Working file C2
Deep retrofit versus heat pump only (UK 2026)
Side-by-side capex, net-after-grants and the fuel-switch running-cost change for an ASHP-only project versus a phased PAS 2035 deep retrofit. Ofgem cap for 1 October to 31 December 2026 baked in.
Live calculator
Live calculator
Q4 2026 cap baked inTotal capex
£36,085 to £63,912
Grant offset
£7,500
Net after grants
£28,585 to £56,412
Annual running-cost change
£101
Heat pump leg, line by line
- Gas standing charge dropped
- £108 / yr
- Heat delivered by the boiler
- 8,443 kWh
- Electricity a heat pump needs
- 3,037 kWh
- Worst case (boiler 92%, SPF 2.55)
- minus £43 / yr
- Best case (boiler 76%, SPF 3.05)
- £249 / yr
- Heat pump leg net of the BUS grant
- £2,300 to £11,000
- Years for running-cost savings to repay that
- 66 yr
Live math, coefficients on /sources/. Gas standing charge comes off the bill first, the rest buys gas at 7.97p, the boiler turns that into heat at the efficiency you set, and the heat pump buys the same heat as electricity at 26.32p divided by the SPF you set. At this cap gas at 85% costs 9.38p per kWh of heat and a heat pump at SPF 2.78 costs 9.47p, which is why the standing charge does most of the work. Payback is quoted on the heat-pump leg alone: the fabric, glazing and solar legs all cut demand too, and this file does not model that, so it publishes no whole-project payback.
The two quotes
Quote A is an ASHP and a radiator upsize. Quote B is a phased fabric-first programme with ventilation, heating, renewables and controls drawn out across a five to ten year medium-term improvement plan. The £-delta is real and worth resolving in writing before either quote is signed.
When heat-pump-only is defensible
- Post-2000 build with U-values already in the 0.18 to 0.28 W/m²K band.
- Cavity-filled 1930s semi where loft is already at 300mm and floors are uncertain.
- Owner-occupier with no realistic five-year horizon to phase a deeper programme.
When deep retrofit pays back
- Pre-1919 solid wall where IWI plus glazing changes the delivered SCOP of the heat pump.
- System-built or no-fines walls where fabric work is a precondition for an MCS-compliant heat-pump design.
- Households on enhanced PTC or LA Flex routes where ECO4 or GBIS strip a real layer off the net cost.
Payback ribbonYears to break even, at the Q4 2026 Ofgem cap.
runs past 20 yr →
ECO4 cliff
BUS end (2030)
0 yr5 yr10 yr15 yr20 yr
Heat-pump leg only: sixteen years at the most favourable end of the published evidence, forty to sixty-seven at the national trial's median SPF, and no break-even where the delivered SPF lands in the bottom quartile. No whole-project figure is published for Quote B, because this file does not model the demand the fabric and solar legs remove.
Surveyed 1 October 2026. Ofgem cap for 1 October to 31 December 2026 applied (gas 7.97p, elec 26.32p, gas standing 29.68p, elec standing 54.83p). Next scheduled review 25 November 2026 (January cap announcement). [ref C2]
By Oliver Wakefield-Smith, Founder, Digital Signet. Corrections to oliver at digitalsignet dot com.