Boiler Efficiency Ratings Explained: ErP, A-Rated and What "Good" Really Means

What is a good boiler efficiency rating?

A good boiler efficiency rating is A-rated, which means 90% or higher seasonal efficiency. In plain terms, an A-rated boiler turns at least 90p of every £1 of gas into heat for your home and wastes less than 10p.

Ideally you want 92% or above. That's the legal minimum for a new gas boiler installed in England under the Boiler Plus rules, so almost every modern condensing boiler sits at 92-94% on paper.

Anything below A (roughly 87% or lower) is older technology. Boilers rated C, D or worse are usually non-condensing units that are well past their efficient years and cost noticeably more to run.

What "efficiency" actually means

Boiler efficiency is simply the ratio of useful heat out to fuel energy in. An 80% boiler converts 80% of the gas it burns into heat and loses the other 20% up the flue and elsewhere.

Modern figures are seasonal efficiency — an average across a typical UK heating year, not a single lab reading. That's more honest, because a boiler behaves differently in a mild October than in a freezing January.

One technical note that trips people up: efficiencies are quoted on a net calorific value basis. That's why you see condensing boilers described as ">90%" rather than the theoretically impossible 100%+ you'd get on a gross basis. You don't need the maths — just compare boilers on the same ErP scale and you're comparing like with like.

The ErP energy label explained

Since September 2015, every boiler sold in the UK and EU must carry an ErP (Energy-related Products) label — the same colour-coded arrow scale you see on fridges and washing machines.

Green arrows mean efficient; red arrows mean wasteful. The space-heating scale runs from A+++ down to G, and you'll still see A+++ quoted on system packages.

There are two kinds of ErP label, and the difference matters:

• Boiler (product) label — rates the boiler on its own. Standalone gas boilers top out at A (typically around 92-94%).
• Package (system) label — rates the boiler plus its controls. Pairing a boiler with a smart, load or weather-compensation control adds a few percent and is the only way to reach above A.

So if you see A+ or higher advertised, that's a package rating achieved by adding controls — not the boiler alone. For example, a 93%-efficient combi paired with the right control class can be uplifted to around 98%, which scores an A+ system. The boiler inside is still an A-rated unit. (The very top A++/A+++ bands mostly apply to systems that include heat pumps rather than a gas boiler on its own.)

ErP band percentage table

Here's the clean reference most guides bury in prose. This is roughly how ErP letter grades map to seasonal space-heating efficiency for boilers (bands are indicative — manufacturers calculate the exact figure):

ErP band	Seasonal efficiency	What it means
A	90%+	Modern condensing boiler — what you want
B	86-90%	Good, but just below today's standard
C	82-86%	Early/older condensing or efficient non-condensing
D	78-82%	Ageing non-condensing
E	74-78%	Inefficient — replacement worth costing
F	70-74%	Old, wasteful
G	Below 70%	Very old — often 25+ years; wasting a third of your gas

SEDBUK ratings explained

Before ErP, the UK used SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK), introduced in 1999 with later versions (SEDBUK 2005 and 2009). It was replaced by ErP in September 2015.

SEDBUK also used an A-to-G scale, where the top band represented the most efficient boilers. It maps closely to ErP, so a SEDBUK A boiler is broadly comparable to an ErP A boiler.

You'll still see SEDBUK quoted on older boilers, paperwork and EPCs, because those units predate the ErP system. If your boiler's documents show a SEDBUK grade, just read it on the same A-to-G logic.

Is an A-rated boiler always 92%+? Boiler Plus & the legal minimum

Almost, in England. The Boiler Plus standard, effective April 2018, made 92% ErP efficiency the minimum for any new or replacement gas boiler installed in an English home.

Boiler Plus also requires an extra energy-saving measure on combi installs — a smart control, weather compensation, load compensation or flue gas heat recovery. The result is that practically every new gas boiler on the market is A-rated at 92-94%.

So when a merchant calls a boiler "A-rated," that's the floor, not a selling point — it's effectively the standard. The real differences between modern boilers are reliability, controls and how well they're installed, not the headline ErP letter.

The catch: why your A-rated boiler probably ISN'T running at 92%

This is the part the brochures skip. Industry specialists estimate that most newly installed condensing boilers run 10-25% below their A-label efficiency in real homes — because they never actually condense, with tests suggesting many perform like a C-E rated boiler (around 75-85%) once installed.

A condensing boiler only reaches its rated efficiency when the water returning to it is cool enough (around 45-50°C) to pull extra heat out of the flue gases. That needs a flow temperature of roughly 60-65°C, not the 80°C factory default most boilers ship with.

The trade-body view is blunt: a large share of installers never adjust this, so the boiler runs hot, rarely condenses, and can deliver closer to 80% instead of the 92%+ on the box.

Two related problems make it worse:

• Oversizing — a boiler too powerful for the home fires hard, satisfies demand fast, then shuts off, then fires again. This "short cycling" hurts efficiency and increases wear.
• No weather compensation — without a control that eases the flow temperature up and down with the outside weather, the boiler runs hotter than it needs to most of the year.

