An air source heat pump (ASHP) installed outside a 1960s house renovation by The DHaus Company. Photo by Matthew White.
Gas central heating is commonplace in the UK. However, with the deadline for phasing out new gas boiler installations by 2035 fast approaching, many of us will soon need to explore low carbon heating alternatives.
This impending transition to low energy heating, particularly the adoption of heat pumps, has become a divisive subject. On one side are the heat pump sceptics who believe that heat pumps are not fit for purpose. On the other side are the heat pump enthusiasts who highlight the benefits while downplaying potential disadvantages.
Our guide aims to provide a balanced assessment of retrofitting heat pumps into houses, helping homeowners decide whether they are right for their particular needs.
Heat pumps operate similarly to a fridge, only in reverse. They absorb heat from the air, ground or water using a liquid refrigerant which first circulates through a looped system of pipes called an evaporator, transforming into a cool gas. The cool gas then passes through a compressor which radically increases its temperature, converting it into a hot gas. It’s then directed through a heat exchanger which transfers the heat to the central heating system. The cooled refrigerant then condenses back into a liquid and flows to the evaporator, ready to start the cycle all over again.
While hydrogen fired boilers are often portrayed as an ideal low-carbon substitute for gas, they currently face commercial unviability due to technological and infrastructure challenges. These hurdles make them unsuitable for widespread use.
The most practical alternative to heat pumps at present is biomass boilers, which utilise wood pellets, chips, or logs for heating and hot water. However, it's important to note that wood boilers take up quite a lot of room and need extra space for fuel storage. Additionally, there is a growing concern that burning wood may impact local air quality, potentially contributing to health issues.
Despite the challenges associated with heat pumps, they remain the most effective low carbon heating system currently available to most homeowners.
A heat pump can be installed without planning permission if certain requirements are met.
Firstly, the design must comply with Microgeneration Certification Scheme (MCS) Planning Standards. Then it must adhere to permitted development guidance (Class G).
If you live in a Conservation Area and World Heritage Site you still may be able to install a heat pump without planning permission.
If you live in a listed building, you will require Listed Building Consent to install a heat pump.
Current MCS noise standards can make it difficult to install a heat pump in a built up area without planning permission.
READ:
Do Heat Pumps Require Planning Permission in the UK?
According to current building regulations, all new heating system installations, whether in new or existing properties, must operate at low flow temperatures. The maximum permitted is now 55°C, contrasting with the previous standard of 75°C.
The regulations do not rule out gas boilers. However, given the plumbing upgrades required to achieve the low flow temperatures, opting for a heat pump becomes a logical choice when you factor in government grants and lower running costs. More on these below.
It's important to note that low flow temperature standards only apply to full installations. You can keep your existing central heating system if you are just replacing the boiler.
For most homes in the UK, an air source heat pump (ASHP) is the most practical choice. Available as either air-to-water or air-to air, they are compact and can be installed in relatively confined areas.
Air-to-water heat pumps work by extracting heat from the outside air and transferring it to a water-based central heating system and hot water cylinder. They are popular in the UK because they can utilise existing plumbing and radiators – continue reading below.
Air-to-air heat pumps operate similarly to air conditioning but can heat and cool the air. In a split multi system, up to five internal fan units can connect to an external unit via flow and return pipes containing liquid refrigerant. Unlike air-to-water systems, air-to-air requires a separate hot water supply.
If you have the space, you could consider a ground source heat pump (GSHP) which absorbs heat from the earth. Installed in shallow horizontal trenches or deep vertical bores, they connect into a water based central heating and hot water system in the same way as an air-to-water heat pump. GSHPs are the most efficient type of heat pump but very expensive to install.
If you live near a large body of water, you may be able to install a water source heat pump (WSHP). However, they are much more involved than the other types of heat pump and very few are installed in the UK.
Air or ground source heat pump installation.
Air-to-air heat pump installation.
A heat pump’s average efficiency is expressed as the Coefficient of Performance (COP) or Seasonal Coefficient of Performance (SCOP) when aggregated over a year.
Gas boilers typically heat water to high flow temperatures, around 60-70°C, and operate at an efficiency of about 80%. Heat pumps operate at much lower temperatures than gas boilers, typically between 35-50°C for heating and 55°C for hot water, and typically achieve efficiencies of 300-500%, or a SCOP of 3-5.
As a result, heat pumps are generally cheaper to run than gas boilers. More on running costs below.
Heat pumps are at their most efficient when operating between 35-45°C. Efficiencies drop off when they need to heat hot water at around 55°C. To address this, heat pumps are often paired with solar panels and home battery storage to improve efficiencies.
Solar thermal panels can meet up to 90% of domestic hot water needs in summer and 25% in winter. Photovoltaic panels (PVs) and home battery storage enable the generation and storage of electricity, which reduces the system’s demand from the grid.
Heat pumps are widely used in places which experience extremely cold winters, such as Scandinavia, and are well equipped to handle the UK's benign climate.
Modern refrigerants boil at around -50°C. In the case of an air source heat pump, if it is -10°C outside, there is a 40°C temperature difference for the heat pump to absorb. Ground source heat pumps benefit from the earth's consistent temperature – typically around 10°C – and can therefore exploit an even greater temperature differential.
Unlike gas boilers which can compensate for poor insulation by heating radiators to high temperatures (albeit at considerable cost), heat pumps operate at much lower temperatures and work best in energy-efficient buildings.
While there are instances where heat pumps have proven effective in uninsulated buildings, ensuring good levels of insulation, and draught-proofing, will guarantee the best overall installation. For example, a well-insulated home will require fewer radiators than a poorly insulated one, resulting in a more practical and aesthetically pleasing internal layout.
