Air-source heat pumps take heat from the outside air and use it to warm your home. They’re one of the main low-carbon alternatives to gas boilers in the UK, and government grants have made the upfront cost more manageable for many homeowners.
This guide covers how the units work, what installation involves, and what to check before committing.
The Best Choice For Your Home?
Heat pumps have grown in popularity for a straightforward reason: run well, they can lower running costs compared with gas or oil heating, particularly in a well-insulated home. The Boiler Upgrade Scheme also reduces the upfront cost, which has made the switch more accessible than it was a few years ago.
They also draw on an electricity grid that’s getting cleaner each year as more wind and solar capacity comes online, so the carbon savings from switching tend to improve over time, not just at the point of installation.
If you’re weighing up whether a heat pump makes sense for your home, start by looking at your current insulation, radiator sizing, and hot water habits; the answer depends more on those factors than on the technology itself.
Harnessing Energy from the Environment
The outdoor unit sits somewhere around your house and draws energy from the air outside. Rain or shine makes little difference; these units are built to handle the UK’s notoriously unpredictable weather.
Inside it, a fan, compressor and evaporator pull in air and absorb its heat via a refrigerant, which functions as the delivery mechanism for thermal energy transfer.
Where it goes matters. Ideally, it lands somewhere with good airflow, away from noise-sensitive areas like bedrooms; a garden or back patio is normally the best spot, and your installer will advise on the specifics for your property.
Most outdoor units these days are monoblock heat pumps, meaning all the required components sit inside the one unit, which simply sends already-heated water into the home.
Professional electrical installation matters too, not just for regulations but to make sure everything runs smoothly from day one; the work spans both outdoor and indoor wiring.
Delivering Warmth Throughout the Home
Heat pumps can often integrate with your existing heating system, saving you from a complete pipework replacement. A buffer tank or low-loss header is commonly used to create hydraulic separation between the new heat pump and the existing system.
Older vented heating systems will typically be upgraded to an unvented mains feed to suit the new setup, and a new hot water cylinder is almost always fitted, sometimes with extra volumiser units to help the two systems work together.
Heat pumps pair well with both well-worn radiators and modern underfloor heating. Some or all radiators may need renewing, but underfloor heating is a particularly good match since it distributes heat evenly.
New builds have it easier; underfloor heating and pre-plumbed cylinders are usually designed in from the start, and the pipework already has the right bore size for heat pump flow rates. A full installation is simply more straightforward on a new build than retrofitting an existing home.
Because installations vary so much in how much of the original system can be reused, the upfront cost depends heavily on what needs adapting or adding to make everything work together.
Air to Water System
Jargon Buster
- Monoblock
- The most commonly used heat pump type, containing all refrigerant cycle components within one unit and sending heated water into the house.
- Split unit
- Looks similar to a monoblock, but sends heated refrigerant to a heat exchanger inside the house to heat air or water there instead.
- Air-to-air
- Sends heated refrigerant from the outside unit into an indoor fan blower unit, warming the house using warm air.
- Air-to-water
- Can be either monoblock or split, sending heated refrigerant to a heat exchanger to heat the water used in the home.
Domestic Hot Water
A hot water cylinder is usually part of an ASHP system, storing heated water so it’s ready on demand rather than heated instantaneously.
Cylinders are reasonably flexible on placement; a garage, airing cupboard, or utility room all work, provided there’s roughly 80 x 80 cm of floor space plus clearance for servicing.
If space is tight, a few alternatives are worth knowing about:
- Hybrid systems pair the heat pump for space heating with a boiler handling hot water on demand, avoiding the need for a full cylinder.
- Heat batteries offer a more compact alternative to a traditional cylinder, storing thermal energy rather than hot water directly.
- Instantaneous water heaters suit smaller, single-outlet needs (e.g., under a kitchen sink) but aren’t a substitute for whole-home hot water.
The right choice depends on your household’s hot water habits; a family running multiple showers a day has very different needs from a couple mostly using a kitchen tap.
