The Short Answer
A semi is one of the easiest homes to put a heat pump in. One shared wall keeps the heat loss down, and a side passage (if you have one) suits the outdoor unit. If the cavity walls aren't filled, fill them first: it means a smaller heat pump and smaller radiators.
On our model a 3-bed semi with filled cavity walls and a decent loft needs only about a 5kW unit. That's about £11,500 installed, or about £4,000 after the £7,500 Boiler Upgrade Scheme grant (England and Wales). At the 1 October to 31 December 2026 price cap it heats the house for about £710 to £770 a year, against £840 to £910 of gas.
Estimate the Saving for a Semi-Detached House
Pre-set for a 3-bed semi-detached on gas with the standard SCOP 3.5. Older semis may need insulation work first to keep flow temperatures down.
1. What heats your home today?
Pick whichever your main heating uses.
Add up last year's bills, or use the typical figure for your home below.
3. What size is your home?
We use this to estimate the size of heat pump you'd need.
4. Do you have solar panels?
Free electricity from your roof cuts what the heat pump costs to run.
5. Your options
Both are on to start with. Leave them on unless you know they don't apply.
Your estimated saving
£407a year
Over 15 years that's around £6,105 off your heating bills.
Now: Gas boiler
£950 / year
With heat pump
£543 / year
You save
£407 / year
What about the upfront cost?
- Heat pump install (DESNZ median for this size)
- £12,200
- Less government grant
- -£7,500
- You pay
- £4,700
- A new gas boiler would cost
- ~£2,750
If your boiler needs replacing anyway, the heat pump costs about £1,950 more upfront (before any radiator upgrades), earned back in roughly 4.8 years from running cost savings. That assumes you have the gas supply disconnected, so you stop paying its standing charge (about £108 a year). Keep gas for cooking and it takes longer. It leaves out a heat pump's dearer servicing.
Like the look of £407 a year?
These are estimates. The only way to know your real saving is a free quote from an MCS-certified installer. Takes about 60 seconds, no obligation.
How we worked this out
Energy prices: Ofgem price cap for 1 October to 31 December 2026 (Direct Debit, GB average): gas 7.97p/kWh, electricity 26.32p/kWh. There is no VAT on domestic electricity from 1 October 2026 to 31 March 2027; the gas figure includes 5% VAT. Heat pump tariff blended at 20p/kWh (an estimate for Cosy Octopus, E.ON Next Pumped or EDF Heat Pump Tracker on a heat pump load profile, not a quoted rate). Heating oil 11.31p/kWh (111.52p/litre plus 5% VAT, over 10.35 kWh/litre; BoilerJuice UK average, 23 October 2026). LPG at the price you enter (we start from 75p a litre, our estimate: there's no published average).
Heat pump efficiency: a seasonal efficiency of 3.5. That sits between two measured figures: a median of 2.80 across 291 air source heat pumps in the government's Electrification of Heat trial (installed in 2020 and 2021, whole system), and an average of 4.01 across the 307 systems on heatpumpmonitor.org with a full year of data when we checked in October 2026, a self-selected group of air and ground source systems, mostly high temperature capable R290 units. The break-even with a 90% gas boiler on a standard tariff is a SCOP of about 2.97.
Typical bill ranges: for a 3-bed using 9,500 to 10,300 kWh of heat a year (DESNZ's median and mean gas use for 3 bedrooms in England and Wales, through a 90% boiler): gas £950 to £1,000 (with the standing charge, which the saving assumes you stop paying by having the gas supply disconnected), oil £1,250 to £1,350, LPG £1,200 to £1,300 on our LPG price, direct electric heating £2,500 to £2,700. At these prices a unit of heat from oil costs about 1.5 times as much as from gas, and from LPG about 1.4 times, which is why the saving from switching is bigger.
Solar: A 4kW system in central England generates around 3,800 kWh a year on our estimate (950kWh per kW). On the government's SAP 10.2 self-use formula, a typical 3-bed's heat pump uses about 350 kWh of that a year without a battery, and about 510 kWh with one (8 kWh usable). Most of its demand falls in winter, when panels make least, and the calculator works it out for your bill.
