The Short Answer
Check the walls first. A 1930s semi probably has cavity walls, and if they're empty we'd fill them, ideally with ECO4 or the Warm Homes: Local Grant paying. Homes built from 1919 to 1944 have the highest median gas use of any age band in England and Wales, and filling the cavity cut measured gas use by a median 11% in homes of that age.
On our model, our example 1930s semi needs a 7kW heat pump as it is, or 5kW with the cavity filled and the loft topped up to 270mm. The DESNZ medians for those sizes are about £12,200 and £11,500 installed, before the £7,500 Boiler Upgrade Scheme grant.
Estimate the Saving for a 1930s Semi
Pre-set for a 1930s semi on gas, using the median gas use of homes built 1919 to 1944 in England and Wales (£1,050 bill). If your cavity walls aren't filled, ECO4 can help with the cost for some households on benefits until 31 December 2026.
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
£442a year
Over 15 years that's around £6,630 off your heating bills.
Now: Gas boiler
£1,050 / year
With heat pump
£608 / year
You save
£442 / 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 4.4 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 £442 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.
Heat Pumps in 1930s Semis: What You're Working With
DESNZ's National Energy Efficiency Data-Framework puts the median gas use of homes built 1919 to 1944 at 11,745 kWh a year, the highest of any age band, above homes built before 1919 (11,007 kWh) and against 10,012 kWh for all homes. The median semi uses 10,574 kWh. Those are weather-corrected figures for mid-May 2024 to mid-May 2025, every house type of that age together, and they include cooking and hot water.
Through a 90% boiler, 11,745 kWh of gas is about 10,600 kWh of heat a year. That's our estimate for a 1930s semi, and the figure the calculator above starts from. Your own bills are a better guide: a year's gas in kWh, times 0.9, is roughly your heat.
What Sets a 1930s Semi Apart
The Energy Saving Trust says that if your house was built after the 1920s, its external walls are probably cavity walls: two layers of masonry with a gap between. Houses built before then are more likely to have solid walls, and in general houses built from the 1990s onwards already have wall insulation; older ones might not. So a 1930s semi is likely to have cavity walls, and they may never have been filled.
To tell, the Energy Saving Trust suggests looking at the brickwork: cavity walls usually show an even pattern with every brick laid lengthways, while solid walls alternate, with some bricks laid across so you see their ends. A brick wall 260mm to 350mm thick (measure it at a door or window) is likely to be a cavity wall. If the walls are rendered, ask a building professional.
It matters because, according to the Energy Saving Trust, about a third of the heat lost from an uninsulated home goes through the walls. In our model, filling the cavity cuts the heat lost through the walls by about 53%, on the standard figures EPC assessors use for a house of this age (a U-value of 1.5 down to 0.7).
What the Numbers Look Like Before and After Insulation
We ran our example 1930s semi (3 bedrooms, 90 sq m, double glazing fitted before 2002) through the same heat loss model as our size calculator, at a -5.1C design day in the Midlands:
| The 1930s semi | Heat loss on the coldest day | Heat pump size | DESNZ median for that size band |
|---|---|---|---|
| As it is: cavity not filled, 50mm in the loft | 6.6kW | 7kW | £12,200 |
| Loft topped up to 270mm | 6.1kW | 7kW | £12,200 |
| Cavity filled | 5.1kW | 6kW | £12,200 |
| Cavity filled and loft to 270mm | 4.5kW | 5kW | £11,500 |
| As it is, with one open chimney | 7.1kW | 8.5kW | £12,700 |
Filling the cavity and topping up the loft take the heat loss down by about 32%. Look at the last column, though: in the DESNZ figures a smaller heat pump doesn't cost much less, so insulating first saves little on the heat pump itself. The case for it is the running costs, and how warm the house feels.
Cavity Wall Insulation
The Energy Saving Trust puts filling a cavity at around £2,200 for a typical home in Great Britain (around £1,000 in Northern Ireland). It says it may not suit a home that's exposed to extreme weather or flooding, in poor condition, at risk of rooms overheating, or with cavities narrower than 50mm, and your installer should check those first.
What it saves in practice comes from DESNZ. It compares the gas use of homes that had one measure installed with similar homes that had no measure recorded in the year before, during or after. For cavity walls filled mid-May 2023 to mid-May 2024, the median saving in the first year was 11% (1,276 kWh) in homes built 1919 to 1944, across 357 homes, and 10% in semis.
The spread is wide: in at least a quarter of homes, gas use rose against the comparison homes. DESNZ says its savings by type of home vary a lot between years and show no systematic differences between types. These are metered savings, so they include people heating their homes more once they're warmer, and DESNZ found the savings from cavity walls held up over five years. Its analysis mainly uses measures fitted under government schemes, and around 80% of those in that period came through the Energy Company Obligation, which helps households on certain benefits and in social housing.
Loft Insulation
The Energy Saving Trust puts insulating an empty loft to 270mm at £750 for a typical home in Great Britain, and topping up from 120mm at £600. DESNZ's measured saving was much smaller than for the walls: a median 3% (310 kWh) in homes built 1919 to 1944, and 0.9% in semis. It also found loft savings about a tenth lower five years on. In our model the loft alone takes the heat loss from 6.6kW to 6.1kW, where the cavity alone takes it to 5.1kW.
