Boiler Efficiency, ErP Ratings and SEDBUK

Every gas boiler sold in the UK since 2015 has to carry an A rating by law, which corresponds to a minimum of 92% ErP seasonal efficiency¹. That single fact explains most of what confuses householders about boiler efficiency: if nearly everything on the market is rated A, the label itself tells you little, and the numbers underneath it matter more. Older boilers still working in UK homes can fall as low as 60% on the energy efficiency scale², and the oldest can have lower than 50% efficiency, while the newest condensing boilers should exceed 90%¹.

This page explains what an efficiency rating actually measures, how the ErP letter bands from A+++ to G work, what the older SEDBUK percentages meant and where they still apply, and how these figures feed into your home's Energy Performance Certificate. It also covers the separate efficiency and emissions thresholds that govern biomass boilers and solid fuel stoves, and what the ratings will mean as fossil fuel boilers are phased out.

What an efficiency rating really tells you

A boiler's efficiency rating is a measure of how much of the energy in the fuel it burns comes out as useful heat, rather than going up the flue. The measurement is not a single snapshot. It has to account for the fact that a boiler spends most of its life running below full power, so ratings are built from efficiency at rated output and efficiency at part load. The earliest UK labelling rules, from 1993, worked exactly this way: a manufacturer could affix a label showing between one and four stars, and the star levels were defined by how the boiler performed at both rated output and part load⁵. A one-star label required efficiency at rated output and at part load equal to or greater than the relevant values for standard boilers, while a two-star label required those figures to be three or more points higher, with a nominal efficiency of at least 87 + 2 log Pn at an average boiler-water temperature of 70°C⁵.

Modern ratings go further and express seasonal efficiency: an estimate of how the boiler performs across a whole heating season, including start-up, standby and part-load running, not just its best-case output. This is why a seasonal figure is always lower than a laboratory peak figure, and why the number on a label can differ from the number in a brochure. The distinction matters to a household because seasonal efficiency is what determines fuel use, and fuel use is what determines both cost and dependence on a supplier.

The measurement regime is enforced through type examination. Under the regulations that govern boiler efficiency in Great Britain, the conformity of series-produced boilers must be certified by examination of the efficiency of a boiler type, with a declaration of conformity to the approved type, and UK marking substituted for CE marking⁶. In other words, the rating on the label is the maker's declared figure for the whole product type, verified through a defined assessment process, not a test of the individual unit on your wall.

The ErP scale from A+++ to G, and why A dominates

Under the ErP standards, boilers are labelled from A+++ to G, with A+++ being the most efficient². The Energy-related Product Directive means all gas boilers have to be rated A to G for energy efficiency¹. The scale is the same one householders see on fridges and washing machines: energy labels rate products from A to G in terms of energy use, with A being the most efficient and G the least efficient⁷.

The practical problem with the scale is compression at the top. Since 2015, all new gas boilers in the UK have to be rated A by law, which is the most efficient band, at a minimum of 92% ErP efficiency¹. Most modern condensing boilers have an A rating². That means the label cannot separate one gas boiler from another: they are all A. Some manufacturers and homeowners still find it preferable to compare boilers using the older percentage system, precisely because the letter bands no longer discriminate between products².

For a householder, the consequence is that the ErP label is a floor, not a selector. It confirms a boiler meets the legal minimum; it does not tell you whether one A-rated model runs more efficiently than another, or how it will behave in your home with your radiators and your flow temperature. Those questions are answered by the seasonal efficiency figures in the product data, and by how the system is set up, which is covered in boiler flow temperature.

SEDBUK: percentage ratings and their remaining uses

SEDBUK stands for the Seasonal Efficiency of a Domestic Boiler in the UK. It expresses efficiency as a percentage, and for the boilers it covered the figures run from 70 to 92%². The system was replaced by ErP ratings but still applies to older boilers made before 2015¹, which is why a boiler installed in the early 2010s carries a percentage while a newer one carries a letter band.

SEDBUK figures still appear in the building regulations of all four nations, because the standards were written when SEDBUK 2009 was the standard measure. The notional dwelling used in the Approved Document L for England has a combination boiler with a SEDBUK 2009 efficiency of 89.5%⁸, and the same 89.5% figure appears in Northern Ireland's proposed notional dwelling³ and in Scotland's modelling of proposed energy improvements for new dwellings⁹. A gas-fired range cooker with an integral central heating boiler in an existing dwelling must reach 75% as defined in SEDBUK 2009, a requirement stated in the English Approved Document⁸, the Welsh consultation version¹⁰, and the current Approved Document L¹¹.