Install costs: DESNZ Boiler Upgrade Scheme statistics (August 2026), median cost of real installs by size: about £11,500 (4-6kW), £12,200 (6-8kW) and £13,900 (10-12kW). New gas boiler about £2,750 (our estimate). The £7,500 grant is the Boiler Upgrade Scheme (gov.uk) for England and Wales; £9,000 for oil or LPG heating with no mains gas connection until March 2027, which this calculator applies when you choose oil or LPG (if you also have a mains gas connection, take that extra off).
Estimates only. Your actual saving depends on the installer, flow temperatures, your home's heat loss and your electricity tariff. Always get a heat loss survey from an MCS installer for an accurate quote.
Semi-detached houses suit heat pumps well. You've got one party wall (less heat loss than a detached house), often decent outdoor space for the unit, and insulation that may already have been done (check before anyone quotes).
Heat Loss: More Than a Terrace, Less Than Detached
A semi sits in the middle of the heat loss spectrum. With one shared party wall you lose less heat than a detached house, but more than a mid-terrace with neighbours on both sides. We work on 9,500 to 10,300 kWh of heat a year for a three-bed home, heating and hot water together. That's DESNZ's median and mean gas use for 3 bedrooms in England and Wales, put through a 90% boiler, and it's the figure every running cost on this site uses. The median semi of any size uses 10,574 kWh of gas. Those are averages, not a measurement of your house. Only a room-by-room heat loss calculation settles it, and an MCS installer has to do one to size the heat pump.
What Size Heat Pump Does a Semi Need?
Probably smaller than you'd expect. We ran a 90 sq m 1950s semi through the same heat loss model as our size calculator (standard RdSAP assumptions, a -5.1C design day in the Midlands):
| The semi | Heat loss on the coldest day | Heat pump size |
|---|---|---|
| Cavity walls filled, 270mm in the loft | 4.5kW | 5kW |
| Cavity walls not filled, 100mm in the loft | 6.3kW | 7kW |
Bigger, older semis run higher, but if a quote for an ordinary 3-bed semi comes in at 9 or 10kW, ask to see the heat loss calculation behind it. An oversized heat pump can cycle on and off rather than run steadily, which costs efficiency. Put your own house in here:
What Size Heat Pump Do I Need?
Describe your home and we'll estimate its heat loss on the coldest design day, using the same default values an EPC assessor uses and the design temperatures MCS installers must use.
Estimated peak heat loss
6.3kW
At -5.1C outside, 20C inside
Heat pump size to look at
7kW
Smallest common size at or above your heat loss
Heat loss per sq m
70W
Lower is easier for a heat pump
What air source heat pumps of 6kW to 8kW actually cost
£12,164
median installed price, before the grant
£4,664
after the £7,500 Boiler Upgrade Scheme grant
Median of 8,054 real installations in that size band in 2025/26. Source: DESNZ Boiler Upgrade Scheme statistics, August 2026 (installations with a paid voucher, England and Wales). The grant applies in England and Wales.
Where the heat goes
What would shrink the heat pump
- Fill the cavity walls: -1.5kW
- Replace the windows with modern double glazing: -0.6kW
- Top the loft up to 270mm: -0.3kW
An estimate, not a design. MCS installers must size a heat pump to cover at least 100% of a room-by-room heat load calculation to BS EN 12831-1:2017 (MCS standard MIS 3005-D), and that survey replaces this number. Our defaults: U-values from BRE's RdSAP 10 specification (June 2026), window area from RdSAP 2012 Table S4, design outdoor temperatures from MIS 3005-D Table 2 column B, a whole-house 20C indoors, a rectangular plan with 2.5m storeys, no thermal bridging allowance, and air leakage from SAP 10.2 with a minimum of 0.5 air changes an hour. Solid floors use the design temperature rather than the annual average MCS specifies, so that part errs high. Flats and homes with extensions or room-in-roof conversions will be less accurate.
