Gas Safe rules for boiler repairs: what you can and cannot do yourself
A boiler has parts you're free to replace and parts the law reserves for a Gas Safe registered engineer. This page sets out where that line falls, component by component.
What you can fix yourself, component by component
The table sets each internal boiler component against the legal position on replacing it, and the regulation behind that position. Where the boiler casing has to come off to reach a part, the law treats that as work on the gas appliance itself, whatever the part actually does.
| Boiler component | Who can do it | Governing rule | What to check first |
|---|---|---|---|
| Room thermostat or programmerA qualified electrician should handle any new circuit. | Usually fine to replace yourself | Sits outside the gas appliance, so gas safety law does not apply; new wiring falls under Part P instead | Isolate the mains supply before you start |
| Expansion vesselSome older systems use an external vessel on the pipework, which changes the picture | Topping up pressure is DIY; swapping the vessel is not | The vessel is usually inside the casing, and opening the casing counts as work on a gas appliance under the Gas Safety (Installation and Use) Regulations 1998 | Check whether your vessel is external and bolted on, or built into the body |
| Circulating pumpA separate in-line pump on open pipework sits outside this restriction | Gas Safe registration required | Gas Safety (Installation and Use) Regulations 1998 covers the casing the pump sits behind | Confirm whether the pump is built into the boiler body or fitted as a separate in-line unit |
| Diverter valveSystem boilers with a separate cylinder can differ, so check the specific model | Gas Safe registration required on most combi boilers | Gas Safety (Installation and Use) Regulations 1998, since the valve sits close to the gas valve assembly on a combi | Ask whether your model routes the valve through the gas-carrying section |
| Pressure relief valveThe set pressure is stamped on the valve body itself | Gas Safe registration required | Gas Safety (Installation and Use) Regulations 1998 covers the casing the valve sits behind | Note the valve's set pressure before removal so the replacement matches |
| Printed circuit boardFitting a compatible board correctly is a separate question from who is allowed to fit it | Gas Safe registration required | Gas Safety (Installation and Use) Regulations 1998, alongside a separate electrical isolation risk that falls to a competent electrician | Isolate the electrical supply fully before the panel is opened |
This is general guidance, and the position can shift with model and installation type. Check the fault code or part page for your own appliance before deciding, since design differs between manufacturers and even between model years.
The Gas Safety (Installation and Use) Regulations 1998
Gas appliances can kill quietly, through a leak or a build-up of carbon monoxide, which is why work on them sits under a specific law. The Gas Safety (Installation and Use) Regulations 1998 set out who is allowed to carry out gas work and what has to be in place before and after it is done.
What the regulations require
Any work on a gas fitting or appliance, including a boiler, must be carried out by someone competent to do it safely. In practice that means a Gas Safe registered engineer working within the categories their registration covers. The regulations also cover the condition of the appliance itself, which must be installed and maintained so that it does not put anyone at risk, and they set out specific duties for landlords around annual checks and record-keeping on the properties they let.
Who the regulations apply to
- Anyone carrying out gas work, paid or unpaid, on a domestic boiler or other gas appliance.
- Landlords, who are responsible for the safety of gas appliances in a let property and for keeping the records the law requires.
- Employers and self-employed engineers, who must hold registration for the specific type of work they take on.
- Homeowners, in that the regulations restrict who may legally carry out gas work on a boiler even where the owner holds the appliance outright.
The line the regulations draw is around gas work itself. A task that does not touch the gas supply, the flue, or a gas-carrying component sits outside that line, and that boundary is what the rest of this guide works through component by component.
What happens if the regulations are not followed
Breaching the regulations is a criminal offence. The Health and Safety Executive enforces them and can prosecute anyone who carries out gas work without the competence the law requires, whether that is an unregistered tradesperson or a landlord who let unsafe work go ahead. A conviction can carry a fine, a prison sentence, or both, and a landlord found in breach also faces the separate consequences that follow from letting a property with unsafe gas appliances.
Questions about Gas Safe rules and DIY boiler work
What actually counts as 'gas work' on a boiler?
Any job that involves connecting, disconnecting, or adjusting a part that carries gas, controls combustion, or affects the flue counts as gas work under the Gas Safety (Installation and Use) Regulations 1998. That covers the gas valve, burner, injectors, and flue joints, along with anything where a mistake could let gas or combustion fumes escape. This work must be carried out by a Gas Safe registered engineer.
Can I replace a part myself if it doesn't touch the gas supply?
Some components sit entirely outside the gas circuit, such as a room thermostat, a circulating pump, or a printed circuit board (PCB) once the boiler is isolated. Isolating the electrics correctly still carries its own risk, and that falls under Part P of the Building Regulations. Whether a given part is genuinely DIY-permissible on your boiler depends on how it's wired into that particular model, which is worth checking before you start on a boiler PCB replacement.
Do landlords have extra legal duties around gas safety?
Yes. A landlord has to arrange an annual gas safety check on every gas appliance and flue in a rented property, carried out by a Gas Safe registered engineer, and keep the record and give a copy to the tenant. That duty applies on a fixed schedule regardless of whether a fault has actually been reported.
What happens if gas work is done by someone who isn't Gas Safe registered?
It's a criminal offence under the Gas Safety (Installation and Use) Regulations 1998, and it can also void a home insurance policy or a landlord's own compliance record. The restriction exists because a badly fitted gas-carrying part can leak gas or let carbon monoxide go undetected, and neither shows up until something has already gone wrong.
Can I reset my boiler or check the pressure myself?
Yes. Resetting a boiler after a lockout, topping up the system pressure, and bleeding a radiator don't involve opening any gas-carrying part, so none of them fall under Gas Safe restrictions. If a fault code keeps coming back after a reset, that usually points to a component that needs identifying, and a fault code lookup is the quicker way to find out which one.
If a repair mixes a gas part and a non-gas part, does the whole job need a Gas Safe engineer?
Usually, yes. A single repair often touches restricted and unrestricted parts together, such as a diverter valve alongside a room thermostat, and where the two are connected the safer approach is to treat the whole job as gas work.
Anyone doing gas work on your boiler must be Gas Safe registered.
A Gas Safe ID card carries the engineer's registration number and lists the categories of appliance they're qualified to work on, so it's worth checking before anyone starts.