Vaillant air source heat pump and gas boiler.
For most homes in the UK, an air source heat pump (ASHP) is the most practical choice for heating. Air-to-water (A2W) heat pumps, in particular, have become fairly commonplace because they work with radiators and underfloor heating, offering a relatively straightforward swap with gas central heating systems.
In this article, we'll explain the A2W heat pump system, summarise installation and running costs versus gas central heating, and outline the types of properties each are suited to.
Air source heat pump external unit . Photo by Veissmann.
Air-to-water heat pump systems consist of an external unit, a hot water cylinder, and emitters - either radiators or underfloor heating. They work by extracting heat from the outside air using refrigerants within the external unit and transferring it to a water-based central heating system. They are popular in the UK because you can utilise existing plumbing and radiators.
Indicative air or ground source heat pump installation.
For a typical 3-bedroom house, an A2W heat pump is a standalone installation. According to recent government statistics, installing a 7kW unit (a size suitable for many households) costs on average £10,800, reducing to £3,300 with the £7,500 Boiler Upgrade Scheme (BUS) grant. This includes new pipework and changes to radiators.
A replacement gas boiler is likely to cost in the region of £2,000.
According to Ofgem, a typical household’s gas consumption is 11,500kWh a year, with 11,213 kWh used for heating and hot water.
Using this as the basis, below is a calculation estimating ASHP and gas boiler running costs. Note that the costs are averages and will vary depending on factors like thermal efficiency and energy usage.
Gas Central Heating (GCH):
From October 2024, the average price cap for gas is 6.24p per kWh.
The annual cost of gas consumption is, 11,213 x £0.0624 = £699.66 (say £700).
The average gas standing charge is, 31.66p per day, or £116 per year.
The total gas bill for heating and hot water then comes to £816/ year.
A2W Heat Pump:
Accounting for the 85% efficiency of a typical combi boilers, the actual heat demand of a typical household is therefore 9,531kWh (11,213 x 0.85).
The calculation for an ASHP is as follows.
Electricity required for an ASHP running at a Seasonal Coefficient of Performance (SCOP) of 4 is, 9,531 ÷ 4 = 2,383kWh
The current price cap for electricity is, 24.5p per kWh (October 2024).
The annual cost of electricity for heating and hot water is, 2,383 x £0.245 = £583.83 (say £584).
An A2W heat pump running at £584/year is therefore about £230/year cheaper to run than a gas boiler.
This excludes the average electricity standing charge of 60.99p a day (£223 a year), as you already pay it.
An air source heat pump was installed as part of a wider series of upgrade works to improve the energy efficiency of a typical mi-terrace house by Collective Works. Photo by Jim Stephenson.
If you have an existing, well-functioning gas central heating system, there's currently no immediate financial incentive to install a heat pump - the payback period would be several years for an A2W system at a SCOP of 4.
However, if you need to replace an existing gas central heating system - either because it's reached the end of its lifespan or you're undertaking major renovation work - an ASHP becomes financially more attractive.
With only a possible £1,300 difference between a new combi boiler and A2W heat pump (after the BUS grant), the payback period is significantly reduced.
Combining an ASHP with solar panels can enhance efficiency and reduce costs. While both photovoltaic (PV) and solar thermal panels can be used, PV is generally preferred as they:
Note that the BUS grant covers ASHP installation but not solar panels.
Aerial view of a refurbished Victorian terrace house with an air source heat pump and solar panels installed on the roof. Project by Bradley Van Der Straeten Architects. Photo by French+Tye.
A2W heat pumps generally fall under permitted development (PD) rights, GPDO Class G, and can be installed without planning permission. However, it can be difficult to meet PD guidance when installing in a built-up area, so careful planning is needed.
Solar panels can also be installed under PD rights. See Part 14, Class A – Installation or alteration etc of solar equipment on domestic premises of the GPDO.
READ:
Do Heat Pumps Require Planning Permission in the UK?
Do Solar Panels Need Planning Permission? PD & Design Guidance
Retrofitting an ASHP poses design challenges. In addition to ensuring thermal comfort, you need to consider the impact on aesthetics and functionality, as well as statutory requirements such as planning.
Before installing an A2W system, consider:
Underfloor heating is the optimal design solution in terms of practicality and aesthetics, especially in open plan spaces, but there is a significant cost attached.
You will need to find space for a hot water cylinder when installing an air source heat pump. Image by Urban Plumbers.
A2W heat pumps are suitable for homes with existing radiators, ideally if they are reasonably well insulated.
They work best when feeding large emitters, such as underfloor heating.
They can also work successfully with radiators, but in leaky, poorly insulated buildings, you'll need more radiators or very large radiators to emit sufficient amounts of heat, making it sometimes impractical.
Pros:
Cons:
While A2W heat pumps present some challenges in terms of installation and initial costs, they offer a viable and increasingly popular alternative to traditional gas boilers.
With government incentives and long-term savings on energy bills, they are becoming an attractive option for many homeowners looking to reduce their carbon footprint and future-proof their heating systems.
Heat pumps work well in homes with radiators or underfloor heating, including period properties. Project by Emil Eve Architects. Photo by Taran Wilkhu.
The information provided in this article is intended for general guidance and educational purposes only. For specific expert advice on your project, consult a skilled residential architect featured on Designs in Detail.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