Considerations when Investing in a Heat Pump
Before committing to a heat pump, it’s worth working through a few practical points with an installer.
Budget: Costs vary by property, but government grants can reduce the upfront outlay significantly. It’s worth checking current eligibility, since scheme terms and funding levels are reviewed periodically.
Existing radiators: Because heat pumps run at lower flow temperatures than gas boilers, some radiators may need upsizing to maintain comfortable room temperatures. This is typically assessed via a room-by-room heat loss calculation, not a guess.
Available space: You’ll need suitable outdoor space for the unit (with adequate airflow, away from bedroom windows where possible) and indoor space for a cylinder if one’s needed.
Your household’s routine: How many people are showering, when, and how much hot water you typically use will influence system sizing. It’s worth discussing this honestly with your installer rather than defaulting to the largest option.
A proper assessment should always include an in-home survey.
Cost Analysis and Financial Support Options
Upfront costs for a heat pump typically start around £4,000 once government grants are factored in, though the final price depends on your property size, existing heating system, and how much groundwork is needed.
Compared with the lifetime running costs of a gas or oil system, many homeowners find the gap narrows over several years, though this depends heavily on your electricity tariff and how well-insulated your home is.
The Boiler Upgrade Scheme is the main government grant currently available for ASHP installations in England and Wales. Some installers also offer financing plans, with monthly payments starting from around £40, spreading the cost rather than requiring the full amount upfront.
If you’re weighing up finance options, it’s worth thinking about how long you plan to stay in the property; the payback period for a heat pump is usually measured in years, so the calculation looks quite different if you’re planning to move soon versus staying long-term.
New Radiators and Underfloor Heating
Getting good performance from a heat pump often means reviewing your existing radiators or fitting underfloor heating, since heat pumps typically run at lower flow temperatures than a gas boiler.
Radiators: Some may need upsizing to release enough heat at these lower temperatures. This is usually a one-off cost during installation, and an installer will calculate room-by-room heat loss to work out which radiators, if any, need replacing.
Underfloor heating: UFH pairs well with heat pumps because it operates at even lower flow temperatures and distributes heat over a larger surface area, suiting the way heat pumps work most efficiently.
Mixing both: It’s common in retrofits to keep underfloor heating downstairs, where it may already exist (kitchens or extensions, for instance), and use upsized radiators upstairs. An installer will typically map this out room by room as part of the design process — not something to guess at without a proper heat loss survey.
Get started with heating upgrades.
Heat Pumps vs Boilers for Hot Water
Heat pumps and gas boilers heat water differently, and it’s worth understanding the difference before you switch.
Heat pump systems typically heat water to around 48–55°C, compared with 60–70°C from a standard gas boiler (some heat pump models can go higher, but efficiency drops at higher output temperatures). In practice, this means slightly less need to mix in cold water when running a bath or shower.
One point worth adding: most systems still need a periodic legionella cycle, where the cylinder is heated above 60°C for a short period, even if day-to-day heating runs cooler. Your installer will normally set this up as part of the system controls, so it’s not something you need to manage manually.
If on-demand hot water matters more than storage, in a smaller property, say, hybrid systems (a heat pump for space heating, a boiler or heat battery for hot water) are worth discussing with an installer, as is a standalone heat battery for lower-usage households.
Compare heat pumps and boilers
Why Choose Air Source Heat Pumps?
If you’re moving away from oil or an ageing gas boiler, a heat pump can meaningfully cut both running costs and carbon emissions, provided the system is sized and installed correctly.
A few things are worth knowing if you’re considering the switch:
- Funding: Grant schemes like BUS are reviewed periodically, and support levels can change; check current eligibility before you plan your budget rather than assuming today’s terms will hold.
- Electricity supply: The grid’s carbon intensity continues to fall as renewable capacity grows, improving the case for electric heating over time.
- Compatibility: Heat pumps can usually integrate with existing radiators or underfloor heating, though some radiators may need upsizing to work efficiently at lower flow temperatures.