Boiler service explained

An annual service is the cheapest thing you can do for a boiler and the first thing most households skip. It is not the same as a repair visit. A service is a set of measurements and checks that confirm the appliance is still burning gas safely and that the parts prone to failure are sound.

What a service actually covers

The engineer opens the case, cleans the burner and heat exchanger where the design allows, inspects the flue and its seals, and measures combustion with a gas analyser instead of judging the flame by eye. Colour is a poor guide, since a flame can look healthy while the appliance produces carbon monoxide. Water pressure, the expansion vessel charge, the condensate trap and the safety interlocks all get checked. Most visits take under an hour, and a service is not the moment to diagnose an intermittent fault; book a separate repair visit for that.

On a sealed system the engineer will usually read the inhibitor level and look at the magnetic filter, because a system full of sludge is what kills heat exchangers. Ask for the report. A proper service records combustion readings and flue gas values, and those numbers are the point of the visit. If the engineer cannot tell you what the readings were, you have paid for a visual look.

• Flue gas analysis - The core measurement. It shows whether the appliance burns cleanly.
• Condensate trap - Cleaned and checked. A blocked trap stops the boiler running.

Why gas work is not a DIY job

Anything inside a gas appliance is work for a registered gas engineer. That covers opening the combustion case, adjusting the gas valve, replacing a fan or a circuit board, and any pipework carrying gas. It applies to cookers, gas fires and wall heaters as well as boilers, and to the pipe feeding them. There is no safe way to test your own work here, because the failure mode is odourless and invisible.

Carbon monoxide has no smell and no colour, and a boiler can run for months while producing it in small amounts. Symptoms look like flu: headaches, tiredness, nausea, and feeling better when you leave the house. Anyone who suspects a leak should turn the appliance off, open windows, leave the building and call the gas emergency line for their area from outside. Resetting a fault code or bleeding a radiator is fine. Everything behind the case is not.

• Safe yourself - Reset a fault code, top up system pressure, bleed radiators.
• Never yourself - Open the case, touch the gas valve, move gas pipework.

Signs to call someone now

A boiler that needs attention rarely fails completely without giving hints. Repeated pressure loss, radiators that need bleeding every few weeks, a pump that whines and a boiler that locks out on cold mornings all point at a system fault rather than a controls problem. A pressure gauge that climbs when the heating is on and drops when it cools usually means the expansion vessel has failed, and one that already reads high when cold points at a filling loop left open.

Some signs carry more urgency. A yellow or orange flame instead of blue, black sooty marks around the case, a smell of gas or a carbon monoxide alarm sounding all mean stop using the appliance. Do not run it again to see whether the fault repeats. Water leaking from inside the case is also a call-now problem, since it usually means a seal has failed and electrical parts sit close by. Persistent headaches that ease when someone leaves the house are a carbon monoxide question, not a medical one.

• Yellow flame - Incomplete combustion. Turn the appliance off and book an engineer.
• Gas smell - Do not touch switches. Open windows, leave, then call from outside.

Choosing and checking an engineer

In the UK, work on gas appliances must be done by someone on the Gas Safe Register, and the register lets you check a specific person rather than a company name. Ask for the registration number and look it up before anyone starts, because a firm can be registered while the individual sent to your house is not. Landlords must have a gas safety record issued every year for each appliance they let, and tenants should be given a copy when the tenancy starts.

Ask what the service includes. Some advertised services are a visual check and a flue reading with no cleaning, which is a different product. Get a written price before the visit and ask what happens if parts are needed. Paying a large deposit up front leaves you with no hold over the firm if the work is delayed. Recent reviews and local references tell you more than a glossy van. Cheap quotes that skip cleaning are common, so ask whether the burner and heat exchanger are included.

• Check the register - Look up the individual engineer's number, not only the company's.
• Keep the paperwork - Warranties usually require annual servicing, and insurers may ask for history.