With the UK government's £7,500 Boiler Upgrade Scheme grant running and gas boiler bans on new homes already in place, the heat pump conversation is unavoidable. Here's our honest engineer's take on when to switch and when to wait.
The cost comparison: like-for-like
Gas boiler replacement (2026) - New combi boiler (A-rated): £2,500–£4,500 fitted - Annual gas bill for heating + hot water (3-bed semi): £1,200–£1,800/year - Expected lifespan: 12–15 years - Carbon: ~2.1 tonnes CO₂/year
Air source heat pump (ASHP) + BUS grant - Gross cost: £12,000–£17,000 fitted - BUS grant deduction: -£7,500 - Net cost to you: £4,500–£9,500 - Annual running cost on Octopus Cosy: £800–£1,100/year (same home) - Expected lifespan: 20–25 years - Carbon: 0.3–0.6 tonnes CO₂/year
At those running costs, the ASHP saves £400–£700/year vs a new gas boiler. With the BUS grant, payback on the extra upfront cost is 6–10 years.
The insulation reality
Heat pumps work best in well-insulated homes. A 1970s semi with inadequate loft insulation and old single-glazing will see higher running costs from a heat pump than a modern boiler.
The rule of thumb: if your home is EPC C or above, a heat pump will likely run efficiently. If it's D, E, F, or G — upgrade insulation first (often fundable via GBIS or ECO4), then fit the heat pump.
We won't fit an ASHP in a poorly insulated home without warning you of the running cost risk. Some installers skip this conversation. We don't.
The new radiator question
Heat pumps run at 35–45°C flow temperature, not 70–80°C like a boiler. If your radiators are undersized for lower flow temperature, they'll struggle to heat rooms.
In practice: about 40% of homes we survey need at least some radiator upsizing (£100–£200 per radiator). We budget £500–£1,500 for this in most quotes. A full underfloor heating install is ideal but expensive (£4,000–£12,000 extra).
Heat pumps with solar panels
Combining ASHP with solar is one of the most compelling home energy setups available:
- Solar generates for free during the day, heating your home in the afternoon cheap window
- Octopus Cosy tariff aligns cheap-rate hours (13:00–16:00) with peak solar generation
- On a sunny afternoon in May, a heat pump running from solar costs effectively zero to run
We've fitted 60+ solar + heat pump combos. The combined savings typically run £1,800–£2,500/year vs using grid electricity for both heating and daytime power.
When to keep the boiler
Don't switch if: - Your home has EPC E, F, or G and you can't fund insulation improvements first - You're in a flat with no outside wall or garden for the ASHP unit - Your gas supply is through a communal heat network (different considerations) - You're planning a major extension or reconfiguration in the next 2 years
Do switch if: - You're replacing a boiler that's already failed or nearing end of life - You heat with oil, LPG, or electric storage heaters (even better economics) - Your home is EPC C or above - You're fitting solar simultaneously (ideal combo)
The gas ban reality
New-build homes can't have gas boilers from 2025 under the Future Homes Standard. From 2035 (current UK policy), new gas boiler sales are banned entirely. The writing is on the wall for gas heating long-term — the question is timing.
For most homeowners, fitting an ASHP when your current boiler fails (rather than replacing like-for-like) is the financially rational decision once you factor in the £7,500 BUS grant.