source Heat Pump
Modern air source heat pump heating systems require specialist design and commissioning to achieve and maintain efficiency. In the age of the ‘app’ and smart heating controls, these systems can easily be tampered with, resulting in lower efficiency and high running costs.
As a result, some manufacturers have developed controls that can be monitored and maintained remotely. This is especially useful in second homes and rental properties, as well as for technophobes, as the systems can be reset and adjusted often without someone coming out to the property.
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 ordering, but monitoring and metering systems can potentially attract an increased RHI payment if compliant components are installed.
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.
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.
Comparatively, installing an air source heat pump is a low-disruption process. 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.
In this case, installation could take as little time as a single day.
Do Air Source Heat Pumps Need Much Maintenance?
No. Most maintenance by the homeowner is visual — checking that the outdoor unit is free from leaves and debris and the pipework is intact.
An annual inspection by qualified engineers is also recommended.
David is an expert in sustainable building and energy efficiency and is also director of Heat and Energy Ltd.
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