How efficient does the new boiler have to be?
Approved Document L sets a minimum efficiency a replacement boiler has to clear before the work can be signed off, and which minimum applies depends on the fuel and the type of appliance going in.
Gas boilers
A replacement gas boiler needs to reach at least 92% efficiency, measured against the efficiency definition used in the Energy related Products (ErP) scheme (Approved Document L, Table 6.2). That 92% is the figure your quote needs to show for the appliance itself.
Oil boilers
Oil boilers are split by type. A regular oil boiler needs to reach at least 91% efficiency. A combi oil boiler, which heats water on demand, needs to reach at least 86% efficiency.
- Gas boiler: at least 92%
- Oil regular boiler: at least 91%
- Oil combi boiler: at least 86%
What to check on your quote
Look for a percentage figure against the boiler itself, tied to the fuel and type you're having fitted. If your quote gives a gas combi, check the figure sitting next to it is 92% or higher. If it gives an oil boiler, check which of the two oil figures applies, since a combi and a regular boiler are held to different minimums. A quote that names no percentage at all gives you nothing to check the boiler against, and that's something to ask your installer to add before you sign.
Replacing like for like, or switching fuel
Replacing a gas boiler with a gas boiler, or oil with oil
Where the new boiler runs on the same fuel as the one it replaces, it should not be less efficient than that old boiler. This runs alongside the usual minimum efficiency figures for a new installation, and it tends to come up on the handful of jobs where meeting the full minimum straight away isn't achievable in that particular house, for instance where the flue route or the space for a condensate pipe limits what can go in.
If your installer is proposing anything below the normal minimum for your fuel, ask what the old boiler was achieving and how the new one compares. A straight answer here is one of the things the quote should name alongside the boiler itself.
Switching from one fuel to another
If you're moving the boiler from oil to gas, or between any two fuels, the switch itself must not increase the emissions coming from the property. This is a separate check from the efficiency figure on the new boiler. A more efficient boiler on a different fuel can still fail this test if the fuel change pushes emissions the wrong way, so it's not enough to look at the efficiency percentage alone when a fuel switch is part of the job.
Ask your installer to confirm, in writing, that the fuel change has been checked against this rule.
The lowest figures allowed, and when they apply
In the narrow cases where neither the usual minimum nor a like-for-like match can be reached, Approved Document L sets an absolute floor: 78% for a boiler on natural gas and 80% for one on LPG. These are not the figures an ordinary replacement is held to. They apply only where the property genuinely cannot take anything better, and your installer should be able to say why that applies to your job.
If a quote shows a figure at or near 78% or 80%, ask what's ruling out the normal minimum before you agree to it.
Questions about the efficiency figure on your quote
Why is the efficiency figure on the manufacturer's page different from the one in my quote?
The figure Approved Document L checks is the ErP efficiency rating for the appliance (Table 6.2), so it should match the manufacturer's own figure for that exact boiler. If the number on your quote is different, it's often because the brochure is quoting a different size or output within the same range. Ask the installer to show you the ErP rating for the precise model and output named on your quote.
Which figure should I be checking my quote against?
Check the ErP efficiency rating for the exact boiler model and output listed on your quote. Under Approved Document L, a gas boiler needs at least 92% and an oil regular boiler at least 91%, both on the ErP rating (Tables 6.2 and 6.3). An oil combi is the exception: its minimum is 86%, measured as SEDBUK 2009 rather than ErP (Table 6.3). If your quote shows a single percentage with no model attached to it, you can't yet tell whether it's the figure for the boiler you're getting.
My quote just says a percentage with no label. What should I do?
Ask the installer to state which model and output the percentage belongs to, and to confirm it's the ErP figure used in Approved Document L. A number sitting on its own doesn't tell you whether it clears the minimum for your fuel type, so it's reasonable to get that confirmed before you accept the quote.
Is a higher percentage always a better boiler?
A higher ErP efficiency rating generally means less fuel burned for the same heat output, so it's a fair way to compare two options. Make sure you're comparing figures for the same kind of boiler, since a gas boiler, an oil regular boiler and an oil combi each sit against a different minimum, 92%, 91% and 86%.
Who decides whether my installed boiler meets the required efficiency?
That's a judgement for the installer or the building control body handling the job. What you can do is check that the figure on the quote names the model and output it belongs to and sits at or above the minimum for your fuel type, so you're asking the right question before the work starts.