Insulation: What Do You Actually Need?
You don't need any insulation to get the grant: the Boiler Upgrade Scheme dropped its insulation condition on 8 May 2024, and since 28 April 2026 you don't need a valid EPC either. You do want it where it's cheap, because it lets the heat pump run cooler and cheaper. Here's how to think about it.
If the cavity walls are filled and the loft is insulated
Good news: you may not need to do anything before installation beyond checking your radiators. A newer or recently updated semi may already be in reasonable shape. Ask your installer to confirm it with the heat loss survey rather than assuming.
If the cavity walls are unfilled
Get them filled, and do it before the heat pump goes in, because it changes the size you need. The Energy Saving Trust puts it at around £2,200 for a typical home in Great Britain (around £1,000 in Northern Ireland). As for what it saves, DESNZ measured a median 10% cut in gas use (1,064 kWh) in semis that had the cavity filled mid-May 2023 to mid-May 2024, against similar homes with no measure recorded. The spread is wide, though. Our 1930s semi guide goes through the figures.
If your loft insulation is thin or missing
Take it to 270mm, but check what's actually up there before you book anyone. The Energy Saving Trust puts a bare loft taken to 270mm at £750 in Great Britain (it used to publish a saving of about £190 a year on a semi). If you already have 120mm, it's a very different proposition: topping up costs about £600 and saves about £17 a year (the Energy Saving Trust's figures for a typical home). DESNZ's measured median for loft insulation fitted in semis mid-May 2023 to mid-May 2024 was a 0.9% cut in gas use.
If the walls are solid (older semis, some 1900 to 1920s stock)
You're in solid-wall territory, like a Victorian house. Internal or external wall insulation is expensive and disruptive, so weigh it carefully. See our heat pumps in old houses guide for the full picture.
The Outdoor Unit: Side of the House Is Usually Ideal
If you have side access, a gate and a passage down to the back garden, it's a great spot for the outdoor unit: out of sight from the front, sheltered, easy to service and close to the house for short pipe runs. If you don't have side access, the back garden works just as well.
In England an air source heat pump on a house is permitted development, so you won't need a planning application, as long as:
- it's the only one on the house
- the unit is no bigger than 1.5 cubic metres
- it isn't on a pitched roof, or above the ground floor on a wall facing the road
- it meets the MCS 020 a) noise limit of 37 dB at your neighbour's window
- the house isn't listed
A conservation area doesn't rule it out, but there the unit can't go on a wall or roof facing the road, or anywhere closer to the road than the house itself. On a corner plot that can rule out the side passage. Our heat pump noise guide shows how the noise check works.
Radiators: Do a Proper Check
Heat pumps run at lower flow temperatures than gas boilers: 35C to 55C, against the 70C to 80C most radiators were sized for. At those temperatures some radiators are too small, and your installer should work it out room by room.
Our estimate for a typical semi is 5 to 7 radiators upgraded, £750 to £2,800. Insulating first can cut that, because each room needs less heat. If you're planning a kitchen or bathroom refit anyway, underfloor heating in those rooms suits a heat pump really well. Our radiators guide has a checker for each room.
Cost Breakdown by Insulation Status
Heat pump costs are the DESNZ median for the size the semi needs (Boiler Upgrade Scheme statistics, 2025/26 installs by capacity). Insulation is the Energy Saving Trust's figure, and radiators are our estimate. Treat the radiators as a possible extra: DESNZ's costs are installers' own reported figures for the system, labour and VAT, and DESNZ doesn't say whether radiator work is in them. It also notes installers may differ on whether the figure includes the grant. Real quotes spread either side of the median.
