A boiler interlock is a wiring and control arrangement that stops your boiler firing when there’s no actual demand for heat — and it’s a Building Regulations requirement, not an optional upgrade your installer is trying to sell you.

What a boiler interlock actually is

A boiler interlock is an electrical wiring arrangement between a boiler’s controls, room thermostat(s), and any cylinder thermostat, designed so that the boiler and pump only run when at least one of the controls is calling for heat. Without an interlock, a boiler can continue firing and circulating hot water even after a room or cylinder has reached its target temperature, wasting gas by heating a space or a hot water store that doesn’t currently need it. The interlock is what ties the boiler’s operation directly to actual demand, rather than leaving it to run on a simple timer regardless of whether any room or cylinder is calling for heat.

Where the requirement comes from

The requirement sits within Approved Document L, Volume 1 (Conservation of fuel and power for dwellings), the technical guidance supporting Part L of the Building Regulations in England. Where a gas or oil boiler is fitted or replaced, the installer is required to fit a boiler interlock along with time and temperature controls, if these aren’t already in place. This isn’t a discretionary best-practice suggestion sitting alongside the regulations — it’s presented as part of what a compliant installation looks like, and a Gas Safe registered installer notifying the work through the Competent Person Scheme route is expected to be delivering an installation that meets it.

How it’s usually achieved in practice

In a typical domestic system, the interlock is achieved through a combination of a room thermostat, thermostatic radiator valves (TRVs) on individual radiators, and, where there’s a hot water cylinder, a cylinder thermostat, all wired back through the boiler’s control circuit so that the boiler only fires when one of these is calling for heat. There’s more than one acceptable wiring configuration, and the specific approach an installer uses can depend on the existing system and the boiler manufacturer’s own control requirements, but the underlying principle, demand-linked operation rather than continuous running, stays the same across approaches.

Why it matters beyond compliance

Setting the regulatory requirement aside, a properly wired interlock has a direct, practical effect on running costs: a boiler that keeps firing and circulating hot water to rooms that have already reached temperature, or to a cylinder that’s already hot, is burning gas for no useful purpose. This is one of the less visible reasons two homes with seemingly similar boilers and similar usage patterns can see meaningfully different gas bills — an older system without a proper interlock, or one where TRVs have been left permanently wide open, can run the boiler far more than a correctly interlocked, correctly set-up system actually needs to.

What this means if you’re replacing a boiler

• If your existing system doesn’t already have a boiler interlock in place, your installer is required to add one as part of a boiler replacement, not offer it as an optional extra.
• This may mean new wiring, a new or repositioned room thermostat, or TRVs fitted to radiators that don’t currently have them, and it’s worth asking your installer to confirm what’s being added as part of the quoted price.
• For a gas combi boiler specifically, Boiler Plus also requires at least one further energy-saving measure on top of the interlock, such as weather compensation, load compensation, flue gas heat recovery, or a smart thermostat with automation features — ask which one is being proposed for your installation.
• Scotland, Wales and Northern Ireland have their own building standards, so if you’re outside England, check the equivalent requirement that applies to your installation rather than assuming Approved Document L applies directly.

FAQs

Do I need to ask for a boiler interlock, or does the installer just include it?

A Gas Safe registered installer working in England is required to fit one as part of a compliant boiler replacement where it isn’t already present, so you shouldn’t need to specifically request it — but it’s reasonable to ask them to confirm it’s included, particularly if your existing system is older.

Does a boiler interlock mean I need a smart thermostat?

Not necessarily — a standard room thermostat combined with TRVs and, where relevant, a cylinder thermostat can satisfy the interlock requirement. A smart thermostat is one of the accepted options for the separate additional measure required on gas combi installations under Boiler Plus, not a universal requirement on its own.

Can an old boiler installation legally lack an interlock?

The requirement applies at the point a boiler is fitted or replaced under current regulations; an older installation that predates the current requirement may not have one, but any boiler replacement carried out now should bring the system up to the current standard.

Sources

This guide draws on the following primary sources, current as of 17 September 2026:

• Approved Document L, Volume 1 (England, 2021 edition incorporating 2023 amendments)
• GOV.UK: Boiler Plus final policy (BEIS, 2017)

Related guides

Best Boiler Deals is an independent guide. We may earn a fee from some links; this never affects what we write. This article describes a Building Regulations requirement in general terms; confirm current requirements with a Gas Safe registered installer before committing to any installation.