An efficiency rating tells you how much of the fuel a boiler burns is turned into usable heat, not how much your household will actually spend on gas. Two boilers with the same letter can still leave you with different bills, because the label covers the appliance in isolation, not how it is controlled, sized or used in your home.

What the label is actually measuring

UK boiler efficiency labelling comes from the EU’s Energy related Products (ErP) framework, retained in UK law after leaving the EU. It applies to gas, oil, LPG and electric boilers up to 70kW, along with hot water storage tanks and water heaters, and requires an energy label with a letter grade at the point of sale.

The figure behind the letter is a seasonal efficiency percentage: broadly, how much of the energy in the fuel is converted into heat across a typical heating season, rather than lost up the flue or through standing losses. It is a laboratory measure of the appliance, tested under standard conditions – not a promise about your specific gas bill.

ErP vs SEDBUK: why you may see both terms

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) is the older UK rating system, also graded A to G, that predates the current ErP label. You will still see SEDBUK figures referenced in older paperwork, some manufacturer technical documents and building-compliance software, because the underlying efficiency data feeds into Building Regulations energy calculations as well as the retail ErP label.

For a buyer today, ErP is the label you will actually be shown on a quote or a boiler’s technical data sheet. SEDBUK figures are worth asking about only if you are comparing an older boiler already installed in a property against a new one, or if a survey or EPC assessment refers to them.

Why almost every new boiler says “A-rated”

Since April 2018, the Boiler Plus standard has set a minimum performance requirement for new domestic gas boiler installations in England: every new gas boiler must achieve at least 92% ErP seasonal efficiency, which sits within the A band. In practice, this means it is now unusual to find a new gas boiler on the UK market that is not A-rated – the label alone no longer separates a good installation from a mediocre one, because almost everything sold today clears the same bar.

This is genuinely useful context when you are comparing quotes: if every combi, system or regular boiler on your shortlist is A-rated, the letter is not the differentiator. What differs between installers and models is everything Boiler Plus added on top of the appliance rating.

The part of the label that still varies: controls

Alongside the 92% minimum, Boiler Plus requires every new gas boiler installation to include basic time-and-temperature control (a programmer and thermostat, if not already fitted and working), and for combi boiler installations specifically, at least one of four additional measures:

• Flue gas heat recovery – recovers some heat from the flue gases to preheat incoming water;
• Weather compensation – adjusts the boiler’s output automatically based on the outside temperature;
• Load compensation – adjusts the boiler’s output based on how close the room is to the target temperature; or
• A smart thermostat with standalone automation and optimisation – learns your household’s patterns and adjusts run times accordingly.

These measures are not reflected in the ErP letter itself, but they materially affect how efficiently the boiler runs in your actual home, day to day. This is a genuinely useful question to ask an installer: not “what rating is this boiler?” – the answer will almost always be A – but “which of the four measures is included, and why that one for my home?”

What to actually compare when you get quotes

With the ErP letter largely equalised across the market, focus comparisons on the things that still vary between installers and models:

• which control measure (of the four above) is proposed, and whether it suits how the household actually uses heating;
• the boiler’s modulation range – how far it can turn its output down, which affects efficiency at part-load, which is most of the time in a well-insulated home;
• correct sizing for the property’s heat loss and hot-water demand, since an oversized boiler run at part-load performs worse in practice than its rated efficiency suggests;
• whether the quoted price includes the full Boiler Plus-compliant control package, or whether it is an optional extra; and
• that installation is carried out and signed off by a Gas Safe registered engineer, which is a legal requirement for any gas boiler work in Great Britain, separate from the efficiency question but essential to check regardless.

Frequently asked questions

Does a higher ErP rating always mean a lower gas bill?

Not on its own. The rating measures the appliance under test conditions. Your actual bill depends on the rating combined with correct sizing, the controls fitted, how the household uses heating, insulation, and gas prices. Two A-rated boilers can produce different real-world bills in the same house.

Is it worth replacing a working boiler purely to get a better rating?

Not as a standalone reason. If your current boiler is a non-condensing model installed before condensing boilers became standard, replacement is likely to improve efficiency meaningfully. If it is already a reasonably modern condensing boiler, the efficiency gain from replacement alone is smaller, and the decision is better made on reliability, parts availability and running costs together – not the label alone.

Do electric boilers carry the same ErP letter scale?

Electric boilers and water heaters fall under the same ErP framework but are assessed differently from combustion appliances, since there is no flue loss to account for. A high ErP letter on an electric boiler does not by itself tell you whether electric heating will cost more or less to run than gas in your home – that depends on your electricity and gas unit rates.

Sources

• Boiler Plus: new standards for domestic boiler installations from April 2018 – GOV.UK factsheet
• Find or check a Gas Safe registered engineer – Gas Safe Register

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