While the take-up of air source heat pumps as heating systems has increased in recent years, in years to come this technology has an important role to play in heating our homes.
As outlined in the government's Future Homes Standard, new homes built in England from 2025 will no longer be able to connect to the gas network — instead requiring suitable levels of insulation and a low carbon heating source, including but not limited to air source heat pumps.
Along with this change to policy in England, the government has made other moves to encourage the uptake of air source heat pumps, including the Clean Heat Grant — a successor to the Renewable Heat Incentive that focuses on heat pump technology — alongside targets to install 600,000 new air source heat pumps every year by 2028.
Whether you're considering an air source heat pump for your self build, or looking to retrofit into your existing home, our ultimate guide has everything you need to know about the pros and cons of these systems, how much maintenance they require and how they work. Check out our guides to air source heat pump costs and installing an air source heat pump too.
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How Does an Air Source Heat Pump Work?
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.
Here's some quick answers to some other frequently asked questions about how air source heat pumps work:
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.
Can Air Source Heat Pumps be Used 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.
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
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 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.
Are There Any Air Source Heat Pumps Grants Available?
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.
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.
Annual inspections by qualified engineers is also recommended.
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.
Where to Buy an Air Source Heat Pump
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David is an expert in sustainable building and energy efficiency and is also director of Heat and Energy Ltd.
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