Must you install a heat pump, or can you replace like for like?
Replacing a boiler with another boiler is still allowed
Approved Document L sets out what any new boiler installation must meet, gas, oil, or otherwise. Nothing in it tells you that you have to install a heat pump when your current boiler needs replacing. If a straight swap suits your home, the rule simply asks that the new appliance, its controls and its flue meet the current standard, the same as it always has.
Switching to a heat pump is your choice.
If you do want to move away from gas or oil, that's a decision you're making. Support toward the cost of a heat pump comes from a separate government grant, the Boiler Upgrade Scheme, with its own eligibility conditions. Qualifying for that grant and meeting the building regulations are two different checks, and passing one says nothing about the other.
A boiler swap and a whole-system replacement aren't treated the same
If your installer is only changing the boiler, that's the end of it as far as flow temperature goes. But if the quote covers the whole heating system, radiators and pipework included, Approved Document L expects it to be designed to run at 55°C or lower. Ask which one your quote describes: a boiler on its own, or the system around it too, because that's what decides whether this applies to your job.
A future standard doesn't apply to this installation
A proposed future edition describes what might be required later, and it has no bearing on a boiler going in now. The rules in force today are what your installer has to meet, and checking which edition applies to your job tells you which set that is. If an efficiency figure in your quote looks unfamiliar, how to read the efficiency figure in a quote sets out what it should show and what it doesn't.
Replacing like for like against switching to a heat pump
Read across to see what each route asks of you, and whether a grant has anything to do with it.
| Route | What it involves | Grant available | Check before you commit |
|---|---|---|---|
| Like-for-like boiler replacementThe boiler still has to meet the efficiency and controls standard in force for your job. | A new gas or oil boiler in the same spot, same fuel, no change to radiators or pipework | No grant applies | Ask which edition of Approved Document L your installer is working to |
| Switching to a heat pumpThe grant is a funding scheme condition. | A heat pump installation, usually with some change to radiators or pipework | £7,500 under the Boiler Upgrade Scheme, plus £1,500 until March 2027 for oil or LPG homes off the gas grid | Check the scheme's own conditions before you commit, separately from the building regulation |
| From 24 March 2027Work already under way is not automatically swept into the new edition. | The 2026 edition of Approved Document L takes effect for work starting from this date | Grant terms are set by the scheme and can change on their own timetable | Ask your installer which edition your quote is written to, and from when |
The grant figures are the Boiler Upgrade Scheme's own conditions, held apart from Approved Document L. Check the scheme's current terms directly before relying on them.
Does the 55°C rule apply to my job?
A heating system designed to run at 55°C or lower is a system built around radiators and pipework sized for that lower flow temperature, usually meaning larger radiators or more of them, so the house still heats properly on less heat from the boiler. That standard applies when the whole system is replaced, not when the boiler alone is swapped out.
Why a straight boiler swap is different
If your installer is taking out the old boiler and fitting a new one onto the existing radiators and pipework, the 55°C design requirement does not apply to that job. The radiators were sized for the old system's flow temperature, and nothing in the work being done changes that sizing, so there is nothing in Approved Document L that asks the installer to redesign the whole system around a lower temperature.
When it does apply
The 55°C design requirement applies when the radiators and pipework are being replaced as well, whether that is because you are moving to a heat pump or because the whole heating system is being renewed alongside the boiler. In that case, ask your installer how the radiators have been sized and what flow temperature the design is based on.
What to check on your quote
- Whether the job is described as a boiler replacement only, or as a system replacement including radiators and pipework
- If radiators and pipework are being replaced, what flow temperature the new system is designed around
- Whether any radiators are being upsized to suit a lower flow temperature, and which ones
What a compliant quote should name item by item is set out on the page on what a boiler installation quote must name.