Part L turns a boiler swap into a system job: minimum efficiency, a system that can run at 55 °C, interlock and controls, plus one extra measure on a combi.
Approved Document L is the statutory guidance on Part L of the Building Regulations in England — conservation of fuel and power — and it is the document that decides what a compliant boiler replacement or new heating system must include. Most of it is about fabric and calculation methods; the parts a heating installer meets on every job sit in the sections on heating and hot water systems. This guide sets out those requirements from the 2021 edition (incorporating the 2023 amendments) that applies today, and what the 2026 edition changes when it takes effect in March 2027.
Which edition applies, and to what
Approved Document L Volume 1 covers dwellings; Volume 2 covers other buildings. The current 2021 edition, incorporating the February 2023 amendments, came into force on 15 June 2022; work covered by a building notice, full plans application or initial notice submitted before that date could continue on the previous edition provided it started before 15 June 2023, as set out in Circular Letter 01/2021. It applies in England only — Wales, Scotland and Northern Ireland publish their own equivalents, and a job across the border follows that nation’s document.
The 2026 edition, retitled Energy and greenhouse gas emissions, takes effect on 24 March 2027 for building work that is not connected with higher-risk building work, and on 24 September 2027 for work that is; its transitional arrangements are in Circular Letter 01/2026. Until then the 2021 edition governs, and a notice given before the changeover keeps a job on the edition it started under, so check the transitional letter before quoting a 2027 job under either set of rules.
Minimum efficiency: 92 % ErP for a wet gas system
Table 6.1 of the 2021 edition sets 92 % (as defined under the Energy-related Products regime) as the minimum efficiency for a gas-fired wet heating system — radiators or underfloor — in a new dwelling, and Table 6.2 repeats it for existing dwellings. Only in exceptional circumstances in an existing dwelling, established through the assessment procedure the document points to, may a lower SEDBUK 2009 figure be accepted: 78 % for natural gas and 80 % for LPG. A range cooker with an integral central-heating boiler has its own lower floor of 75 % SEDBUK 2009. Every modern condensing boiler clears the 92 % line comfortably; the figure matters when a customer asks why a non-condensing replacement is not on the table.
Design the system for 55 °C
The rule most likely to change how you quote is the flow-temperature requirement. All parts of a wet space-heating system, including pipework and emitters, should be sized so the system can operate effectively and meet the dwelling’s heating needs at a maximum flow temperature of 55 °C or lower. In an existing dwelling where that is not feasible — the document’s own examples are insufficient space for larger radiators and a distribution system fed at higher temperature from a low-carbon district heat network — the system should be designed to the lowest temperature that will meet the demand.
The reasoning is condensing efficiency and heat-pump readiness: a boiler returns from a 55 °C circuit well below the dew point of its flue gases, and a radiator circuit that can heat a home at 55 °C is one a heat pump can later run without a rip-out. In practice a boiler swap becomes a radiator check: the output of an existing radiator falls sharply at a lower mean water temperature, so size each room’s emitter against its heat loss at the lower temperature difference before deciding whether it stays. The radiator output calculator shows the correction from the standard ΔT50 rating to a lower ΔT.
Controls the job must leave behind
- Boiler interlock whenever work is carried out on a system that includes a boiler (paragraph 5.25): the boiler fires only when a room, cylinder or zone control calls for heat.
- Separate time control for space heating and hot water (5.24), unless the existing system already controls both from a single time controller, in which case that may continue.
- A replacement hot-water cylinder needs an electronic temperature control such as a cylinder thermostat (5.26).
- A replacement boiler brings the controls up to the Section 6 standard for the system type (5.27): a thermostat in a room the circuit serves plus an individual thermostatic control such as a TRV on every emitter outside that room — never a TRV in the thermostat’s own room — or a thermostat for each room or heating zone, or networked controls on each emitter.
- New dwellings use time and temperature zone control with thermostatic radiator valves; a single-storey dwelling whose living area is more than 70 % of the floor area may use a programmer and room thermostat instead. The document’s notional dwelling assumes an interlocked system with ErP Class V controls.
