Nobody talks about this boiler fault

You don’t notice a gas boiler when it’s behaving. It’s the quiet heart of the house: pushing hot water to taps, keeping radiators steady, making mornings possible. The problem is the hidden failures - the ones that don’t trip a dramatic error code, but slowly bleed efficiency, comfort, and safety until the day it finally won’t fire.

Most people talk about pressure drops, frozen condensate pipes, or a noisy pump. Hardly anyone talks about the fault that sits in plain sight: the boiler works, but it’s lying to you about how well it’s burning.

The fault nobody mentions: “it still heats” combustion drift

Combustion drift is when the gas/air mix and flame pattern move away from where they should be. The boiler may still run, still make hot water, and still pass a quick “does it turn on?” test. But underneath, it can be burning less cleanly, less efficiently, and with more stress on components.

This is why it’s easy to ignore. There’s no puddle on the floor and no obvious bang. Instead you get little domestic clues that feel like weather, habits, or “this old place”.

Think of it like a car that starts every time but is quietly misfiring. You can drive it-right up until you can’t.

What it looks like at home (because it rarely announces itself)

The signs are annoyingly ordinary, which is why they’re so easy to write off:

• Hot water that goes warm–hot–warm in a single shower, especially when another tap opens.
• Heating that feels “spiky”: radiators hot fast, then the house cools sooner than it used to.
• A boiler that seems to cycle on and off more often, even in mild weather.
• A faint new smell near the boiler cupboard when it first lights (not always gas; sometimes “hot metal” or a dusty singe).
• Condensate that seems heavier than usual, or a plume outside that looks thicker than you remember.

None of these prove combustion drift on their own. The point is the pattern: small changes that arrive together, then become the new normal.

If you find yourself adapting-shorter showers, nudging the thermostat up, timing the washing up for “when it’s hot”-that’s the house telling you something.

Why it happens (and why the boiler keeps soldiering on)

A gas boiler is designed with tolerances. It can cope with slight variations in gas quality, airflow, and demand. Over time, though, a few common things stack up:

• Dust and debris affecting air intake or burner surface.
• Worn seals or gaskets allowing tiny leaks in the wrong places.
• Flue issues (including partial restrictions or poor terminal conditions outdoors).
• Ageing electrodes/sensors giving less reliable feedback.
• Incorrect settings after a previous repair, conversion, or part swap.

The boiler doesn’t “know” comfort; it knows signals. If those signals are slightly off, it can keep trying, cycling, and compensating-right up to the point where efficiency drops and wear increases.

The three-minute pause that catches it early

Before you book a service or start turning dials, do a tiny audit. It’s not a fix; it’s a way to stop guessing.

1. Listen at ignition. When it fires, does it sound smooth and consistent, or does it flutter and surge?
2. Watch one full hot-water run. Open a hot tap for two minutes. Does temperature stay stable without you fiddling?
3. Check the cupboard space (safely). Is it unusually warm, stuffy, or smelling “hot”? Don’t block vents or poke inside-just notice the air.

Write down what you observe. Engineers love clear symptoms more than vague dread.

What to ask for at the next service (so it isn’t just a quick clean)

A proper annual service matters, but you can still end up with a “yep, it runs” visit if the appointment is rushed. When you book, ask for specifics that relate to hidden failures, not just obvious breakdowns:

• Flue gas analysis with a calibrated analyser, and the readings recorded.
• Burner and combustion chamber inspection/clean where the manufacturer allows.
• Electrode and flame-sensing checks (condition, positioning, corrosion).
• Case seal and flue integrity checks (especially on room-sealed appliances).
• Confirmation that settings match the manufacturer’s spec after any parts have been replaced.

If an engineer says “it’s fine” but can’t show you readings or explain what they checked, that’s a sign to ask one more question.

A quick story: the boiler that “never failed” until it did

A neighbour in Leeds told me their boiler had been “reliable for years”, just a bit temperamental with showers. They’d learned the routine: run the hot tap first, don’t use the kitchen sink mid-shower, keep the thermostat a notch higher in winter.

When it finally locked out, the fix wasn’t exotic. The engineer found evidence of poor combustion and a set-up that had drifted; the boiler had been compensating for ages. The big change wasn’t the repair cost-it was the relief of not managing the house like a machine.

Hidden failures don’t always cost you in one dramatic invoice. They cost you in constant little adjustments.

Don’t DIY this part (and what you can do safely)

Combustion isn’t a “have a go” zone. Leave internal adjustments and anything involving the gas train, burner, or flue to a Gas Safe registered engineer.

What you can do:

• Keep the area around the boiler clear, so ventilation isn’t accidentally restricted.
• Note error codes, cycling behaviour, and when symptoms happen (hot water only vs heating).
• Check your boiler pressure only if your manual shows you how, and only adjust via the approved filling loop procedure.
• Fit a carbon monoxide alarm in the correct location and test it regularly.

If you ever suspect fumes, feel unwell near the boiler, or your CO alarm activates: turn off the appliance if safe, ventilate, and seek urgent help.

The point: it’s not about panic, it’s about catching the quiet drift

The most expensive boiler fault is often the one you normalise. A gas boiler can appear “fine” while hidden failures make it dirtier, thirstier, and more fragile.

The fix is rarely dramatic. It’s attention: a short audit, a better service request, and the confidence to say, “It runs, but it doesn’t feel right.”

FAQ:

• Is this the same as low boiler pressure? Not necessarily. Low pressure can cause heating issues, but combustion drift is about how the burner is mixing and burning gas and air. They can coexist, but they’re different problems.
• Will my boiler show an error code if combustion is off? Sometimes, but not always. Many boilers will keep operating within broad limits, which is why this can become a hidden failure.
• Can a dirty system (sludge) cause similar symptoms? Yes. Poor circulation can mimic cycling and uneven heat. A good engineer will separate “system-side” issues from “combustion-side” ones.
• How often should combustion be checked? Typically at the annual service, and any time parts affecting combustion are replaced. Ask for flue gas analysis readings to be recorded.
• Do I really need a carbon monoxide alarm if the boiler is serviced? Yes. Servicing reduces risk; a CO alarm adds a vital backstop, especially because some problems develop between visits.