Most boilers fail in November. Not because November is special, but because that's the first month of the year where the boiler is running flat-out for hours every day after seven months of mostly idling. Anything that's been quietly drifting out of spec since spring shows up the moment heavy demand hits.
That's why annual servicing exists — and why the right time to book one is late summer or early autumn, before the weather turns and every engineer in South Yorkshire is fully booked with breakdown callouts.
Most boilers fail in November — the first month of the year where they're running flat-out for hours every day after seven months of mostly idling. Anything that's been quietly drifting out of spec since spring shows up the moment heavy demand hits.
A service is £80 with us. Customers occasionally ask what they're getting for that. Here's the full list.
The 14-point service inspection
1. Visible inspection — boiler, flue, terminations
Before we touch anything, we look. Soot marks around the casing, water staining, paint discolouration on flue pipes, and corrosion at the terminations all tell a story before we open anything up. Most of the boilers we condemn for safety reasons fail at this step.
2. Gas pressure and gas rate
We use a manometer to measure inlet gas pressure (should be ~21 mbar standing, ~19+ mbar working). If it's significantly low, the boiler can't fire properly and we trace back to find the issue — meter regulator, pipework size, or a partially-closed valve we'll find buried in a kitchen unit.
We then check the gas rate by timing how much gas the boiler burns over a known period. This must match what the manufacturer specifies for that model. If it's outside spec, the boiler is either over-firing (waste, hotspots) or under-firing (incomplete combustion, soot, CO).
3. Flue gas analysis
Two readings: combustion ratio and CO content. The combustion ratio (CO/CO2) tells us how cleanly the boiler is burning. CO content tells us how much unburnt fuel is reaching the flue.
A healthy modern condensing boiler runs at around 0.001–0.004 ratio. Anything above 0.008 is a fail. CO above 200 ppm is a fail. We log every reading on our service report — you get a copy.
4. Heat exchanger inspection
We open up the casing and look at the heat exchanger directly. We're looking for soot deposits (incomplete combustion), white powder deposits (limescale carryover), green corrosion (combustion chamber leaks), and any visible cracks. Soot can be cleaned with a brush; cracks mean the boiler is end-of-life.
5. Burner clean
The burner is taken out, brushed off, and reseated. On older boilers especially, the injection ports clog with combustion debris and reduce flame quality. Five minutes of brushing makes a real difference to combustion ratio readings on the way back in.
6. Condensate trap and drain
The condensate trap collects acidic water from the heat exchanger. It needs to be at the right level — too low and combustion gases escape into the room; too high and the heat exchanger floods. We empty, flush, and refill it with fresh water. We also check the condensate drain runs freely (frozen condensate drains are the single most common cause of "boiler stopped working" callouts in January).
7. Expansion vessel
A pressurised vessel inside the boiler that absorbs the expansion of heated water. The internal diaphragm fails over time, usually after 6–8 years. We test pressure with the system depressurised — should match manufacturer spec, typically 0.75–1.0 bar. If it's low or zero, the vessel is failed and needs re-pressurising or replacing.
8. System pressure and PRV operation
System (heating circuit) pressure should be ~1.0–1.5 bar cold, ~2.0 bar hot. We check it's holding overnight by comparing with the previous service report. We also test the pressure relief valve (PRV) by gently lifting it — if it drips after, it needs replacing.
9. Pump operation
We listen to the pump under load. Bearings make a different noise when they're nearing the end. We also check the pump head matches what the system needs — undersized pumps cause radiators to be hot at one end and cold at the other.
10. Diverter valve / 3-way valve
On a combi, this switches between heating and hot-water modes. They stick. We test by running heating, then a hot tap, and listening/feeling for the switchover. A sticking diverter shows up as "lukewarm shower while heating is on" or vice versa.
11. Magnetic filter inspection
If you have one (most modern installs do), we open and inspect. The amount of magnetite trapped tells us how the system is wearing internally — black sludge means iron oxide from radiators, which corrodes pumps and heat exchangers. Heavy build-up means we recommend a chemical flush.
12. Thermostat and controls
We check the room thermostat is reading accurately, the boiler responds to it, and any timers/programmers are working as set. If you've got a smart thermostat, we check it's still talking to the boiler reliably.
13. Radiator survey
Quick walk-around. Check none are cold at the top (air bleeding needed) or cold at the bottom (sludge build-up). Note temperature differential across each — if some are cooler than others, the system is unbalanced.
14. Service report and Gas Safe sticker
Everything we tested gets recorded. You get a service report by email and a dated sticker on the boiler. Manufacturers require this for warranty validity — without an annual service record, most warranties are voided.
What we'd flag before it becomes a callout
The most common things we find on services that aren't urgent today but will become callouts within 12 months:
- Sludge build-up in the magnetic filter — chemical flush recommended (~£300, vs a new pump at £400 + labour later)
- Expansion vessel pressure low — re-pressurise now or replace within the year
- System pressure dropping overnight — small leak somewhere; we'll trace it
- PRV slow drip — needs replacing before it sticks open and dumps your system pressure
- Condensate drain near a frost point — lag it before December
We'll tell you what's urgent versus "fine for another year". A service isn't a sales pitch — most of ours come away with a report saying "all good, see you next year".
When you should book
Late August through October is the sweet spot. By November we're juggling breakdown callouts; by January we're booked up two weeks out. If you book early autumn, you also catch any issues before you actually need the heating.
Book an annual service — £80, takes about 45 minutes, you get the report and the sticker, and you stop worrying about a November surprise.
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