Boiler Energy Efficiency Ratings
A boiler's efficiency rating tells you how much of the gas it burns is converted into useful heat. A 90%-efficient boiler means 90p of every £1 spent on gas heats your home; the other 10p is wasted.
The A–G Efficiency Scale
Highest efficiency. All modern condensing boilers should achieve this.
Good efficiency. Some older condensing boilers.
Average efficiency. Older boilers nearing replacement age.
Below average. Consider replacing soon.
Poor efficiency. Replacement recommended.
Very poor. Significant energy waste.
Worst efficiency. Replace immediately to save money.
How Much Could You Save?
Annual gas bill savings from upgrading to an A-rated boiler, based on a typical 3-bedroom semi-detached house:
| Upgrading From | Upgrading To | Annual Saving |
|---|---|---|
| G (below 70%) | A (90%+) | £200–£580/year |
| F (70–74%) | A (90%+) | £170–£440/year |
| E (74–78%) | A (90%+) | £130–£350/year |
| D (78–82%) | A (90%+) | £100–£275/year |
| C (82–86%) | A (90%+) | £60–£165/year |
Based on Energy Saving Trust estimates for a gas-heated, 3-bed semi-detached house. Your savings may vary.
What Is a Condensing Boiler?
Since 2005, all new gas boilers installed in the UK must be condensing boilers. These recover heat from exhaust gases that would otherwise be lost up the flue. This is what allows them to achieve 90%+ efficiency.
The "condensing" process produces water (condensate) that needs draining — this is why modern boilers require a condensate pipe leading to a drain. In cold weather, this pipe can freeze, which is the most common condensing boiler fault.
ErP vs SEDBUK Ratings
You may see two types of efficiency rating:
- ErP (Energy-related Products) — The current EU/UK standard. Uses A+++ to G scale. All new boilers should be A-rated or above.
- SEDBUK — The older UK standard (Seasonal Efficiency of Domestic Boilers in the UK). Shows a percentage figure. Useful for comparing older boilers.
Tips to Maximise Efficiency
- Annual servicing — A serviced boiler runs 3–5% more efficiently than a neglected one
- Smart thermostat — Can cut heating bills by 10–15% through optimised scheduling
- TRVs on radiators — Thermostatic radiator valves let you heat rooms individually
- Insulation — Even the most efficient boiler wastes money in a poorly insulated home
- System inhibitor — Prevents sludge buildup that reduces heat transfer
- Reduce flow temperature — Running your boiler at 60°C instead of 80°C can improve efficiency by 5–10%
Want to see how much a new efficient boiler costs?
Check our full cost guide with brand comparisons and finance options.