How efficient is my boiler, really?

Every boiler sold since 2015 carries an ErP energy label, A to G. A modern condensing boiler is A-rated and around 92% efficient. A twenty-year-old non-condensing one might be 60% to 70%.

That gap is the whole argument for replacement, so it is worth understanding it properly.

What the percentage means

70% efficient means that for every £100 of gas you buy, £30 goes out of the flue as heat you never see. At current prices, on a typical detached home, that is real money every single winter.

92% means £8 of every £100. Energy Saving Trust currently puts the saving from replacing a G-rated gas boiler that has no heating controls with an A-rated boiler and a full set of controls at around £420 a year in Great Britain. Read that basis carefully, because it is doing a lot of work: some of the saving is the controls rather than the boiler, so if you already have a programmer, a room thermostat and thermostatic radiator valves you should expect less. It is a market figure, not a quote, and your number depends on your house, your usage and your tariff.

Source: Energy Saving Trust, How to improve your EPC rating

How to find yours

Look for the manufacturer's plate on the boiler, usually behind the front panel or on the underside. Note the make and model and search for its ErP rating.

Rough rule: pre-2005 and non-condensing is likely D or worse. 2005 to 2010 condensing is likely B or C. Post-2015 is almost certainly A.

Why your real efficiency is lower than the badge

The lab figure assumes the boiler is condensing properly, which requires return water temperatures under about 55C. Most UK systems run their flow temperature far too high, which means the boiler never condenses and never reaches its rated efficiency.

Turning the FLOW temperature down to 60C (not the room thermostat - the flow dial on the boiler) can lift real-world efficiency by several percent for free. It is the cheapest efficiency gain available to almost every household in the country, and almost nobody does it.

When efficiency justifies replacement

Going from 90% to 94% saves you almost nothing and will never pay back. Going from 65% to 92% is a different conversation entirely. If your boiler is pre-2005 and non-condensing, the fuel you are wasting is a substantial part of the cost of replacing it.