Heat Pumps
Miss any of the four and the running cost lands closer to a gas boiler than the brochure suggested. The order matters as much as the numbers.
01 Fabric first
The house has to lose less heat than the old boiler was papering over.
Draught-proofing, loft insulation to current standards, and cavity or solid-wall insulation where the fabric warrants it. Skip this and every downstream sizing decision gets more expensive.
The existing gas or oil system is coming up for replacement.
A heat pump into an under-insulated home works harder and costs more than it should. Bringing the loft to a proper standard reduces the size of the heat pump you need and improves how it runs for the next fifteen years.
Draught-proofing costs very little and lowers the heat-loss number the installer designs against. Cheap, quick, and materially changes the sizing conversation.
For solid-wall houses where the fabric can't be brought up to standard in one pass, a hybrid keeps the gas boiler for the coldest days and lets the heat pump carry the rest of the year. Not the destination, but often the honest next step.
If you've considered these and a heat pump is still the right answer for your home, here's how to choose well.
With the fabric in a reasonable state and a proper room-by-room heat-loss survey behind it, a well-designed heat pump is the cheapest, quietest and lowest-carbon way to heat almost every British home built after about 1930.
The decision, in six questions
Seven questions decide whether a heat pump is a strong candidate for your home, worth considering with caveats, or probably the wrong answer today.
Is your loft insulated to at least 270mm and are the main draughts sealed?
The fabric can carry a properly sized heat pump.
Strong candidate
Fix the loft and the worst draughts first; a heat pump into an uninsulated roof is money wasted on both sides.
Worth considering
Do you have external space for an outdoor unit that isn't right against a bedroom wall?
Siting stops being the reason to say no.
Strong candidate
Talk to an installer about acoustic siting; a well-placed unit is almost always achievable but sometimes needs creativity.
Worth considering
Is your existing boiler within five years of end-of-life?
The natural decision window is open; a heat pump becomes the honest replacement.
Strong candidate
A working boiler still has value; plan the fabric work first and revisit when the boiler starts to fail.
Worth considering
Are you willing to accept slightly larger radiators in one or two rooms?
The design will work without heroics.
Strong candidate
Some rooms will underperform on the coldest days; consider a hybrid or hold off until you're ready to accept the radiator upgrade.
Worth considering
Is your home under 60m² with a combi boiler and no space for a cylinder?
A heat pump needs a cylinder, and flats or micro-terraces often can't fit one; this is a genuine constraint.
Probably not for your home
Cylinder space is manageable in almost every other UK home type.
Strong candidate
Are your walls solid stone or single-skin with no plan to insulate them?
Peak load may exceed what a sensible heat pump can deliver; a hybrid or fabric-first plan is a better first move.
Worth considering
The fabric will let a well-designed heat pump carry the winter load.
Strong candidate
Do you plan to move within three years?
The payback horizon shortens; make sure the buyer is likely to value the install, or defer.
Worth considering
You'll be in the house long enough to enjoy the running-cost saving.
Strong candidate
What usually changes the answer
Change one of these and our recommendation often changes with it.
Your loft is genuinely under-insulated.
What changes
Your gas boiler is failing this winter.
What changes
You have a solar and battery system already, or plan to install one.
What changes
You are on LPG or oil rather than mains gas.
What changes
You have specific noise-sensitive neighbours or a listed building constraint on the outdoor unit.
What changes
You are on Economy 7 or a heat-pump-specific tariff.
What changes
If this were our house
For a 1930s three-bed semi with a working but ageing gas boiler
We would insulate the loft properly first if it wasn't already, and we would have a proper room-by-room heat-loss survey done before we accepted a single quote. That survey costs a few hundred pounds and is the single biggest determinant of whether the system delivers. We would ask specifically for the calculated flow temperature and the radiator schedule that follows from it.
Assuming the numbers came back in the usual range for a semi of that age, we would fit a 7kW to 9kW unit, upsize the radiators in the coldest two rooms, take the £7,500 grant, and run the cylinder overnight on a heat-pump tariff. We would not accept an install without a written commissioning report showing the weather compensation curve and the cylinder schedule at handover.