For oil, the SEDBUK thresholds are lower. The boiler SEDBUK 2009 efficiency for oil-fired combination boilers of the condensing type should be not less than 86%, according to Scotland's Domestic Building Services Compliance Guide¹². In exceptional circumstances in existing buildings, an oil-fired wet heating system with a regular boiler may be accepted at 84% SEDBUK 2009¹⁰.

The lesson for a household with an older boiler is not to mix the two systems. A SEDBUK percentage and an ErP band measure the same underlying performance but on different scales, and a boiler rated under one cannot be read as if it were rated under the other. If your boiler predates 2015, its percentage is the figure to look for.

The 90 per cent efficiency zone and what the rules require

The numbers regulators actually work with cluster in a narrow band just above 90%. The final Part L 2021 standard for England sets a gas boiler at 88% SEDBUK 2009, or 92% ErP, with 92% ErP efficiency as the requirement¹³. Northern Ireland's building regulations discussion document proposes that the ErP seasonal efficiency of a gas boiler should be a minimum of 92% in an existing dwelling for a new or replacement installation, and for oil-fired boilers to set the minimum seasonal efficiency at 91% ErP³.

Independent guidance puts the same requirement in plainer terms: building regulations specify that if you are replacing an old boiler, the new one must be A-rated for energy efficiency¹⁴. Since the A band begins at 92% ErP for gas boilers¹, the practical rule is that any replacement gas boiler installed today in any UK nation will sit at or just above that figure.

The gap between the legal minimum and the oldest stock in homes is where the savings are. Older boilers can have lower than 50% efficiency, while the newest condensing boilers should exceed 90%¹, and older models still in use can fall as low as 60% on the energy efficiency scale². A household moving from a boiler at the bottom of that range to one at the top is, in effect, recovering fuel that was being wasted, which reduces both running cost and the volume of gas or oil that has to be bought in. Independent advice treats ten years as the age at which replacement is worth considering¹⁴.

Finding the rating: on the label, data plate and in databases

The rating is carried on the appliance and in the paperwork that comes with it. The 1993 regulations provided for an optional system under which a manufacturer of a boiler may affix a label showing between one and four stars, reflecting the level of energy efficiency of that boiler⁵. That star system is long superseded, but the principle survives: the maker affixes the label and stands behind the declared figure, with conformity of series-produced boilers certified through type examination and a declaration of conformity to the approved type⁶.

For boilers made before 2015, the SEDBUK percentage may appear on the data plate, in the installation manual, or in the manufacturer's product literature rather than on a label at all. Where the paper trail has been lost, the product characteristics database maintained under the National Calculation Methodology holds the efficiency data used in energy assessments, and is the reference source for a boiler's seasonal efficiency figure¹⁶. It is the database assessors and building professionals draw on, so it is the most reliable place to check a figure for an older model.

A practical sequence for identifying an old boiler's rating:

1. Find the model name on the data plate on the casing.
2. Check the manufacturer's literature for that model for a SEDBUK percentage or ErP figure.
3. Search the model in the product characteristics database if the literature gives no figure¹⁶.
4. Treat a boiler more than 10 years old as a replacement candidate regardless of its stated rating, since efficiency falls with age and the oldest stock can sit far below current minimums¹⁴.

System labelling for the boiler and its package

A boiler's efficiency is a property of the appliance, but a heating system's efficiency is a property of everything around it: the controls, the cylinders, the emitters. Energy labels rate products from A to G in terms of energy use, with A being the most efficient and G the least efficient⁷, and the same labelling logic extends beyond the boiler itself to the package of components that make up the system.

Where components are bought separately and assembled on site, the paperwork responsibility falls on the installer. When works have been completed, the installer should produce a commissioning certificate, such as a Benchmark certificate, and notify the local authority building control department, either directly or, if a member of a Competent Person Scheme, via the scheme operator; the local authority should then supply a building regulations completion certificate¹⁷. The same requirement is stated for Wales¹⁸. An installer registered with a scheme can self-certify that their work complies with building standards¹⁹, and independent guidance confirms that an engineer on a Competent Person Scheme can self-certify that the installation meets building regulations¹⁴.

For the householder, the documents to expect at handover are the commissioning certificate and, in due course, the completion certificate from the local authority. These matter beyond formality: they are the evidence that the installation was registered, which supports warranty claims and any later grant applications, and they record the declared efficiency of what was actually installed.

From boiler rating to home rating via the EPC

The boiler's efficiency figure does not stay on the label; it feeds into the home's Energy Performance Certificate. To determine the energy efficiency rating of a home, EPC assessors use the standard assessment procedure, SAP, with reduced SAP, RdSAP, used for existing homes where information may not be available²⁰. SAP is the Standard Assessment Procedure for Energy Rating of Dwellings, and it supports the production of EPCs¹⁶. The government approved version 10.3 of SAP for the energy performance of new dwellings in England on 24 March 2026²¹.

