Live Direct Debit rates for UK average since 1 October 2026. Same warmth; typical home.
Why old boilers cost more
Every boiler wastes some of its gas as heat up the flue. Old non-condensing boilers, common before about 2005, are often around 70% efficient or less. A modern condensing boiler reaches about 85% in real homes, according to a government field study.
That means an old boiler needs much more gas to give you the same warmth.
The numbers for a typical home
A typical home needs about 8,075 kWh of heat a year. Here's the gas each boiler needs to supply it:
| Boiler | Efficiency | Gas a year | Cost a year (gas only) |
|---|---|---|---|
| Old non-condensing boiler | about 70% | 11,536 kWh | £919 |
| Older condensing boiler | about 80% | 10,094 kWh | £804 |
| Modern condensing boiler | about 85% | 9,500 kWh | £757 |
The saving from replacing an old boiler is about £162 a year. A bigger, colder house saves more.
Does a new boiler pay for itself?
A new gas boiler typically costs a few thousand pounds fitted, so the savings alone usually take many years to pay it back. It makes most sense when:
- your boiler is old and breaking down often,
- it's a non-condensing model (it has no plastic pipe dripping water outside),
- or you're thinking about a heat pump instead, which could get a £7,500 grant. See the Boiler Upgrade Scheme.
Get more out of the boiler you have
- Lower the flow temperature.
- Get it serviced once a year.
- Bleed your radiators and fit thermostatic radiator valves.
Sources: Government in-situ boiler study, Ofgem typical consumption values, Octopus Energy tariff data.