| Insulation status | Insulation | Heat pump | Total before grant | After the £7,500 grant |
|---|---|---|---|---|
| Already insulated (cavity filled, 270mm loft) | £0 | £11,500 (5kW) | £11,500 | £4,000 |
| Fill the cavity and top up the loft first | £2,800 | £11,500 (5kW) | £14,300 | £6,800 |
| Insulate first, plus radiator upgrades | £2,800 | £11,500 (5kW) plus £750 to £2,800 radiators | £15,050 to £17,100 | £7,550 to £9,600 |
| Skip the insulation, size up instead | £0 | £12,200 (7kW) | £12,200 | £4,700 |
Skipping the insulation is about £2,100 cheaper upfront. The catch is that you pay for the extra heat loss every winter, and some radiators may need to be bigger to match. DESNZ's measured median for a filled cavity in a semi is worth about £70 a year of heat pump electricity on a standard tariff at the 1 October to 31 December 2026 cap. The savings calculator above uses £12,200, the 6 to 8kW median, as its typical 3-bed install. If your semi is already insulated, your real figure is likely nearer £11,500.
Running Costs
At the 1 October to 31 December 2026 cap (26.32p a kWh for electricity, 7.97p for gas) and a SCOP of 3.5, on 9,500 to 10,300 kWh of heat:
- Heat pump, standard tariff: £710 to £770 a year
- Heat pump, heat pump tariff: £540 to £590 a year
- 90% gas boiler: £840 to £910 a year
So the heat pump is £130 to £140 a year cheaper on a standard tariff and £300 to £320 on a heat pump tariff. Those are fuel costs only. If the heat pump means you have the gas supply disconnected altogether, you also stop paying about £108 a year of gas standing charge.
We used to say these were level pegging. On the April 2026 cap gas was in fact cheaper (6.38p a kWh of heat against 7.05p from a heat pump); the July cap tipped it the other way (heat pump 7.46p, gas 8.14p). The October 2026 cap widened the gap: gas up 8.7% while electricity rose 0.8% and lost its VAT until 31 March 2027. A heat pump now beats a 90% gas boiler at any SCOP above 2.97.
A heat pump tariff is where the bigger saving is. Cosy Octopus, the one built for heat pumps, has three cheap windows a day (04:00 to 07:00, 13:00 to 16:00, 22:00 to midnight) at about half the day rate, and a 16:00 to 19:00 peak about 50% above it. Octopus prices it by region, so check the rates for your area. Our heat pump tariff guide compares it with the others.
We'd fit one when the boiler is dying anyway, and go on a heat pump tariff. As an investment on its own the sum is still slow, but at that point you're comparing the heat pump against the £2,750 you'd spend on a new combi, not against nothing. The insulated semi's 5kW heat pump costs about £1,250 more than that, which pays back in about 5 years on a heat pump tariff, servicing included. On a standard tariff it's about 9 to 10 years on fuel alone, or 18 to 21 counting a heat pump's dearer servicing (about £70 a year).
| The semi | Extra over a new combi | Heat pump tariff, with servicing | Standard tariff, fuel alone | Standard tariff, with servicing |
|---|---|---|---|---|
| Insulated, 5kW | £1,250 | about 5 years | 9 to 10 years | 18 to 21 years |
| Needing 6 to 8kW (our typical 3-bed) | £1,950 | about 8 years | 14 to 15 years | Past the heat pump's life |
The calculator above shows a shorter payback because it also counts the gas standing charge you stop paying.
Is Your Semi a Good Candidate?
Ask yourself:
- Are your cavity walls filled? (Look for the drill holes in the mortar, or ask your installer to check.)
- Is your loft insulation at 270mm or more?
- Have you got side or rear garden space for the outdoor unit?
- Is your boiler more than 10 years old?
If you said yes to most of those, you're in good shape: a small heat pump, few radiator changes, and a grant that covers most of the gap to a new boiler.
Ready to fit one in your semi?
Get free quotes from MCS-certified heat pump installers near you.
More: How the £7,500 heat pump grant works | Heat pump running costs | Do you need new radiators for a heat pump?
Frequently Asked Questions
A solid match for a heat pump
Predictable costs, modest prep work, and the BUS grant takes £7,500 off the bill. Get free quotes from MCS-certified installers in 60 seconds.