When we'd say no
We would rather lose the sale than take you somewhere the numbers don't stand up.
You are being sold a heat pump without a proper heat-loss survey. The sizing will be wrong and the system will disappoint. Change installer.
Your loft is genuinely bare and there is no budget to insulate. Put the money into the fabric first; the heat pump will be a better decision in two years.
You live in a flat or micro-terrace with no plausible location for a cylinder or an outdoor unit that isn't a nuisance to neighbours.
You have less than three years in the property and the local market doesn't yet reward a heat pump install on resale.
You have been quoted a flow temperature above 55°C to avoid upgrading radiators. That system will run inefficiently for its entire life; insist on a proper design or walk away.
What most people get wrong
The trade at a glance
A five-step editorial scale, not a rating. Green weight is a gain; grey weight is a cost you carry.
Running cost vs gas
In a reasonably insulated home on a heat-pump tariff, annual bills usually undercut a modern condensing boiler by a meaningful margin.
Comfort in cold weather
A properly sized system delivers steady, even warmth through the coldest weeks; the myth that heat pumps 'don't work when it's cold' comes from undersized installs.
Hot water performance
Cylinder-based, so no combi-style instant hot water; overnight reheat handles a typical household without a wait.
Carbon reduction
Roughly three-quarters lower carbon emissions than a gas boiler on the current grid mix, improving each year as electricity decarbonises.
Noise
A well-sited modern unit is quieter than a fridge from a few metres away; audible up close but not intrusive at a bedroom window if placed sensibly.
Installation complexity
Outdoor unit, indoor hydraulics, cylinder, controls and radiator work. A two to five day install; not a boiler swap.
Up-front cost
Even after the £7,500 grant, the out-of-pocket cost is materially higher than a boiler replacement; the payback is measured in years, not months.
What it costs
MCS-accredited install including outdoor unit, hydraulics, cylinder and standard radiator upgrades. Before the £7,500 grant.
Larger unit, more radiator work, occasionally a phased-fuse upgrade. Before grant.
Applied by the installer at the point of quote; England and Wales, one per property.
On a heat-pump tariff with sensible cylinder scheduling. Compares to £1,100 to £1,700 for a modern gas boiler on standard credit tariffs.
Ranges are triangulated from DESNZ heat-pump cost analyses, published MCS installer data, and Energy Saving Trust reference figures. Refreshed quarterly. Your figure depends on the fabric, the radiator schedule and your electricity tariff.
You've got the answer
For most UK homes with a decently insulated loft and a garden or side return, a properly designed heat pump is the right answer today. The £7,500 grant is doing a lot of work in the maths; it will not last forever.
You can stop here if you're still deciding. The rest of this page is for readers who have already committed and want to spend the money well.
If you're going ahead
This half of the page is written for the reader who has decided. It reads in the order the decisions come at you.
Which version
One outdoor unit contains the refrigerant loop; only water flows inside the house. Simplest to install, cheapest to service, and the version that most UK installers know best. Almost every mainstream install falls into this category.
When we'd choose it
Unless there is a specific reason otherwise, this is the version we would fit in a UK home.
The refrigerant loop crosses into the house to an indoor unit that transfers heat to the water circuit. Slightly higher efficiency in cold weather because the refrigerant condenses indoors, but requires an F-Gas engineer and adds pipework complexity.
When we'd choose it
Cold-region installs where the marginal efficiency gain justifies the extra install work, or where the outdoor unit needs to be small.
A monobloc designed to deliver flow temperatures around 70°C, closer to a boiler. Sold as a way to keep existing radiators, at the cost of running efficiency; the SCOP is materially lower and the annual bill higher.
When we'd choose it
A last resort in properties where radiator upgrades are genuinely impossible; not a substitute for proper design.
Your house
The outdoor unit is roughly the size of a large chest freezer laid on its side and needs about a metre of clearance on the intake side. It should not sit tight against a bedroom wall, a neighbour's boundary within a metre, or any surface that will reflect noise back at a habitable room. On most UK semis it lives beside the side return or under the kitchen window; on terraces it needs more thought but is almost always achievable. Walk the outside of the house with the surveyor before signing.