When insulating old buildings, it’s important to have a robust vapour strategy in place to avoid creating damp and mould issues. See our other guides below for more information.
READ:
External Wall Insulation: A Guide to Retrofitting Homes
Is Retrofitting Internal Wall Insulation Worth It?
Retrofitting an air-to-water or ground source heat pump will require some new pipework and radiators or underfloor heating in your home. The extent depends on your property’s specifics.
It’s well documented that heat pumps work more efficiently with large bore pipework (28mm diameter +), whereas many houses in the UK with gas boilers are fitted with micro bore pipes (less than 15mm diameter). However, the three main factors determining the need of plumbing upgrades are the thermal performance and the size of the property and the layout the existing heating system.
If you live in a reasonably well-insulated property that contains micro-bore pipes, it’s perfectly feasible to install a heat pump with minimal disruption. The demands on the heating system will be lower, and there is a good chance you will be able to keep much of the existing plumbing, potentially only requiring higher output radiators.
Further, in cases where the existing system has a simple layout with short, efficient pipe runs, there's a good chance you will be able to integrate much of the micro bore pipework into the new installation. However, larger properties or properties with complex heating layouts pose more challenges when using micro bore pipes, as they can reduce the rate of water flowing through the system. Installing a buffer and secondary pump can mitigate this issue, albeit with the loss of some efficiency.
Note that air-to-air heat pumps require a completely new central heating system as pipework is filled with liquid refrigerant, not water.
Commissioning an experienced heating engineer is essential for a successful heat pump installation. They will appraise the site and carry out detailed heat loss calculations to ensure the proposed system meets the particular demands of the building.
Heat Geeks currently set the benchmark for heat pump installations and provide a database of certified heating engineers across the country.
However, there is a shortage of qualified engineers in the UK. While the numbers are improving, many areas lack adequate service, posing a barrier to good installation.
As heat pumps are classified as a low carbon technology, installations do not incur VAT and are eligible for government loans and grants.
In broad terms, if you’re about to undertake a major renovation project which requires an entirely new central heating system, opting for a heat pump is likely to be the most cost-effective solution when you factor in VAT relief and grants.
However, if you live in an uninsulated but well finished home, installation costs may pose a significant barrier to adopting a heat pump. This is especially true when considering the need for new insulation and draughtproofing for efficient operation and the repair of substrates and finishes disturbed by the works.
The table above gives an approximate idea of the cost to supply and install a heat pump, excluding home improvements necessary for its efficient running. Please note that estimates for air-to-air systems exclude domestic hot water.
Most homeowners in the UK have access to government grants or loans to offset the costs of installing heat pumps.
The Boiler Upgrade Scheme offers grants in England and Wales, offering £7,500 for both air and ground source heat pumps. However, hybrid systems that use fossil fuels, such as a combination of heat pump and gas boiler, and air-to-air heat pumps are ineligible.
In Scotland, grants of £7,500 (increasing to £9,000 for homes in remote areas) are available through Home Energy Scotland. Additionally, homeowners can get an interest free loan of up to £7,500 to cover the remaining costs of installation.
Low income households can also benefit from grants through the ECO4 government scheme in England, Nest in Wales and Warmer Homes Scotland.
To qualify for these schemes, the heat pump installation must be carried out by an MCS certified heating engineer.
A well designed and installed heat pump is generally cheaper to run than a gas boiler.
According to Ofgem, the average UK home consumes 11,213kWh of gas for heating and hot water per year (the annual energy use). Gas boilers operate at an average efficiency of 80%, turning 8,970kWh of the gas into heat (the annual heat demand).
The price cap for gas is currently 7.42p/kWh (from January 2024), so the annual cost of the average gas boiler is £832 (11213 x 0.0742).
For an air source heat pump operating at a SCOP of 3.5 (350% efficiency) to deliver an annual heat demand of 8,970kWh, the annual energy use will be 2,563kWh (8970 / 3.5).
The price cap for electricity is currently 28.62p/kWh, so the annual cost of an air source heat pump is £733.53 (2563 x 0.2862).
Off peak rates can reduce electricity costs by 20%, taking the tariff to 22.90p/kWh. This could bring heat pump running costs down to £586.93 (2563 x 0.2290) per year.
Further savings can be made by installing solar panels and battery storage which reduce demand from the grid.
Generally speaking, heat pumps are low maintenance.
Most are installed with a 10-year manufacturer’s warranty and a 2-year workmanship warranty. To ensure the validity of the warranty, manufacturers recommend the heat pump is serviced once a year by an MCS certified heating engineer, or equivalent.
A well maintained heat pump can last over 20 years, whereas a gas boiler typically lasts around 10 years.
Low carbon heating is something we will all need to consider soon. Currently, heat pumps stand out as the only viable solution.
The decision to install a heat pump now or later will depend on your current circumstances. If you need to replace an old gas heating system as part of a major refurbishment, installing a heat pump becomes a cost-effective choice, especially when you factor in government grants and VAT relief.
If you live in a reasonably well insulated home, a heat pump may be installed with minimal disruption.
However, for older, uninsulated properties, the immediate cost of installing a heat pump might pose a barrier, particularly when accounting for the home improvement works recommended to achieve an efficient, well-laid-out system. In such cases, adopting a phased approach, where improvements are carried out gradually, will ensure that your home is "heat pump ready" when the opportune time for installation arrives.
About the author
Aron Coates is an architect with over two decades of experience, encompassing projects ranging from the restoration of historic buildings to the design of contemporary homes.
