Boiler 14 August 2026

What Does a Boiler Service Actually Include? A Gas Safe Engineer's Checklist

A proper boiler service is a combustion test, not a look. It means taking the case off, measuring the gas rate at the meter, putting an analyser probe in the flue and reading what the boiler is actually burning — then cleaning the parts the manufacturer specifies and checking every safety device works. If nobody opened your boiler and nobody produced a set of readings, you did not have a service.

That distinction matters more than it sounds, because “boiler service” is not a protected term. We are called out to plenty of boilers across Kent that were serviced a few months earlier and are running badly rich, or have a perished case seal, or a condensate trap full of sludge. In almost every case the previous visit lasted a quarter of an hour and produced nothing in writing.

Here is what our engineers actually do on the visit, in the order we do it.

What Happens During a Boiler Service?

1. The boiler’s history, before anything is touched

We take the make, model and GC number off the data badge, and ask when it was last serviced and whether anything has been playing up — intermittent lockouts, a pressure drop you keep topping up, radiators that need bleeding every few weeks. Those symptoms usually point somewhere specific, and knowing them before the case comes off saves working blind.

2. Gas tightness and working pressure

We check the gas installation for soundness and take the standing and working pressure at the appliance. A boiler starved of gas because of an undersized or partly blocked supply will never burn correctly, no matter how clean the burner is. This is also where a leak on the pipework upstream of the boiler gets found — nothing about that is a DIY job, and it is the reason all gas work must be done by a Gas Safe registered engineer.

3. The gas rate

We time the meter dial against the appliance’s rated input from the data badge. If the boiler is over-gassed it will sooth and stress the heat exchanger; under-gassed and you get poor hot water and nuisance lockouts. Very few 20-minute “services” ever measure this.

4. Flue gas analysis

The analyser probe goes into the flue sampling point and we read carbon monoxide, carbon dioxide and the CO/CO₂ ratio, at both low and high fire. This is the single most important measurement in the whole visit, because it tells you whether combustion is safe and whether the appliance is burning cleanly — and it is the one thing a visual check cannot fake. The readings get written down, so next year’s engineer can see whether anything has drifted.

5. Case seal and combustion chamber

Modern boilers are room-sealed: the case seal is a safety component, not a dust cover. If it has hardened or been refitted with a screw missing, combustion products can spill into the room. We check the seal, the seal face and that every fixing is present, then inspect the burner, injector, electrodes and heat exchanger, and clean them where the manufacturer’s instructions call for it.

6. Flue and terminal

We check the terminal outside for clearance from windows, doors and boundaries, look for staining or damage, and inspect the flue run itself. Where a flue passes through a ceiling or a void, inspection hatches are required so the joints can be seen — if they are missing, that has to be dealt with rather than ignored.

7. Safety devices

Flame supervision, overheat thermostat, pressure relief valve, and the expansion vessel charge pressure checked against a drained system. A flat expansion vessel is behind a huge proportion of the “pressure keeps dropping and then blowing off” calls we get, and re-charging it takes minutes during a service versus a separate visit later.

8. The water side

System pressure, condensate trap cleaned and refilled, condensate route checked for frost risk before winter, and the magnetic filter emptied and rinsed if one is fitted. What comes out of that filter tells us a lot about the health of the system — heavy black sludge means the inhibitor has given up and the heat exchanger is on borrowed time.

9. Test, and a written record

The boiler is run through both heating and hot water, final readings taken, and you get a record of what was checked and what was measured, plus anything we think will need attention before next year.

How Long Does a Boiler Service Take?

Done to that standard it is not a ten-minute job — we cover the realistic timings and what stretches them in how long does a boiler service take. The short version: allow a decent slot, and be suspicious of anyone who is back in the van almost immediately.

What a Boiler Service Is Not

A gas safety certificate (CP12) is not a service. It is a safety inspection a landlord must have carried out every 12 months, and it does not include cleaning or the maintenance work above. Plenty of rental properties get a certificate every year and never actually get serviced, which is why we usually recommend landlords book both together.

A service is also not a repair. If we find a fault during the visit — a failed diverter valve, a scaled heat exchanger, a dying pump — we will tell you what it is and what it needs, but putting it right is separate work. Those jobs are covered under boiler breakdowns and repairs, charged at £95 + VAT for the first hour and £47.50 + VAT per hour after that, with any materials agreed with you before we fit or order them.

And if the boiler turns out to be beyond sensible repair, we will say so plainly rather than keep patching it, and give you a free fixed written quote for a new boiler.

What It Costs With Us

A boiler service is a flat £95 + VAT, whatever the make, age or where it is hidden. If we service a second gas appliance on the same visit — a gas fire, a cooker, a second boiler — that one is half price. Booking them together is worth doing; it is the same journey for us, and gas fires are the appliance most often left unchecked for a decade.

Book It Before Everyone Else Does

Autumn is our busiest stretch, because the whole county switches the heating on in the same fortnight and finds out what went wrong over the summer. Servicing in spring or summer means easier appointments, and if something does need fixing you have months to sort it rather than days.

Call 01732 792 454 or book a boiler service and we will sort a slot that suits you. We are Gas Safe registered under number 951167.