Boiler efficiency ratings explained

The short answer

Boiler efficiency ratings tell you how much of the gas you pay for becomes heat in your home rather than going out of the flue. In the UK, new gas boilers carry an ErP energy label, and almost all of them are A rated for heating, with seasonal efficiency in the low 90s per cent. Because nearly every modern boiler lands in the same band, the label is a minimum standard more than a way to choose between models. What decides how efficient your boiler is in real life is whether it condenses, and that comes down to flow temperature, sizing and controls.

The two rating systems you will see

ErP stands for Energy-related Products. The label is calculated on a seasonal basis, which means it tries to reflect a year of real use rather than a single lab reading. A gas boiler on its own tops out at A. Higher grades such as A+ appear on a “package label” when the boiler is sold with certain controls or renewable kit, which is why you may see different letters for what seems to be the same boiler.

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) was the older system. You will still see it on boilers fitted before the ErP labels and on some manufacturer data sheets.

What the rules require

In England, the Boiler Plus standards set a minimum ErP efficiency of 92% for new gas boilers, and require time and temperature controls to be fitted. A combi installed in England must also come with one extra energy saving measure from a short list: weather compensation, load compensation, a smart thermostat with automation and optimisation, or flue gas heat recovery. Wales, Scotland and Northern Ireland have their own Building Regulations. Your installer has to meet whichever apply, so check the current rules on gov.uk if you want the detail.

What condensing means

Every new gas boiler is a condensing boiler. When gas burns, it produces water vapour. A condensing boiler cools its exhaust gases enough to turn that vapour back into water, and the heat released in the process goes back into your heating. The water drains away through the plastic condensate pipe you see running to a drain.

A condensing boiler only condenses when the water coming back from your radiators, called the return temperature, is cool enough. Above roughly 55°C, it stops condensing and behaves much more like an old non-condensing boiler. This is the part the label does not show you. Two identical A rated boilers can perform quite differently depending on how they are set up.

Flow temperature: the setting that matters

Flow temperature is the temperature of the water the boiler sends out to the radiators. Many boilers are left at a high factory or installer setting, often around 70°C to 80°C. At that level the return water is usually too hot for the boiler to condense well.

Turning the radiator flow temperature down, often to somewhere around 60°C or below on a combi, lets the boiler condense more of the time. The trade-offs:

1. Rooms take longer to heat up, so you may need the heating on a little earlier.
2. In a draughty house with small radiators, a low setting may not keep you warm on the coldest days.
3. On a system or conventional boiler with a hot water cylinder, the cylinder still needs to reach a safe temperature to control bacteria. Change only the radiator setting, or ask your engineer to set it up.

We are not going to give you a savings percentage, because it depends on your house, radiators and habits. The honest summary is that a correctly set condensing boiler usually uses less gas than the same boiler run hot, with no new equipment.

Ask your installer to set the flow temperature on commissioning day and explain how to change it. It costs nothing and it is the easiest efficiency gain you will get.

What makes a real difference

• Right size. An oversized boiler cycles on and off, which wastes energy and wears parts. See what size boiler do I need?
• Weather or load compensation. These controls lower the flow temperature automatically when it is mild outside or the house is nearly warm.
• Clean system water. Sludge and scale act as insulation on the heat exchanger. A flush and magnetic filter protect efficiency.
• Thermostatic radiator valves. They stop rooms overheating.

Is an old boiler worth replacing for efficiency alone?

If your boiler is non-condensing, it wastes noticeably more of your gas than a modern one, and replacement usually makes sense even before it fails. If it is a condensing boiler that is working well, efficiency alone is rarely a reason to replace it. Adjusting the flow temperature and controls may get you most of the way. Our guide to the signs your boiler needs replacing helps with that decision, and the boiler replacement hub shows typical fitted prices if you do go ahead.

If you are weighing a boiler against a heat pump, efficiency is measured differently there, as a ratio of heat out to electricity in. Our heat pump installation hub explains how the two compare.