An Open Chimney
If the house still has an open chimney, our model puts the heat loss at 7.1kW, against 6.6kW with it closed, which is enough to move it up a size, to 8.5kW instead of 7kW.
Sizing the Heat Pump
On the model, our example needs a 7kW heat pump as it is, 6kW with just the cavity filled, and 5kW with the loft done too. Only a room-by-room heat loss calculation settles it for your house, and an MCS installer has to do one to size the heat pump. If a quote for an ordinary 3-bed 1930s semi comes in much bigger, ask to see that calculation. Put your own house in here (it starts from different settings to our example, so set the age, walls and loft to match yours):
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.
Radiators: Some May Need Upsizing
Heat pumps run at lower flow temperatures than gas boilers: 35C to 55C, against the 70C to 80C a gas boiler supplies. At those temperatures some radiators are too small, and your installer should work it out room by room. Insulating first means each room needs less heat, so fewer radiators fall short.
Our estimate for a typical semi is 5 to 7 radiators upgraded, £750 to £2,800. Our radiators guide has a checker for each room.
The Full Cost Breakdown
Heat pump costs are the DESNZ median for the size our example needs (Boiler Upgrade Scheme statistics, 2025/26 installs by capacity), insulation is the Energy Saving Trust's, and radiators are our estimate. Real quotes spread either side of the median.
| The job | Before grant | After the £7,500 grant |
|---|---|---|
| Heat pump as the house is (7kW) | £12,200 | £4,700 |
| Fill the cavity and do the loft first, then a 5kW heat pump | £14,300 to £14,450 | £6,800 to £6,950 |
| Radiator upgrades, if needed (our estimate) | £750 to £2,800 | £750 to £2,800 |
Treat the radiator row as a possible extra rather than a certain one. 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. The insulation range runs from the Energy Saving Trust's loft top-up from 120mm (£600) to an empty loft (£750), plus the cavity. Our example has 50mm, which the EST doesn't price, so expect nearer the top. Insulating first costs about £2,100 to £2,250 more upfront, unless a scheme below pays for it.
Help With the Cost
- Boiler Upgrade Scheme: £7,500 towards an air source heat pump in England and Wales, if you own the house and you're replacing gas, oil, electric or LPG heating. There's no insulation or EPC condition.
- ECO4 (Great Britain): your energy supplier may help with the cost of insulation, cavity walls and lofts included, if you get certain benefits and own your home (rated D, E, F or G on its EPC) or rent privately (E, F or G, with the owner's permission), or if you live in social housing rated E, F or G. It runs until 31 December 2026.
- Warm Homes: Local Grant (England only): if you're eligible and your council has funding, it arranges and pays for the work, which can include wall and loft insulation and an air source heat pump. It's for homes rated D, E, F or G on their EPC, and the household income limit is usually £36,000 a year.
Our heat pump grants guide covers Scotland, Wales and Northern Ireland.
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 our 10,600 kWh of heat:
- 90% gas boiler: about £940 a year
- Heat pump, standard tariff: about £800 a year
- Heat pump, heat pump tariff: about £610 a year, at our estimate of a 20p blended rate
Those are fuel costs only; if you have the gas supply disconnected altogether, you also stop paying about £108 a year of gas standing charge. The savings calculator above counts that standing charge in your gas bill, which is why its saving is bigger than these figures.
If filling the cavity saves the DESNZ median for a house of this age, the heat falls to about 9,500 kWh: about £840 a year on gas, or £710 with a heat pump on a standard tariff. That's about £100 a year on gas, so a £2,200 cavity fill takes about 22 years to pay for itself on the median saving alone. With a heat pump on a standard tariff the same heat is worth about £90 a year, so about 25 years.
It's a much better deal if a scheme pays for it, or if your house is one of the half that saves more than the median.
Where the Outdoor Unit Goes
In England a heat pump on a semi is permitted development, so you won't need a planning application, if:
- it's the only one on the house
- the outdoor unit is no bigger than 1.5 cubic metres
- it meets the 37 dB noise limit at your neighbour's window
- it isn't on a pitched roof, or above the ground floor on a wall facing the road
- the house isn't listed
In a conservation area or World Heritage Site it can't go on a wall or roof facing the road at all, or anywhere closer to the road than the house. Our semi-detached house guide goes through the rules and where the unit can go.
Best Bang for Buck
The order we'd do it in:
- Find out whether you have cavity walls, and whether they're filled. Check the brickwork and the wall thickness, and ask an insulation installer to look if you're not sure.
- If the cavity is empty, see whether ECO4 or the Warm Homes: Local Grant will pay. Free, it's an easy yes. Paying yourself, it's slower to pay back than you might expect, so weigh how cold the house feels as well as the bill.
- Top up a thin loft while you're at it. It's cheap, though DESNZ's measured savings for lofts are small.
- Get the heat pump when the boiler is due. If your boiler is more than 10 years old, start getting quotes, and ask each installer for the heat loss calculation and which radiators they'd change.
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More: Heat pumps in semi-detached houses | Heat pumps in old houses | How the £7,500 heat pump grant works
Frequently Asked Questions
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