How to find out YOUR boiler's efficiency rating

You don't need to guess or rely on a salesperson. You can usually look up your exact model's official efficiency in a couple of minutes, for free.

1. Find the data plate. Look at the front panel/cover of your boiler (no tools needed, no gas parts touched) for a sticker or label showing the make and model. Write down the full model name and number.
2. Open the PCDB. Go to the official Product Characteristics Database at ncm-pcdb.org.uk — the database (managed by BRE for DESNZ) used for energy assessments.
3. Search your model. Enter the manufacturer and model. The database returns the seasonal efficiency percentage the manufacturer submitted — useful even if your boiler's own label has faded or fallen off.

This works for many older boilers too, which is the point: it turns a vague "is mine any good?" into a precise number. If your model isn't listed, the manufacturer may simply not have submitted it, and a heating engineer can advise from the make and age.

Boiler efficiency by age: how much it drops over time

Even a boiler that was efficient when new loses ground as components wear, scale builds up and technology moves on. As a rough guide:

Boiler age	Typical efficiency	Likely band
Brand new	92-94%	A
10-15 years	~80-85%	B-C
~20 years	~75%	D
25+ years	~60-70%	F-G

These are rough averages — a well-maintained unit holds up better than a neglected one. A 25-year-old G-rated boiler can waste roughly a third of every unit of gas you pay for. Our guide to how long a boiler lasts and when efficiency drops goes deeper on the timeline.

How much money does a good efficiency rating save?

The savings from upgrading are real, but they depend heavily on your old boiler, your home and your bill. Headline figures from the industry:

• Old G-rated to a new A-rated boiler: the Energy Saving Trust estimates roughly £500 a year in a detached house, around £320 in a semi and about £130 in a flat — so the figure varies a lot by property type.
• British Gas cites savings of up to 23% on annual gas bills for a typical household swapping an older D-rated boiler for a modern A-rated one.

Two things to weigh against that. First, a full boiler replacement cost runs into the thousands, so payback on efficiency alone can be many years — replacement usually makes sense for reliability and peace of mind as much as the gas saving.

Second, the cheapest gains need no new boiler at all. Turning the flow temperature down is free, and a weather-compensation control typically costs around £100-£160 in parts plus a modest labour charge — a far faster payback than a whole new unit.

How to improve your existing boiler's efficiency

Before you spend thousands, get more from your current boiler. In rough order of cost vs. payback:

• Turn the flow temperature down to around 60°C so it actually condenses (free, and safe to do on the control panel — you're not touching gas parts).
• Add weather/load compensation so it only heats as hard as the weather demands (fitted by a Gas Safe engineer).
• Fit a magnetic system filter to keep sludge out of the system.
• Power flush a sludgy system and keep inhibitor topped up so heat moves freely.
• Service it yearly using a Gas Safe registered engineer to keep combustion clean.

We've gathered the full list in 11 ways to make your boiler more efficient, plus broader tips in more ways to cut your gas bill. Anything involving gas, the burner, flue, gas valve, PCB or sealed circuit is for a Gas Safe registered engineer only.

Does efficiency affect boiler cover or insurance?

Yes — and this is where efficiency can hit your wallet a second time. An old, low-rated boiler is generally harder and more expensive to cover than a modern A-rated one.

Many boiler cover and insurance products set an age limit — it's common for providers to refuse new cover on boilers over about 15 years old, or to restrict it, and some won't renew once a boiler passes a certain age. Older units break down more often, parts can get scarce, and that risk is priced in.

So a worn-out G-rated boiler can cost you three ways: more gas, pricier (or harder-to-find) cover, and a higher chance of an out-of-the-blue breakdown. If you're in that position, see boiler cover for older, less efficient boilers and weigh it against replacement.

Should you replace just for efficiency?

Usually not for efficiency alone. As the payback maths shows, the gas saving on its own rarely justifies a several-thousand-pound replacement quickly.

Replacement tends to make sense when efficiency stacks up with other reasons: your boiler is 12-15+ years old, breaking down, expensive to repair, hard to insure, or no longer reliable. At that point a modern A-rated unit pays back in comfort and predictability as much as in pounds.

If you're on the fence, work through whether to repair or replace your boiler, and if you're leaning towards new, browse the most efficient combi boilers in 2026 and the most reliable boiler brands — reliability matters at least as much as the ErP letter.

This guide is general information, not financial or gas-safety advice, and not a personalised recommendation. Never work on the gas, burner, flue, gas valve, PCB or sealed circuit yourself — that's for a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, call the National Gas Emergency line on 0800 111 999.

This article is general information, not financial or gas-safety advice. We compare a selected panel of providers, not the whole market, and may earn a commission if you buy through our links. Always have gas appliances checked and repaired by a Gas Safe registered engineer; in a gas emergency call 0800 111 999. Prices are indicative UK guides for 2026 — confirm current prices on the provider's own site.