Free Eco Home Calculators

Boiler Replacement Calculator

1 min read Last updated 20 May 2026
Quick answer Replacing a 20-year-old non-condensing boiler (around 60-65% efficient) with a modern A-rated condensing model (94% efficient) saves roughly £200-£350 a year on gas at 2026 rates. Install cost is typically £2,500-£3,500 for a like-for-like swap, giving a 7-12 year payback.

The Boiler Replacement Calculator estimates how much you could save each year by upgrading from an old non-condensing or back-boiler to a modern A-rated condensing model. It also recommends a kW output based on bedrooms and bathrooms, and gives a payback period at 2026 gas rates.

Boiler Replacement Calculator

Calculate potential savings from upgrading your boiler.

See 20 real-world examples

Each link below runs this calculator with real UK city and property inputs and publishes the exact computed savings, payback time, install cost and CO2 figures.

Browse all calculator examples →

How this calculator works

We start with the manufacturer's rated efficiency for each boiler type (non-condensing 72%, back-boiler 55%, condensing 88%) and apply an age penalty of 0.6% per year of degradation. New A-rated condensing boilers run at 94% on the SEDBUK scale. Annual gas demand is estimated from property type and bedroom count. Cost is gas demand ÷ efficiency × 7.2p per kWh. Recommended output uses standard British Gas / Worcester sizing tables: 24 kW for small homes, 28-35 kW for larger or multi-bathroom properties.

Sources & references

Frequently asked questions

How long does a new boiler last?

A modern condensing combi boiler is typically warrantied for 7-10 years and has a working life of 12-15 years if regularly serviced. Premium models from Vaillant, Viessmann and Worcester Bosch often last longer.

Combi or system boiler — which is best?

Combi boilers heat water on demand and don't need a hot water cylinder, so they're cheaper to install and save loft space. They struggle in homes with two or more bathrooms used simultaneously. System boilers (with a stored cylinder) cope better with multi-bathroom demand.

What size boiler do I need?

A useful rule of thumb: 24 kW for 1-2 bed flats, 28-30 kW for 3-bed family homes, 35 kW for 4+ bed homes or multi-bathroom properties. The calculator uses this rule but a Gas Safe engineer should verify with a heat-loss calculation.

Should I get a heat pump instead?

If you plan to live in the property for 10+ years, a heat pump combined with the £7,500 BUS grant often beats a new boiler on lifetime cost — and aligns with the 2035 ban on new gas boilers in new-build homes. Run our Heat Pump Sizing Calculator for a comparison.

Are there grants for new boilers?

Not usually for like-for-like gas-boiler swaps. Low-income households can sometimes get grants under ECO4 if their existing boiler is broken and they receive qualifying benefits. The Boiler Upgrade Scheme covers heat pumps, not gas boilers.

Related guides

Tool: Boiler Replacement Calculator. Last reviewed: . Figures based on Ofgem price cap, gov.uk grant guidance, and Energy Saving Trust advice. Verify scheme eligibility with your installer before commissioning work.