An EPC provides an assessment of your home's energy efficiency²², and the boiler's seasonal efficiency is one of the inputs that determines the rating. The boiler figure is adjusted for how the system is designed to run: the space heating efficiency of a condensing boiler is increased by the applicable efficiency adjustment given in Table 4c of the SAP 2012 specification where the system has been designed to operate at a lower temperature¹⁵. This is the formal recognition that a condensing boiler run at lower flow temperatures performs better than the same boiler run hot, and it is why flow temperature settings show up in assessments.

The ratings that result vary across the housing stock. In Scotland's 2024 house condition survey, mean SAP 2012 ratings ranged from 65.8 in dwellings owned outright to 73.5 in housing association dwellings, and the mean was 69.5 for urban dwellings against 61.4 for rural dwellings²³. Retrofit programmes target these figures directly: Edinburgh's high-rise retrofit programme aims for each home, bar a few exceptions, to meet a minimum energy performance rating of EPC B, or SAP 81²⁴.

For a household, the connection is direct. Replacing an old boiler with an efficient one lifts the EPC, because the assessor records the seasonal efficiency of the installed appliance. But the EPC also captures everything else, insulation and heating controls included, so a new boiler alone moves the rating less than a household might hope if the rest of the fabric is poor.

Biomass and solid fuel: different thresholds and emissions rules

Biomass boilers and solid fuel appliances are governed by their own efficiency thresholds, set by appliance category rather than by the ErP seasonal efficiency scale that applies to gas and oil. The Welsh consultation version of Approved Document L sets out the categories: an independent boiler burning wood logs only must reach 75% gross calorific value, an open fire and high output boiler must reach 63%, and a cooker with a heating boiler exceeding 3.5kW must reach 60% for wood fuels¹⁰. The English Approved Document states the 60% figure for that cooker category⁸.

Biomass boilers are eligible technology under the Boiler Upgrade Scheme²⁵, but eligibility carries emissions conditions. The emissions certificate must show that NOx did not exceed 150 g/GJ net heat input and particulate matter did not exceed 30 g/GJ net heat input⁴, limits repeated in the scheme's installer guidance²⁶. Where a biomass boiler's performance is metered, the metering has its own accuracy standards: heat metering should meet Class 3 of the Measuring Instruments Directive, electricity metering Class A, and efficiency estimates should be done as accurately as possible, with better than 20% recommended²⁷.

For stoves, the marker to look for on a new appliance is the Ecodesign standard, which ensures the appliance meets strict standards for emissions and efficiency²⁸. These rules bite at the point of sale, not retrospectively: an existing stove or boiler is not pulled up to the new thresholds, but a new purchase must meet them. Households in smoke control areas face additional rules about what may be burnt, and the guidance on those rules is separate from the efficiency thresholds²⁸.

The independence angle is different here from gas. A biomass boiler or stove burning UK-sourced wood reduces dependence on imported fossil fuel and on a gas supplier, but it swaps that dependence for a fuel supply chain of its own, and the emissions limits exist because burning solid fuel has a local air quality cost that a gas boiler does not. The thresholds and the emissions certificate are the regulatory answer to that trade-off.

Ratings and the phase out of fossil boilers

Efficiency ratings were designed for a market in which buying a better boiler was the natural response to a poor one. That market is narrowing. Scotland's New Build Heat Standard prohibits polluting heating systems, like gas and oil boilers, in new buildings²⁹, and the Future Homes Standard consultation response for England set the Part L 2021 gas boiler requirement at 88% SEDBUK 2009, or 92% ErP, as part of the transition toward low-carbon heating in new homes¹³.

The support schemes already point the same way. The Boiler Upgrade Scheme is for homes replacing a fossil fuel heating system³⁰, and ECO4's measures table includes boiler upgrades to non-renewable and renewable heating systems, repairs of broken heating systems, first time central heating, and heating controls, alongside insulation and solar PV³¹. In other words, public money is shifting from making fossil boilers more efficient to replacing them.

The tax position reflects this too. Energy efficient boilers do not qualify for the reduced rate: heating equipment that is not funded through an energy efficiency grant, and energy efficient boilers, sit on the list of non-qualifying products at 20% VAT³².

For a household weighing a replacement now, the ratings still matter, but as a floor rather than a destination. An A-rated boiler at 92% ErP wastes less fuel than the 60% or sub-50% appliance it replaces², and that saving is real for as long as the boiler runs. What the rating cannot do is protect the investment from policy: a boiler installed today is being bought into a market with a stated direction of travel away from fossil fuel heating, and the gas boiler ban question, heat pumps compared with boilers and hydrogen-ready claims are all part of that same picture. The most durable efficiency decision is one that reduces fuel use per unit of heat, whatever fuel the home ends up using.