Boiler efficiency explained in one sentence: it is the share of the energy in the gas that ends up as heat in your home. A boiler rated at 90 percent turns 90 kWh of every 100 kWh of gas into useful heat and loses the other 10 kWh up the flue and through the casing. The number matters because a lower one means you buy more gas for the same warmth.
In this guide
From efficiency to gas use
Gas needed is the heat you want divided by the efficiency.
Gas cost = gas needed × gas price
A home that needs 12,000 kWh of heat a year uses 13,333 kWh of gas with a 90 percent boiler. At 0.08 per kWh, that costs 1,067. The same home with a 75 percent boiler uses 16,000 kWh and pays 1,280. The gap is 213 a year for the same warmth, and it grows when gas gets dearer.
What the upgrade is worth
If a replacement boiler costs 2,500 and saves 213 a year, the simple payback is 11.7 years. That is a slow return, which is why a boiler replacement is usually justified by age and reliability and not by savings alone. The result moves with your prices. At a gas price of 0.12 the saving is 320 a year and payback falls to 7.8 years. Run your own numbers with the heat pump vs gas boiler calculator, which also shows what a heat pump would do.
Why the label and real life differ
A condensing boiler recovers extra heat by cooling the flue gases until the water vapour in them condenses. That only happens when the water returning to the boiler is cool enough, typically below about 55°C. If the system runs hotter than that, the boiler stops condensing and its efficiency drops towards the level of an older model.
Many homes run their heating at a flow temperature that is higher than necessary. Lowering it, by turning down the boiler's flow temperature setting, often brings the boiler back into condensing mode. The change makes rooms warm up more slowly, so check comfort over a few days and adjust. Weather compensation, which lowers the flow temperature as it gets milder outside, does this automatically.
Other factors that reduce real-life efficiency include short cycling, where an oversized boiler switches on and off often, poor servicing, scale in the heat exchanger and badly balanced radiators.
Finding your boiler's efficiency
Look for the energy label or the model name and search the manufacturer's specification. In Europe, the label gives a seasonal efficiency rating on an A to G scale, and the data sheet gives the percentage. Real-life figures are lower than test figures by a few points. For a quick estimate, use 85 to 90 percent for a modern condensing boiler, and 70 to 80 percent for an older non-condensing one.
You can also test your own boiler. Over a year, divide the heat you think you used by the gas you bought. This is hard to do accurately, because heat use is unknown, so treat the label as the starting point.
Heat demand
The heat your home needs does not depend on the boiler. It depends on the size of the home, its insulation and the weather. The gas bill shows the gas used, so multiply it by the boiler efficiency to find the heat demand. If your bills show cubic metres, use the gas m³ to kWh calculator first. A home that used 13,300 kWh with a 90 percent boiler needed about 12,000 kWh of heat.
Cutting heat demand with insulation and draught-proofing saves gas at any efficiency, and it makes a future heat pump cheaper to run because the pump can be smaller.
Efficiency numbers above 100 percent
A heat pump can deliver more than 100 percent of the electricity it uses, because it moves heat from outside instead of making it. The comparable measure is the seasonal coefficient of performance, which is usually between 2.5 and 4. The guide to SCOP explains it. Electricity costs more per kWh than gas, so a SCOP of 3 does not mean the heat is three times cheaper.
Frequently asked questions
Is a 90 percent efficient boiler good?
Yes for a gas boiler. Condensing models reach the high eighties and low nineties in real use when the system is set up well. Higher figures on brochures often reflect ideal conditions.
Does turning the flow temperature down save gas?
Often, if it brings the boiler into condensing mode. The effect depends on your radiators, so test it in mild weather first.
Should I replace a working boiler to save money?
Rarely on savings alone. Replace it when it is old, unreliable or unsafe, and choose the replacement with the efficiency of the whole heating system in mind.