A combi boiler needs one extra measure
Paragraph 6.2 carries forward the 2018 Boiler Plus standard: when a gas-fired combination boiler is installed in an existing dwelling, at least one of four energy-efficiency measures appropriate to the system must be installed — flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. Weather or load compensation also happens to be the control that lets the boiler modulate to the low flow temperatures the previous section asks for, so on many installations one choice satisfies both requirements.
Notification, commissioning and the paperwork
Replacing a boiler or installing a heating system is controlled building work under the Building Regulations, so it must be notified. A Gas Safe registered installer normally self-certifies through the Register’s competent person notification, which reaches the local authority and produces the compliance certificate the householder keeps; otherwise a building notice and inspection are needed. Commissioning is part of compliance: the system is set up and tested to the manufacturer’s instructions, and the Benchmark commissioning checklist — the industry record the document recognises — is completed and left with the customer, together with the operating instructions the regulations require to be provided.
What the 2026 edition changes for heating work
For an existing dwelling the 2026 edition keeps the same minimum efficiencies — Table 5.3 repeats 92 % ErP, with the exceptional SEDBUK 2009 fallbacks — and the same 55 °C design rule, now expressed as a maximum flow temperature of 55 °C with the lowest achievable design temperature where that is not feasible. The material change is in controls: paragraph 4.22 asks that when a heat generator such as a boiler is replaced in an existing dwelling, or in any new-dwelling system, each heated room is given thermostatic room controls able to adapt the output in that room, with zone control only where justified under paragraph 4.23 and no controls required in unheated rooms. Notes acknowledge cases where that is not technically feasible, such as dwellings with very low heat demand.
For new dwellings the gas-fired guidance is written for existing homes only; new homes are assessed against the Future Homes Standard 2026 notional dwelling, which is heat-pump based, so a gas boiler is not the expected route for a new home built under the 2026 edition. Heating installers will feel that first through developers’ specifications rather than through the boiler-replacement work that this guide is about.
Revise it alongside CENWAT and the heating notes
CENWAT assessment questions on commissioning, controls and interlock lean on the same Part L logic, and the central heating revision notes cover S-plan and Y-plan wiring, interlock and balancing that turn the requirements into a working system. Read the requirement, then trace how the installed controls satisfy it: that is what an assessor, a building control officer and a customer’s next engineer will each do.
Quick answers
Do I have to fit TRVs when I replace a boiler?
Under the 2021 edition, replacing a boiler brings the controls up to the Section 6 standard: a room thermostat plus an individual thermostatic control, usually a TRV, on every emitter outside the thermostat’s room, or a thermostat per room or zone, or networked emitter controls. From 24 March 2027 the 2026 edition asks for thermostatic room controls in each heated room when a boiler is replaced, with zoning only where justified.
Does the 55 °C rule mean I must replace the radiators?
It means the system must be designed so it can meet the heating need at a flow temperature of 55 °C or lower. Where an existing radiator cannot deliver the room’s heat loss at that temperature and there is space for a larger one, the larger one is the compliant answer; where there is no space, the document allows the lowest feasible design temperature. Size each emitter against the room’s heat loss at the lower ΔT before deciding.
Is a boiler replacement notifiable under the Building Regulations?
Yes. It is controlled building work. A Gas Safe registered installer self-certifies through the competent person notification, which informs the local authority and generates the compliance certificate for the householder; without that route a building notice is required.
What counts as the extra energy-saving measure on a combi?
One of four: flue gas heat recovery, weather compensation, load compensation, or a smart thermostat with automation and optimisation. It applies when a gas-fired combination boiler is installed in an existing dwelling.
Which edition applies to a job that starts in 2027?
The 2026 edition takes effect on 24 March 2027 for work not connected with higher-risk buildings and on 24 September 2027 for work that is. Work notified before the changeover normally continues under the 2021 edition; Circular Letter 01/2026 sets out the transitional arrangements, so check it before quoting.
Does Approved Document L apply in Scotland, Wales or Northern Ireland?
No. The approved documents apply in England. Wales has its own Approved Document L, Scotland uses the Building Standards technical handbooks, and Northern Ireland uses Technical Booklet F. The physics of condensing boilers and low-temperature design is the same everywhere, but the specific requirements and dates are not.
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