Air Source Heat Pumps (ASHP) have been suggested as a potential alternative to gas boilers for space and water heating. Whilst currently used mainly for heating properties with no gas connection, the UK Committee on Climate Change (CCC) has even proposed banning gas boilers in new build homes from 2026, by not allowing a gas connection.
Types of Heat Pumps and Basic Principle
Heat pumps are designed to absorb heat from an external heat source and use it for space and water heating in the home. The external heat could be taken from the:
- Air (from outside air)
- Ground (usually from a pipe buried in a horizontal trench or a vertical borehole)
- Water (using a flowing river or open water)
A heat pump can be used to extract far more energy from the heat source (the air, the ground or a body of water) than it consumes. Heat pumps are measured according to their coefficient of performance or COP (sometimes CP or CoP) and typically produce a COP of 2.5-3.5. That means a heat pump using 1 kWh of electrical energy can deliver say 3.0 kWh of heat, making it competitive in the right circumstance with fossil fuel sources that may initially appear cheaper per kWh.
The average energy gains over the year are often described as the Seasonal Performance Factor (SPF). For a well installed ASHP a typical SPF for space heating would be 3.0 with a performance for water heating at 2.3. This is because water heating requires a higher temperature to be achieved, upwards of a 55°C gain, and the ASHP is less efficient when delivering these higher temperatures. In comparison, space heating can be operated with lower temperature gains, typically 35°C.
At March 2019 prices, domestic electricity was circa £0.18/kWh with gas at circa £0.04/kWh, meaning that a heat pump would have to deliver an SPF of above 4.5 to deliver a cost benefit on operating costs. If fossil fuel prices rise faster than electricity in the future (driven by higher gas prices or a carbon tax?) this would deliver a cost benefit at a lower COP. One off installation costs also need to be taken into account; it is much easier to do a new installation in a new house than change an existing gas boiler central heating system to use a heat pump.
| Air and Ground Source Heat Pumps may be eligible for the Renewable Heat Incentive (RHI) which pays incentives, for seven years, for those using renewable heat. (Read Ofgem’s Essential Guide For RHI applicants). To be eligible the system would have to be installed and registered by an MCS certified installer. |
Why Air Source Heat Pumps (ASHP)?
Air source heat pumps are generally cheaper and easier to install in the home than ground or water source heat pumps although their “Seasonal” COP will be lower. This is because in winter using the ground or flowing water as a heat source gives access to warmer and more consistent temperature than the air. All three types of heat pump are best installed in new homes that are well insulated, where space heat demand is low. Ground source heat pumps can be costlier to install and require the drilling of a deep borehole (typically around 20-50m in depth but can be deeper). Pipes can also be laid in shallow trenches but that requires a much larger area of land, not available to the average home. Water source heat pumps are associated with large bodies of water such as a lake or river and again these are often not accessible for the average home. Due to the much higher capital cost, ground and water source heat pumps are better suited to larger systems that serve a number of homes or a commercial building.
A Typical Air Source Heat Pump System
The ASHP itself looks much like a stand-alone air conditioning unit (see simplified image) and sits on the outside of your home. Like a gas boiler it provides heat to supply hot water and space heating. Not shown in the image are the control systems used to manage the heat flows to the space and water heating. The operating noise of the ASHP is relatively low but it is wise not to position it near an open bedroom window.
Image courtesy of Cool Energy
To reduce system cycling and improve efficiencies some manufacturers and installers will sometimes recommend use of a 60-100 litre buffer tank to increase the heated water in the heating system. This means that a flow of water is available, even when the ASHP is not operating. The ASHP can continue to run for a few extra minutes, even when there is no call for heat from space heating or water heating, which reduces system cycling. Buffer tanks are more likely to be recommended in larger houses with many separately controlled heating zones.
Differences From A Gas Boiler
Heat pumps work most efficiently when delivering a flow of water at around 35°C, rather than the 60°C typically delivered from a gas boiler. As the water temperature is lower, radiators need to be larger than those used for gas boilers. Pipework also needs to be bigger, as flow rates are higher to compensate for the lower water temperature. Underfloor heating, which needs a much lower temperature anyway, is a good option if it can be installed as part of the concrete floor slab. These differences make it generally more cost effective to install a heat pump in a new home, rather than having to change over an existing gas central heating system, However, if a house with an existing gas boiler has been upgraded with better insulation then the overall heat demand will be lower and will make an air source heat pump more attractive.
With a heat pump it is best practice to leave the system on for longer, as the instantaneous heat output is lower than for a gas boiler. The home will require more time to get to the desired temperature from cold than a gas boiler. Provided this is accounted for, the home can be as warm as with a gas boiler. On a cold day the ASHP heating may be on continuously, rather than the intermittent bursts delivered by a gas boiler. Experience has shown that leaving heat pumps on continuously generally gives the lowest running costs, as heat pumps work best when they are delivering lower temperatures. With a gas boiler, a radiator may be at 60°C, whereas with an ASHP it could be 20°C cooler.
Air source heat pumps are installed outside the home, so there are some considerations related to where the pump can be mounted (no closer to the property boundary than one metre) and to avoid visual intrusion (particularly in listed buildings).
During frosty weather the system will occasionally have to go into a defrost routine to avoid ice build up on the heat exchanger.
Air Source Heat Pump Output Compared To Gas Boiler
Domestic ASHPs are typically available in the 4-15kW output range, compared to 12-30kW output for gas boilers.
Heat pumps are most suited to installation in better insulated quality built homes that demand lower heat output. That said, people are most suited to well insulated quality built homes as well! It is smart to make your home as energy efficient at possible before you decide on the type and size of a space heating system.
