Guide 01 · Boilers & gas
What a boiler service actually involves.
“Service” covers everything from a proper hour's work with a flue gas analyser to a man wiping the casing and writing you a certificate. Here is what the real thing looks like, so you can tell which one you have been sold — by Matt or by anybody else.
Step by step
Six things, in this order.
The paperwork before the spanners
A Gas Safe registered engineer should offer you their ID card without being asked. The front shows their photo, licence number and expiry date; the back lists the specific appliance types they are qualified to work on — boilers, cookers, fires, LPG and so on. An engineer registered for cookers is not automatically registered for your boiler. Thirty seconds of looking is the single most useful thing a homeowner can do.
Visual inspection and the flue
Before anything is taken apart: is the boiler installed where it should be, are the clearances right, is the flue properly sealed and terminating somewhere safe, and is there adequate ventilation. A surprising number of faults are installation faults that have been quietly getting worse since the day someone else fitted it.
Opening up and cleaning
The casing comes off and the combustion chamber, burner, heat exchanger and condensate trap are inspected and cleaned as needed. A blocked condensate trap is one of the most common reasons a modern condensing boiler cuts out — and one of the cheapest to put right if it is caught at a service rather than at seven on a January morning.
Combustion analysis
A flue gas analyser is put into the flue and the readings compared against the manufacturer’s figures. This is the test that says whether the boiler is burning cleanly and safely, and it is not something that can be judged by eye. If an engineer services a boiler without using an analyser, you have not had a service.
Pressures, controls and safety devices
Gas pressure and gas rate are checked against the data badge, the system pressure and expansion vessel charge are checked, and the safety devices, thermostat and controls are tested. Then the boiler is run and watched — a fault that only shows up under load will not show up standing still.
The handover
You should be told what was found, what was done, and what is likely to need attention next — in language you can repeat to someone else. Anything unsafe must be reported to you and, if it is dangerous, the appliance is turned off and labelled. That is not the engineer being difficult; it is the law working as intended.
Warning signs
Six symptoms worth a phone call.
None of these mean disaster. All of them mean the system is telling you something, and all of them get more expensive the longer they are ignored.
The pressure keeps dropping
A system that needs topping up every few weeks is losing water somewhere — a small leak, a weeping radiator valve, or a failed expansion vessel. Topping it up forever quietly corrodes the system from the inside.
Radiators cold at the bottom
Sludge, not air. Bleeding will not fix it. Depending on how far it has gone you need a chemical clean or a power flush, then a magnetic filter so it does not come back.
Banging, kettling or gurgling
Kettling — a noise like a kettle coming to the boil — usually means scale or debris on the heat exchanger. It gets worse, and it shortens the life of the most expensive part of the boiler.
Hot water that runs lukewarm
On a combi this often points at the diverter valve or a scaled plate heat exchanger. Both are repairable; neither fixes itself.
Repeated lock-outs
A boiler that needs resetting more than very occasionally is telling you something. Resetting it repeatedly is not a fix — and on some faults it is actively unwise.
Any smell of gas
Not a servicing matter. Open windows, do not touch switches, leave the building and ring the National Gas Emergency Service on 0800 111 999 straight away.
If you smell gas
Do not ring a plumber first. Open doors and windows, do not operate any electrical switches, turn the gas off at the meter if you can reach it safely, leave the building and ring the National Gas Emergency Service on 0800 111 999. They are free, they are available 24 hours, and they will make it safe. Matt can come afterwards and put it right.
Repair or replace?
The honest version of a question with a vested answer.
Anyone quoting you for a new boiler has an interest in the answer, which is exactly why the reasoning should be laid out rather than asserted. The rough shape of it: if the boiler is relatively young, parts are still available, and the failure is a single component, repair. If it is old enough that parts are scarce, has had multiple different faults in a short period, or the heat exchanger itself has gone, replacement usually wins on cost over any sensible timescale.
The bit people miss is the system around the boiler. Dropping a new boiler onto a system full of sludge is how you buy a second problem: the new heat exchanger blocks, the warranty gets awkward, and the fault comes back wearing a different hat. A proper clean and a magnetic filter as part of the install is not an upsell, it is the difference between a boiler that lasts and one that doesn't.
When Paul Charlish had his boiler replaced after the old one developed what he called a terminal problem, the part of the review worth noticing isn't “did an excellent job” — it's “explained everything clearly”. That is what this section is for.
Five questions to ask any gas engineer
- 01Can I see your Gas Safe ID card — and does the back cover this appliance type?
- 02Will you be using a flue gas analyser as part of the service?
- 03Is the fault repairable, and what would the repair cost against a replacement?
- 04If you are recommending a new boiler, why this model and this size for this house?
- 05What have you found that is not urgent today but will need doing next?
Matt would rather be asked all five. An engineer who bristles at any of them is telling you something useful.
Book a service, or get a straight answer.
Natural gas or LPG, service or repair or replacement. Ring Matt, or send a photo of the boiler's data badge on WhatsApp and he can tell you a lot before he even sets off.