Gas Boiler Efficiency Ratings Explained: ErP, SEDBUK and What They Mean for Your Bills

Why Boiler Efficiency Ratings Matter in London

London households spend more on gas than any other region in the UK on a per-household basis, partly because of the high proportion of older Victorian and Edwardian housing stock that is more difficult to insulate, and partly because gas prices have risen substantially since 2021. In this context, the efficiency rating of your boiler — how much of the gas it consumes is converted to useful heat — is a direct determinant of how much you pay each year. Understanding the ErP rating system and what the seasonal efficiency figures actually mean allows you to make an informed comparison between boiler models when a replacement is due.

The ErP Rating System: A to G

The Energy-related Products (ErP) Directive, implemented in the UK from September 2015, established a standardised efficiency labelling system for boilers using an A to G scale, similar to the system used for white goods and electrical appliances. An A-rated boiler achieves a seasonal space heating efficiency of 90 percent or above; a G-rated boiler achieves below 86 percent under the ErP classification (note that G-rated under ErP is still technically a condensing boiler — the pre-2015 G-rated non-condensing boilers achieved seasonal efficiencies of 65 to 75 percent, which was a fundamentally different and much more wasteful technology). Under the current ErP system, all boilers sold in the UK for domestic use must achieve at least an A rating, meaning non-condensing boilers can no longer be sold for domestic installation.

Within the A-rated band, boilers are further classified as A, A+, and A++ where applicable. Most high-quality modern combi boilers from Worcester Bosch, Vaillant, Viessmann, and Ideal achieve A or A+ ratings. The differences in running cost between an A and an A+ rated boiler of the same size are relatively modest — typically 2 to 4 percent seasonal efficiency difference — and should not be the primary driver of a choice between brands. More significant differences arise from correct sizing, system design, and the quality of the installation.

SEDBUK: Seasonal Efficiency of Domestic Boilers in the UK

Before the ErP system, UK boiler efficiency was measured using SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK), a calculation methodology developed by the Building Research Establishment. SEDBUK figures expressed efficiency as a percentage — SEDBUK 2009 figures typically ranged from 86 to 91 percent for modern condensing boilers. The SEDBUK database (now hosted at boilers.org.uk) lists efficiency figures for all boilers approved for sale in the UK, and SEDBUK figures are still referenced in the Standard Assessment Procedure (SAP) used for EPC calculations. If you want to compare boilers using a consistent methodology for energy modelling purposes, SEDBUK figures are the most useful reference.

What Seasonal Efficiency Actually Means

A seasonal efficiency figure represents the proportion of the energy content of the gas consumed that is converted to useful heat over a representative heating season, accounting for different operating conditions including part-load operation, standby losses, and condensing versus non-condensing operation at different return temperatures. A boiler with a seasonal efficiency of 92 percent converts 92 pence of every pound of gas consumed into useful heat; 8 pence is lost as heat through the flue, casing, and standby losses. The "seasonal" component is important — condensing boilers operate more efficiently at lower return water temperatures, so a boiler installed in a well-designed system with lower flow temperatures will achieve its rated seasonal efficiency more consistently than one operating at higher temperatures on an older system.

Running Costs: The Real-World Numbers for London

A typical London household with a three-bedroom house uses approximately 12,000 to 16,000 kWh of gas per year for space heating and hot water. At a gas unit rate of approximately 6.5 to 7.5 pence per kWh (the indicative rate in early 2025, subject to change), this represents an annual gas bill of approximately £780 to £1,200. Replacing a pre-2005 non-condensing boiler with a seasonal efficiency of 72 percent with a modern A-rated boiler at 92 percent efficiency reduces the heating gas consumption by approximately 22 percent — a saving of £170 to £260 per year on a bill of this size. Over the 12 to 15 year life of a new boiler, this saving more than offsets the installation cost even without accounting for avoided repair costs on the old unit.

Boiler Sizing: Why Oversizing Reduces Real-World Efficiency

The efficiency figures quoted by manufacturers and used in ErP calculations assume correctly sized boilers. An oversized boiler — one rated at 30 or 35 kW for a property that only requires 15 to 18 kW of heat output — operates inefficiently because it spends most of its time short-cycling: firing for a short period, reaching temperature, turning off, and repeating the cycle. Each ignition and warm-up cycle is inherently less efficient than sustained steady-state operation. Short-cycling also causes accelerated wear on the ignition electrode, heat exchanger, and pump. A properly sized boiler for a London terraced house or flat is significantly smaller than most people expect — commonly 18 to 24 kW for most properties — and a correctly sized installation will consistently outperform a larger boiler in real-world efficiency terms.

Smart Controls and Their Effect on Efficiency

Modern A-rated boilers are designed to work with smart thermostats and weather compensation controls. Weather compensation adjusts the boiler flow temperature based on external temperature — when it is milder outside, the boiler runs at a lower flow temperature, which increases the proportion of time the condensing boiler spends in condensing mode (where the latent heat in flue gases is recovered). Weather compensation typically improves real-world seasonal efficiency by 5 to 8 percentage points compared with a fixed high-temperature control, and is strongly recommended for new installations in London. Smart thermostats such as Nest, Hive, and Worcester Bosch EasyControl provide room-by-room and time-based control that can reduce heating consumption by 10 to 20 percent compared with basic programmer and thermostat arrangements, depending on the household's usage patterns.