Typical Space Heating Demand Of A Home
Whilst it is hard to be specific, the table below shows some typical levels.
| Description | Typical space heat demand per m² of floor area | Heat demand for 3-bedroom house (100m² floor area) | Heat demand for 4-bedroom house (150m² floor area) |
| Passivhaus | less than 10W/m² | 1kW | 1.5kW |
| New home (high level of insulation) | 40W/m² | 4kW | 6kW |
| Older house (poorly insulated) | 100-150W/m² | 10-15kW | 15-23kW |
Air source heat pumps could provide heating for a wide range of different homes, although would struggle with poorly insulated larger homes. With Passivhaus , space heating requirements are minimal and many occupants would never need the space heating on at all.
Water Heating With A Heat Pump
As heat pumps operate at lower temperatures than gas boilers, stored water may only be heated to around 50°C. This is often fine for household use but to ensure there is no risk of Legionella build up (a type of bacteria) an immersion heater is used once a day to heat the whole tank from 50°C to 60°C.
Air Source Heat Pump Costs: Typical New System Installation
The table below shows the total cost to the builder of installing an air source heat pump system versus a gas boiler system (both systems using a hot water cylinder and radiators), based on 2020 prices (see Centre for Sustainable Energy report: Cost of carbon reduction in new buildings )
| Home type | Air Source Heat Pump System Cost | Gas Boiler System Cost | Difference |
| Detached house | £9,200 | £7,750 | +£1,450 |
| Semi-detached or terraced house | £8,000 | £5,500 | +£2,500 |
| Low rise flat | £6,400 | £5,600 | +£800 |
Air source heat pump costs for a new installation are a little higher than for a gas boiler (between £800 and £2,500 depending on house type). This is primarily down to the costs of the heat pump itself versus a gas boiler. The table below shows indicative pricing available from UK plumbers merchants for a typical gas boiler and for different power output heat pumps.
| Heating type | Typical price |
| Gas boiler 12-30kW | £750 – £2,000 |
| 5kW Air Source Heat Pump (Vaillant Arotherm) | £3,400 |
| 6kW Air Source Heat Pump (Grant AERONA3) | £3,400 |
| 8kW Air Source Heat Pump (Vaillant Arotherm) | £3,800 |
| 10kW Air Source Heat Pump (Grant AERONA3) | £4,900 |
| 11kW Air Source Heat Pump (Vaillant Arotherm) | £4,400 |
| 15kW Air Source Heat Pump (Vaillant Arotherm) | £5,900 |
| 16kW Air Source Heat Pump (Grant AERONA3) | £5,900 |
As well as the heat pump or gas boiler, other items used in the system will include a hot water cylinder, costing between £750-£1,500 depending on storage capacity (usually 150-330 litres), and radiators for each room. Radiators for the heat pump system will tend to be sized to be a little larger than for a gas boiler system, as the system operates at lower temperature (35°C rather than 60°C). Pipework will also have to be bigger to allow for greater flow rates. However, heat pumps are ideal for use with underfloor heating which, although more expensive to install than radiators, does free up wall space and also delivers a more even temperature throughout the room. The other components of the two systems are similar in costs and installation time (pipework, controllers and control valves).
Typical Cost Of Replacing An Existing Gas Boiler With An Air Source Heat Pump System
Deciding whether to replace an old gas boiler with an air source heat pump depends on a number of factors. If you own an old draughty home which costs a lot to heat then installing decent levels of insulation and cutting down on draughts would be essential. Ideally, for the ground floor, underfloor heating would also be used, rather than increasing the size of the existing radiators.
The table above shows that the air source heat pump is likely to be circa £2,000 more expensive than the gas boiler but there are other factors to take into consideration. If the boiler is quite old then there may be other parts of the system that also need to be upgraded (see table below) as gas safety regulations change over the years.
| Other potential upgrades required | Approx Cost |
| New condensate pipework (if old boiler is not a condensing boiler) | £150-£250 |
| New gas flue | £200-£500 |
| Replace gas supply pipe (if supply pressure insufficient) | £150-£300 |
A site survey would need to be done by a Gas Safe Registered Installer, to identify if any or all of this work needed doing.
Upgrading of any electrical wiring, the hot water cylinder and the control system will probably need to be done with either a replacement gas boiler or a heat pump. A key question to answer is whether the radiators are of sufficient size to heat the room using the lower water temperature provided by the heat pump. If not then these will need to be upgraded in size. Pipework with an increased diameter will also be needed. It may be smarter to spend the money on upgrading the insulation in your home, reducing the energy required to heat each room; in this case the old radiators and pipework may then be suitably sized to heat the better insulated room with a heat pump.
Warranty
Air source heat pumps have warranties of 3-5 years. If a new hot water cylinder is required these often come with a 25-year warranty.
| Product | Warranty Period |
| Vaillant Arotherm | 5 years |
| Grant AERONA3 | 5 years* |
*when installed by a G1 installer. Subject to T&C’s
Air Source Heat Pump Maintenance
To qualify for RHI payments and to maintain warranty on the product the system should be serviced annually by a MCS certified installer. The ASHP maintenance would include:
- Cleaning filters
- Checking glycol levels
- Check manifolds on underfloor heating
How Do I Find An Installer For An Air Source Heat Pump System?
There are a variety of routes to try. A simple one is to use the Renewables Guide Quote Service. Alternatively you can find individual Microgeneration Certification Scheme (MCS) certified installers by visiting the MCS website. Feedback from those with heat pump already operating indicates that the quality of the design and installation of the system are key to getting an effective system.
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