What a gas boiler service should include, and how to tell it did not happen

Inspection before maintenance

The sequence matters. What the initial checks reveal determines what the rest of the visit involves – and cleaning before measuring destroys the evidence of how the appliance has actually been running.

1. Check the surroundings before touching the applianceFlue route and terminal position, ventilation provision, clearances, compartment condition, and whether anything has changed since last time – a new extractor, a blocked airbrick, a boxed-in flue.
2. Run it and observeIgnition sequence, flame picture where visible, controls, any unusual noise, and how it behaves through a cycle.
3. Combustion analysisAn electronic flue gas analyser measures the ratio of carbon monoxide to carbon dioxide. This is the single most important output of the visit – an objective measure of whether the appliance is burning correctly.
4. Gas rate and operating pressureChecked against the appliance data plate. An appliance running outside its rated gas rate is not set up correctly, whatever else looks fine.
5. Flue integrityAlong its accessible length, including inspection hatches where they exist. A flue that has separated in a loft void discharges combustion products into the building.
6. Safety device operationFlame supervision, overheat protection, and any other protective device the appliance relies on.
7. Then clean and adjustCondensate trap cleaned, heat exchanger and burner cleaned to the manufacturer’s specification, seals checked and replaced as needed, electrodes inspected.
8. Recommission and recordAnalyser reading taken again after cleaning, and the findings written down.

The condensate trap, which gets skipped

Worth its own mention because it causes several apparently unrelated faults.

A condensing boiler’s condensate trap collects acidic condensate before it drains. Over a year it accumulates a sludge of combustion residue. When that trap partially blocks, the boiler cannot drain properly, and the resulting fault codes are frequently not obviously condensate-related – ignition lockouts, fan or air pressure faults, and flame loss during operation all appear.

Cleaning the trap is a standard part of a service and it is one of the more commonly omitted steps, because it is dirty and fiddly. If your boiler has a history of intermittent lockouts and the trap has never been cleaned, that is worth mentioning.

The readings, and what they mean

An analyser prints a ticket. Ask for it and keep it, because the year-on-year trend is more informative than any single visit.

Service versus safety check

These are routinely conflated and they are different pieces of work.

A landlord’s legal duty is the check. A manufacturer’s warranty condition is the service. Most landlords want both, and they are commonly done in one visit – see landlord gas safety certificates.

How to tell it did not happen

1. It took a few minutes. A proper service cannot.
2. You were given no readings, and no ticket from an analyser.
3. Nothing was opened up. A service produces waste – old seals, soot, condensate trap contents.
4. The engineer never looked at the flue, or at any inspection hatches.
5. Ventilation was not mentioned, and no airbricks were checked.
6. The benchmark or service record was not completed, or was signed without entries.
7. Nothing was said about the system – pressure, filter, inhibitor.
8. You received a sticker and an invoice and nothing else.

None of these individually proves bad faith. Together they describe a visit that did not do what a service does, and it is entirely reasonable to ask about them.

What else should get mentioned

A service visit is the natural moment to look at the things that will cause the next call-out. A good engineer covers these without being asked:

• System pressure and whether it has needed topping up – which if it has, means a leak.
• The magnetic filter, emptied and what came out of it reported – see magnetic system filters.
• Inhibitor level, tested rather than assumed.
• The condensate route, particularly any external section, before winter rather than during it.
• Controls – whether they are set sensibly, and whether the flow temperature could come down.
• Any CO alarm in the property, present and in date.
• Radiator performance, if anything is obviously cold.

Those are five minutes of attention that prevent a disproportionate number of winter call-outs.