Which boiler part can you replace yourself, and what is work for a certified gas engineer? Where the line lies per part

Short answer: anything outside the sealed combustion section of the boiler that does not touch gas, flame or flue gas, you may in principle replace yourself: room thermostat, thermostatic radiator valve heads, condensate trap, air vent, an external expansion vessel, filling hose and the filters of your ventilation. Everything involving the gas valve, burner, ignition and ionisation probe, fan, heat exchanger, flue and air supply, and recommissioning after such a repair, is work for a certified gas engineer; in the Netherlands, for example, this has been a legal requirement since 1 April 2023. In between lies a grey area of pump, sensors, three-way valve and PCB: technically feasible for an experienced DIYer, but with risks for warranty and insurance, which is why we recommend an installer there.

The rules in one paragraph

In many countries only certified companies or engineers may work on gas appliances: boilers, gas water heaters, gas heaters, gas fires and the associated flue and air supply; check the rules in your country. In the Netherlands, for example, since 1 April 2023 only companies with a valid CO certificate (based on BRL 6000-25) may carry out this work. Work here covers installing, commissioning, servicing and repairing. The rules are aimed at companies; the aim is that uncertified engineers no longer work on gas appliances. For you as a resident it means in practice: if you call in an engineer, check their certification; if you do something to the combustion section yourself, you act against the intent of the rules, lose the manufacturer's warranty and an insurer may refuse to pay out in case of damage. Carbon monoxide is the reason: an incorrectly refitted gasket or a misaligned electrode cannot be seen, but you only notice it when it is too late.

Three levels, per part

What "installer recommended" means

For the middle group the law does not forbid you anything and the part is not dangerous in itself. Still, we recommend an installer, for three reasons. One: the boiler has to be drained and, after fitting, refilled, bled and brought up to pressure, and a leaking O-ring on a pump drips straight onto the PCB below it. Two: sensors and PCBs come in versions per year of manufacture; the wrong NTC gives a boiler that heats too hot or keeps cutting out. Three: the manufacturer's warranty on the appliance lapses if unauthorised repairs have been carried out, and in case of water damage an insurer may ask who replaced the pump. Weigh that against the labour cost of an hour of an installer's time.

If you do it yourself anyway, the basic rules apply: unplug the boiler, close the gas tap, let the boiler cool down, release the pressure via the drain cock, take photos of every connector before you disconnect it, use only the original specified part and fit new O-rings or gaskets from the set. Order the matching gasket set at the same time; it is listed per brand under gasket sets.

What is always allowed: the diagnosis

The line lies at working on it, not at looking. You may always look up the code yourself, read the water pressure, check the thermostat, listen to what the boiler does on start-up, feel which pipes get warm and photograph the rating plate. With that information you determine which part is probably faulty and order it in advance, so that the engineer can finish in one visit. Our guides are built around this: first rule out what is free (air, pressure, thermostat, condensate trap), then the part. Start at Troubleshooting a boiler fault.

How to find a certified engineer

In many countries every certified company or engineer is listed in a public register with a certificate number you can check; for the Netherlands, the Dutch government explains how on the page about carbon monoxide. Instalpunt does not install; have the repair carried out by a qualified local installer installation service. Order the part from us in advance and give the engineer the part number; that saves a second visit.

Frequently asked questions

Can I work on my own boiler as a private individual?

Rules on gas work differ per country, and in many countries only qualified, registered professionals may work on gas appliances. Either way, working on the combustion section without measuring equipment and know-how is dangerous, voids your warranty and can affect your insurance cover if damage occurs. That is why we never recommend it.

Can a non-certified installer replace my pump?

A pump is outside the gas combustion section, so strictly speaking, yes. But a company that works on boilers without the proper qualification is a bad sign; most reputable installers are certified these days.

Does my warranty lapse if I replace the condensate trap myself?

No. The condensate trap is a maintenance part that the manufacturer itself describes as a user task, just like topping up and bleeding. The warranty lapses when the appliance itself is worked on by unqualified parties.

How do I know whether a part is in the combustion section?

Everything inside the sealed casing behind the burner door, or anything that gas or flue gas passes through: gas valve, venturi, fan, burner, electrodes, heat exchanger, flue bend. If you are unsure about a specific part, ask us and quote the part number.