A heat pump needs a hot water cylinder, typically 180 to 250 litres for a family home. It usually lands in the airing cupboard, the utility, or occasionally a repurposed under-stairs space. Combi households need to plan for this; the space is real. The upside is that airing-cupboard cylinders return you to the era of usable warm-drying space, which people who grew up with them tend to miss.
A well-designed heat pump runs the radiators cooler than a boiler, which means the surface area has to be larger to deliver the same heat. In practice, one to three radiators in the coldest rooms usually need upsizing; the rest stay. Microbore pipework occasionally needs replacing on one or two runs; the surveyor will flag this before the quote. Underfloor heating pairs beautifully but is not a prerequisite.
In use
A properly commissioned heat pump varies its flow temperature against the outdoor temperature; it runs cooler on mild days and warmer on cold ones, always at the lowest temperature that keeps the house comfortable. This is called weather compensation and it is where most of the SCOP comes from. A system left on a fixed flow temperature will run efficiently on average and badly at the extremes.
Ask the installer to show you the weather compensation curve at commissioning and to explain how to adjust it if the house feels cool.
The cylinder should reheat in one or two scheduled overnight runs on the cheapest tariff hours, plus a top-up if the household is unusual. This is the single biggest lever on annual running cost after the compensation curve. A cylinder set to reheat every time it drops a few degrees will keep the heat pump running at inefficient temperatures all day. Ask for the schedule to be set at commissioning, not left to a default.
A well-sized heat pump keeps up on the coldest UK design day; the 'they don't work when it's cold' myth traces back to undersized installs that used gas-boiler rules of thumb. Modern R32 and R290 units retain useful efficiency down to about −7°C, which covers almost all UK weather; below that, the immersion element in the cylinder provides a modest boost, not a full backup. The system does not switch off when it snows.
Installation
Day 1
Outdoor unit base laid, cable and pipe routes cored through the wall, existing boiler isolated.
Day 2
Cylinder positioned, primary pipework run between outdoor unit, cylinder and existing manifold.
Day 3
Radiator upgrades fitted room by room where the survey requires them; system filled and pressure-tested.
Day 4
Electrical connection, controls wired, weather compensation and cylinder schedule configured.
Commissioning
Balancing across the radiator circuit, printed commissioning report with flow temperature, compensation curve, cylinder schedule and MCS certificate for the grant claim.
Common mistakes
Sizing calculation done from floor area rather than a room-by-room heat-loss survey; the resulting system either overspends or underdelivers.
Flow temperature set high to avoid upgrading radiators; the SCOP collapses and the annual bill overshoots the quote.
Cylinder scheduling left on factory defaults; the heat pump reheats water at expensive rates through the day.
Outdoor unit sited against a bedroom wall or a neighbour's boundary; a small acoustic assessment at the survey stage would have avoided the complaint.
Weather compensation not commissioned; the system runs at one flow temperature all year and never touches its rated SCOP.
MCS certification paperwork incomplete, delaying or invalidating the £7,500 grant claim.
The installer
The gap between the best and worst installer on this list of trades is larger than the gap between products. Judge the person, not the brochure.
Maintenance
Annual service by an MCS-registered engineer, mostly to check refrigerant charge, condensate drainage and system pressure. Budget around £150 to £250 a year. Filters and outdoor unit fins want a quick inspection each spring; twenty minutes with a hose and a soft brush prevents the vast majority of efficiency drift. Weather compensation and cylinder schedule are worth reviewing whenever your electricity tariff changes.
Real questions
For most UK homes built after 1930, a properly designed air source heat pump is the cheapest way to heat the house and the lowest-carbon by a wide margin. The £7,500 grant is doing meaningful work in the payback. The single decision that matters is the quality of the heat-loss survey; that is where a good install is separated from a mediocre one.
Quote teardown
A representative quote for a mid-sized 1930s semi, worked against public installer benchmarks so a reader can see what is doing the work and what is padding.
Read the investigation