Why Boiler Efficiency Matters
Your boiler accounts for around 55β60% of your home's total energy bills. The efficiency of your boiler β how much of the fuel it burns actually converts into useful heat β has a direct and significant impact on what you pay to heat your home every year.
Replacing an old G-rated boiler (as little as 60β70% efficient) with a modern A-rated condensing boiler (90%+ efficient) can save a typical household hundreds of pounds per year on gas bills. Over a boiler's 15-year lifetime, that saving can comfortably exceed the cost of installation.
What Is SEDBUK?
SEDBUK stands for Seasonal Efficiency of Domestic Boilers in the UK. It's the primary efficiency rating system used for gas and oil boilers in the UK, expressed as a percentage. A SEDBUK rating tells you what percentage of the energy in the gas (or oil) your boiler burns is converted into useful heat over the course of a typical UK heating season.
SEDBUK ratings are divided into bands:
| SEDBUK Band | Efficiency Range | Typical Boilers |
|---|---|---|
| A | 90% and above | Modern condensing boilers (all new installations) |
| B | 86β90% | Older high-efficiency non-condensing boilers |
| C | 82β86% | Older efficient boilers |
| D | 78β82% | Mid-range older boilers |
| E | 74β78% | Older standard boilers |
| F | 70β74% | Older, less efficient boilers |
| G | Below 70% | Old boilers, cast iron systems |
Since April 2005, all new boiler installations in England and Wales must be condensing boilers β meaning all new boilers achieve at least an A-band SEDBUK rating. If your existing boiler is rated D, E, F, or G, you're wasting a significant proportion of the gas you're paying for every time your heating runs.
What Is the ErP Rating?
ErP stands for Energy-related Products β a European Union energy labelling framework that was introduced for boilers in 2015. The ErP label is the coloured A+++ to G label you now see on boilers, similar to the energy labels on white goods and appliances.
For boilers, the ErP rating is calculated using a seasonal efficiency measure called Ξ·s (eta-s), which takes account of standby losses, cycling losses, and ignition energy in addition to the raw combustion efficiency. The vast majority of modern condensing boilers achieve ErP A ratings.
ErP is the label you'll see displayed on boiler marketing materials and specification sheets. SEDBUK is the underlying calculation used by heating engineers when specifying and comparing boilers.
What Is a Condensing Boiler?
All modern gas boilers fitted in UK homes are condensing boilers. A condensing boiler achieves its high efficiency by recovering heat from the flue gases that would otherwise escape up the flue as waste.
In a conventional (non-condensing) boiler, flue gases leave the appliance at temperatures of 180β250Β°C. That's a lot of energy going up the flue and out of the house. A condensing boiler has a larger heat exchanger that cools the flue gases down to around 50β60Β°C, causing the water vapour in the gases to condense back into liquid. This phase change releases additional latent heat which is captured and transferred into the heating system β hence the term "condensing".
The condensate (slightly acidic water) produced by this process is drained away via a condensate pipe β typically to an internal drain or outside. In very cold weather, this pipe can freeze, which is a common boiler fault in winter.
What Efficiency Ratings Do Different Boiler Brands Achieve?
Worcester Bosch Greenstar
SEDBUK: 89β94% | ErP: A
The Greenstar 4000 and 8000 ranges achieve up to 94% SEDBUK efficiency. Class-leading in its price band.
Worcester Bosch Installation βVaillant ecoTEC
SEDBUK: 89β94% | ErP: A
The ecoTEC Plus and Exclusive achieve up to 94% SEDBUK. Quiet modulating burner optimises part-load efficiency.
Vaillant Installation βViessmann Vitodens
SEDBUK: 92β94% | ErP: A+
The Vitodens 200-W achieves up to 99% efficiency at partial load. Best-in-class for homes with variable heating demands.
Viessmann Installation βIntergas Combi Compact HRE
SEDBUK: 92β93% | ErP: A
Comparable efficiency to mainstream brands. Advantage is in reliability and lower lifetime running costs.
Intergas Installation βHow Much Can I Save With a New A-Rated Boiler?
The savings from upgrading to a new high-efficiency boiler depend heavily on what you're replacing. The Energy Saving Trust provides the following estimates for replacing an old G-rated boiler with a new A-rated condensing boiler in a typical semi-detached home:
- Gas bill savings of approximately Β£150βΒ£300 per year
- COβ emissions reduction of around 1,200β1,500 kg per year
- Potential improvement in your property's EPC rating by 1β2 bands
These savings are on top of the additional efficiency improvements from correct sizing (avoiding oversized boilers that cycle inefficiently) and smart controls (which can save a further 10β15%).
Modulation: The Efficiency Factor Most People Miss
Published SEDBUK efficiency figures are measured at full load β when the boiler is running at maximum output. But in practice, your boiler rarely runs at full output. On a mild autumn day, it may only need 20β30% of its rated capacity.
A boiler that can "modulate" β turn down its output to match the actual heating demand β is far more efficient in real-world use than one that can only run at full capacity and switches on and off repeatedly (known as "cycling"). Cycling is inefficient and increases wear on components.
Premium boilers like the Viessmann Vitodens 200-W can modulate down to just 19% of maximum output. This is why their real-world efficiency is often significantly higher than the lab-measured SEDBUK figure.