How Efficient Must a New Oil Boiler Be? Part L Explained

The minimum efficiencies

For an oil-fired heating system in an existing home, Approved Document L sets these minimums:

• A regular oil boiler: 91%, as defined in the ErP regulations.
• An oil combi boiler: 86%, as defined in SEDBUK 2009.
• An oil range cooker with an integral central heating boiler: 80%. It should also have two independently controlled burners, one for cooking and one for the boiler.

Lower figures in exceptional circumstances

The two boiler figures are measured under different rating methods: ErP for a regular boiler and SEDBUK 2009 for a combi. In exceptional circumstances in an existing home, a lower figure is allowed: 84% for a regular boiler and 82% for a combi. Those circumstances are defined in a separate government assessment procedure for condensing boiler installations.

A replacement can't be worse than the old one

A replacement has to reach the minimum above, and if it uses the same fuel, its efficiency must not be worse than the boiler it replaces. If you switch fuel, the new system must not produce more carbon dioxide or use more primary energy per unit of heat than the old one.

What else a replacement has to include

Approved Document L also covers the controls and setting up that go with a new boiler:

• Thermostatic controls in each heated room, or for each heating zone where that is justified (two adjacent rooms with a similar use, such as a kitchen and utility room, can count as one zone). A room thermostat with thermostatic radiator valves on the radiators in the other rooms is one way to meet this.
• A boiler interlock, whenever work is done on a system that includes a boiler.
• Separate time control for the heating and the hot water, unless they shared one timer before the work.
• If the hot water cylinder is replaced too, a cylinder thermostat or other electronic temperature control on the new one.
• Before the new boiler goes in, the heating and primary hot water circuits cleaned and flushed, with an inhibitor added to protect against scale and corrosion. The system should be prepared and commissioned to BS 7593.
• Commissioning in line with the manufacturer's instructions, and operating and maintenance instructions handed to the owner.

When the whole system is replaced

Where a wet heating system is newly installed, or fully replaced including the boiler, radiators and pipework, Approved Document L says it should be sized to heat the home properly at a maximum flow temperature of 55°C or lower. Where that is not feasible, for example because there is no room for larger radiators, it should be designed for the lowest temperature that still meets the home's heating needs.

Who signs off the work

Installing or replacing a heating system is covered by the Building Regulations 2010. Installers registered with a government-approved competent person scheme can self-certify that their work complies, instead of getting building regulations approval. For oil appliances, the schemes on the government's list include OFTEC.

OFTEC says this means no site inspection by a building control officer or approved inspector is needed. A notice that commissioning has been done must still reach building control within 30 days of the work being completed, and the registered installer can give it.

OFTEC has warned that changes under the Building Safety Act increase the potential penalties for anyone who fails to notify building work, including heating installations. Servicing and repairs are not covered by the building regulations, although OFTEC says they should always be done by a competent person.

A carbon monoxide alarm comes with it

A different part of the building regulations guidance, Approved Document J, says that where a new or replacement oil boiler is installed in a home, a carbon monoxide alarm should be provided in the same room.

The 2026 edition keeps the same figures

A new edition of Approved Document L takes effect in England on 24 March 2027 (24 September 2027 for higher-risk building work). It keeps the same minimums for oil heating in existing homes: 91% for a regular boiler and 86% for a combi. The rule that a same-fuel replacement must be no less efficient than the old one stays too.

Approved Document L is guidance for use in England, which is where this article applies.

Related advice

• The 2035 Oil Boiler Ban: What Is Actually Law?
• Carbon Monoxide Alarms and Oil Boilers: What the Rules Say
• How to Cut the Cost of Heating Your Home With Oil