What do the regulations say must be fitted with a new boiler?

Where do these rules come from?

Replacing a gas boiler is controlled work under the Building Regulations 2010. The practical guidance is in Approved Document L (conservation of fuel and power, Volume 1: Dwellings, 2021 edition, with 2023 amendments) and Approved Document J (combustion appliances, 2022 edition). The rules below apply to existing homes in England. Wales and Scotland have their own building regulations.

Minimum boiler efficiency

Approved Document L sets a minimum efficiency of 92% (as defined under the ErP rules) for gas-fired wet central heating in existing homes. A modern condensing boiler meets this. Only in exceptional circumstances, defined by a government assessment procedure, can a lower-efficiency appliance be accepted.

Extra measures for combi boilers

If a gas combi boiler is installed in an existing home, Approved Document L says at least one of these energy efficiency measures, appropriate to the system, should be fitted:

• Flue gas heat recovery.
• Weather compensation.
• Load compensation.
• A smart thermostat with automation and optimisation.

It also says a combi boiler should be chosen so that it can modulate down to the typical heating load of the home, so it is not oversized for most of the year. If you are deciding between boiler types, see our guide to combi vs system boilers.

Room-by-room temperature control

When a boiler is replaced in an existing home, Approved Document L says each room should be given thermostatic room controls so the heat in each room can be adjusted separately. In practice this usually means thermostatic radiator valves (TRVs) on radiators, with no TRV in the room that has the main room thermostat. Rooms with no heating do not need controls. Zone control instead of room control can be justified in some cases, such as a single-storey open-plan home or two adjacent rooms with a similar use, like a kitchen and utility room.

Other control requirements

• A boiler interlock should be installed, so the boiler and pump switch off when there is no demand for heating or hot water.
• If the hot water cylinder is replaced, the new cylinder should have electronic temperature control, such as a cylinder thermostat.
• The system must be commissioned by testing and adjusting it so it uses no more fuel and power than is reasonable.

What about the 55°C flow temperature rule?

Approved Document L says that where a wet heating system is newly installed or fully replaced, including the boiler, radiators and pipework, it should be sized to heat the home at a maximum flow temperature of 55°C or lower. A straightforward boiler swap that keeps the existing radiators and pipework is not a full system replacement, but a lower flow temperature can still help a condensing boiler run efficiently where the home stays warm enough.

A carbon monoxide alarm

Since 1 October 2022, Approved Document J says a carbon monoxide alarm should be fitted in the same room when a new or replacement gas boiler is installed in an existing home. It should meet BS EN 50291. See our guide to carbon monoxide alarm rules for the type and position.

Notification and paperwork

A Gas Safe registered installer can self-certify the work. The installation is notified to the local authority through the Gas Safe Register, and you should then receive a Building Regulations compliance certificate. You should also be given the completed commissioning checklist for the boiler and the manufacturer's instructions. Keep them together: you will need them for the warranty, for future services and when you sell the house.

What the regulations do not require

The Building Regulations do not require you to replace a working boiler because of its age, and they do not set a price. A magnetic filter, a system flush and upgraded thermostats may be recommended, or required by the manufacturer for its warranty, and are quoted as paid extras. Ask for a written list of what is included so you can compare quotes fairly. For local prices, see our boiler installation cost guide.

This guide is general information, not a substitute for an on-site assessment by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.