Air source heat pumps (ASHPs) are the most discussed home heating technology of the decade. Government policy has made them central to the decarbonisation of domestic heating, and the Boiler Upgrade Scheme has put them within financial reach for many homeowners who wouldn't previously have considered them. They're genuinely excellent technology when correctly specified and installed. They're also the subject of a lot of misinformation: both from enthusiastic advocates who overstate the savings, and from gas industry interests who overstate the problems.
Here's an honest assessment of the technology, the costs, and the suitability questions you need to work through before making a decision.
How an Air Source Heat Pump Works
An ASHP extracts heat energy from outside air and transfers it into your home's heating system. It works on the same refrigerant cycle as a fridge or air conditioning unit, but in reverse: the refrigerant absorbs heat from outdoor air (even at temperatures well below 0°C), compresses it to increase the temperature further, and transfers the heat to your water heating circuit.
The key metric is the Coefficient of Performance (COP): how much heat energy the pump delivers per unit of electrical energy consumed. A COP of 3 means 3kWh of heat for every 1kWh of electricity. Modern heat pumps achieve seasonal COPs of 2.5-4.5 depending on outdoor temperature, flow temperature, and how well the system is designed.
Heat pumps operate most efficiently at lower flow temperatures (35-45°C) than conventional gas boilers (typically 70-80°C). This is a critical point for suitability: if your home needs high-temperature radiators to stay warm, a heat pump will work less efficiently and the running costs may be higher than expected.
Is Your Home Suitable?
The question of suitability is where honest assessment matters most. A heat pump in a poorly insulated, draughty house with small undersized radiators will not perform well. In a well-insulated house with large radiators or underfloor heating, it will be excellent.
Insulation. A heat pump works best in a well-insulated building envelope. Loft insulation to 270mm, cavity wall insulation (if you have cavity walls), good windows and doors, and minimal draughts. You don't need Passivhaus standards, but you do need to have addressed the obvious energy losses.
Heat emitters. Your radiators need to be capable of heating the room at lower flow temperatures. The calculation is: take the room heat loss (in watts), apply the flow temperature the heat pump will run at, and check whether your current radiators are large enough. Many homes need radiator upgrades when installing a heat pump. Underfloor heating, which naturally operates at low temperatures, is ideal for heat pumps.
Hot water cylinder. A heat pump heats water more slowly than a gas boiler and needs a hot water cylinder (not a combi-boiler arrangement). If you currently have a combi boiler, you'll need to install a hot water cylinder, which requires space. Factor this into your planning.
Outdoor space. The outdoor unit (roughly the size of a large air conditioning unit) needs to be placed outside with adequate clearance on at least two sides. It makes noise (a low hum): check the positioning relative to your neighbours and any permitted development restrictions on placement near boundaries.
Costs and the Boiler Upgrade Scheme
A typical ASHP installation in England in 2025 costs £10,000-£18,000 installed, including the outdoor unit, indoor controls, hot water cylinder (if needed), and any radiator upgrades required. This is before any grant support.
The Boiler Upgrade Scheme (BUS) provides a £7,500 grant towards the cost of an ASHP installation. The grant is available through registered MCS-accredited installers, who claim it directly and reduce the quoted price by that amount. So a £14,000 installation effectively costs £6,500 to the homeowner.
The BUS grant has been extended multiple times and was confirmed at £7,500 per household through to 2028. Availability in any given period depends on whether the scheme's annual budget has been exhausted, so check current availability at gov.uk before planning around it.
Only use MCS-accredited installers. MCS (Microgeneration Certification Scheme) accreditation is a prerequisite for Boiler Upgrade Scheme grants and demonstrates that the installer has met defined competence standards. Check an installer's MCS accreditation at mcscertified.com before getting a quote.
Running Costs: The Honest Picture
Running cost comparisons between heat pumps and gas boilers depend heavily on current electricity and gas prices, the efficiency of the heat pump installation, and how well insulated the house is. As of 2025:
- Electricity: approximately 24p/kWh (Ofgem price cap)
- Gas: approximately 6p/kWh
- A heat pump running at COP 3.0: effective cost 24p ÷ 3 = 8p per kWh of heat delivered
- A gas boiler at 90% efficiency: effective cost 6p ÷ 0.9 = 6.7p per kWh of heat delivered
At current prices, gas is cheaper per unit of heat than a heat pump, unless the heat pump achieves a very high COP. The picture changes if you have solar panels: using self-generated solar electricity to power a heat pump reduces the effective cost dramatically. It also changes if electricity and gas prices shift (the electricity/gas price ratio has narrowed since the 2022 energy crisis and may narrow further).
The long-term case for heat pumps rests on decarbonisation, not necessarily on immediate cost savings. Running a heat pump rather than a gas boiler significantly reduces your home's carbon emissions. If carbon pricing changes, or gas prices rise relative to electricity, the economics shift further in heat pump favour.
What to Expect from Installation
Installation typically takes 2-3 days for a straightforward replacement of a gas system, longer if significant radiator work or new pipework is required. The process involves:
- Outdoor unit installation (footing or wall-mount depending on manufacturer recommendations)
- Internal pipework connection to existing heating circuit
- Hot water cylinder installation (if not already present)
- Controls installation and configuration (the heat pump controls are more complex than a gas boiler thermostat)
- Commissioning and optimisation of flow temperature settings
- Handover: understanding how to operate the system, including any weather compensation controls
The commissioning and optimisation stage is important. A poorly commissioned heat pump will run at unnecessarily high flow temperatures, degrading efficiency. Ask your installer about weather compensation (adjusting flow temperature based on outdoor temperature) and ensure it's set up correctly.