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Old boiler running cost estimator
An old boiler can easily burn £300–£500 of extra gas per year compared to a modern A-rated one. Work out your own saving below, based on 2026 UK gas prices.
Estimated annual saving with a new A-rated boiler
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Based on Ofgem 2026 price cap and typical household gas use. Real savings vary with insulation, thermostat use, and hot water demand.
Why the difference is so big
Old non-condensing boilers lose heat up the flue. Modern condensing boilers capture that heat from the exhaust and re-use it — that's where the 20%+ efficiency gain comes from. On a pre-2005 boiler you're roughly paying for 4 cubic metres of gas to get 2.6 cubic metres of heat into your home. A new boiler gets 3.7 cubic metres into your home from the same 4.