Ah, your boiler’s decided today is the day to go on strike - no ignition, no firing up, and the house feels like a fridge.
Annoying, but don’t panic: most “won’t ignite” dramas come down to something boring (and fixable) like power, settings, a safety lockout, low pressure, a frozen condensate pipe, or one tired part refusing to cooperate.
In this guide, we’ll keep it safe and simple: the quick checks you can do at home, what those lights/fault codes actually mean, the most common reasons boilers fail to fire, and the point where it’s officially an engineer job.
And if your boiler’s an ageing legend that’s costing more in callouts than it’s worth, we’ll also cover when replacement is the smarter move.
⚠️ Safety first:
Before you do anything, let’s keep this sensible (and safe).
If you smell gas, don’t start troubleshooting.
If it’s safe to do so, turn off the gas at the meter, open windows, and avoid switches, plugs, naked flames, or anything electrical.
Then call the National Gas Emergency Service on 0800 111 999 straight away.
If you suspect carbon monoxide - think headaches, dizziness, nausea, or a CO alarm sounding - get everyone outside immediately and call emergency help.
And one clear rule: if the “fix” involves opening the boiler casing, stop. That’s Gas Safe engineer territory.
If you’re not sure, don’t guess - a Gas Safe engineer can test it properly and safely.
🔑 Key Takeaways:
Start with safety - no gas smells or CO worries? Proceed.
Run the quick checklist - power, gas, pressure, condensate, air.
Decode any fault codes via your manual.
Match symptoms to causes for clues.
If resets fail or issues repeat, engineer it is.
Old boiler blues? Consider a replacement for peace of mind.
Not sure? A Gas Safe Engineer sorts it safely.
Quick Checks:
Right - let’s get practical. This is a safe, no-drama checklist you can work through without taking the boiler apart.
Go one step at a time, and don’t skip ahead.
Power & controls:
Boiler switch: Is it definitely switched on at the wall and on the boiler front panel?
Fuse box: Has a fuse/MCB tripped? If it has, reset it once. If it trips again, stop and call an electrician/engineer.
Thermostat/programmer: Make sure it’s actually calling for heat (turn the thermostat up a few degrees and check the schedule).
Smart thermostat: Check it’s online, not in Away/Holiday, and the batteries (if it uses them) aren’t dead.
Reset (once): Try a single reset only, following your manual. If it locks out again, that’s the boiler telling you it needs attention.
Gas supply:
Do other gas appliances work? If your hob/cooker works, the gas supply is probably fine.
Prepay meter: Check you’ve got credit/emergency credit.
Isolation valve: Make sure the gas isolation valve is open (handle typically inline with the pipe when open).
Nothing gas works? That points to a supply/meter issue - contact your supplier.
Water pressure (sealed systems):
Check the gauge: Many boilers like roughly 1.0–1.5 bar when cold (varies by model - your manual wins).
Low pressure: You may need to top up via the filling loop - follow the manufacturer instructions carefully and don’t overdo it.
Keeps dropping? Repeated pressure loss usually means a leak or component issue (PRV/expansion vessel, etc.) - engineer time.
Frozen condensate pipe (classic UK winter issue):
Typical signs: Boiler tries to start, maybe gurgles, then locks out, often during/after freezing weather.
Safe thaw: Use a hot water bottle or warm (not boiling) water along the outside pipe. Then try one reset.
Stop it happening again: Insulate the pipe or have an engineer reroute/upgrade it if it’s a repeat offender.
Air in radiators / circulation quirks (heating side):
This doesn’t always stop ignition completely, but it can cause poor flow and heating issues.
If radiators are cold at the top, bleeding may help (radiator key + cloth).
But if you’re hearing strange pump/fan noises or it’s repeatedly failing to fire, don’t keep poking - call a Gas Engineer.
🎥 You may be interested in Gas Engineer Allen Hart explaining the most common boiler problems and how you can resolve them in the video below:
What your boiler is telling you: fault codes & lockouts
Boilers look like dumb white boxes, but they’re actually drama queens with a safety conscience.
If yours won’t fire, it may have gone into lockout - basically the boiler saying: “Nope. Something isn’t right, and I’m not risking it.”
So don’t guess. Check the display for a fault code, grab the manual (paper copy, PDF, or a quick search), and match the code to what it’s complaining about.
The usual suspects:
Ignition lockout / flame failure – it can’t light the burner, or it lights but can’t “see” the flame (think ignition/flame sensor, gas supply, or related controls).
Fan / air pressure switch – the boiler can’t prove it’s moving air safely (flue/airflow safety checks), so it won’t ignite.
Low pressure – if system pressure is too low, many boilers refuse to fire because it’s not safe to run.
Overheat / poor circulation – the boiler’s getting too hot too quickly (often circulation/pump issues, sludge, blockages, or a heat exchanger problem).
Condensate issue – a blocked or frozen condensate pipe can trigger lockouts, and it’s not just a winter thing.
Important: don’t get stuck in the “reset roulette” loop. Try one reset (following the manual). If the fault code comes straight back, that’s your cue - stop pressing buttons and book a Gas Safe engineer.
Common causes: symptoms & likely suspects
Boiler not igniting? Here’s the cheat sheet. Match what you’re seeing to the most likely culprit (without turning it into a full-time hobby).
Nothing happens at all
Usually power or controls: switched off at the wall, tripped fuse/MCB, dead thermostat batteries, programmer not calling for heat, smart stat stuck in Away.
Fan runs, you hear clicking, then it gives up
Classic ignition / flame-sensing territory: gas supply issues, ignition leads/electrodes, flame sensor, or the PCB (the boiler’s brain) having a moment.
It starts… then cuts out… then tries again
Often a condensate problem (blocked/frozen), an airflow/flue safety check failing, or a sensor/pump/circulation issue causing it to shut itself down.
Hot water works, but heating doesn’t
Combi: diverter valve could be stuck (it can’t “choose” radiators).
System/regular: could be a zone valve, programmer, room stat wiring, or motorised valve issue.
Pressure is low (or keeps dropping)
Often a leak somewhere, a PRV dripping, or an expansion vessel that’s lost its charge. If you top up and it drops again, that’s not “one of those things” - it needs investigating.
When it’s an engineer job
Here’s the line in the sand. If you’ve done the safe checks and it’s still not firing, don’t keep poking.
Call a Gas Safe engineer if:
It locks out again after one reset (no reset marathons).
You smell gas, feel unwell, or a CO alarm goes off.
Pressure drops repeatedly or you’re topping up often.
Fault codes return immediately.
You suspect fan/flue issues or anything gas valve related.
The boiler’s getting old and breakdowns are becoming a recurring subscription.
With gas appliances, “better safe than sorry” isn’t a cliché - it’s the correct setting.
Do You Need A New Boiler?
In some cases, replacing a boiler can make more financial sense than continuing to repair it. Here are the key factors to consider:
Age of the Boiler - If your boiler has surpassed its average lifespan (typically beyond 15 years) and is no longer covered by the manufacturer’s warranty, replacing it may be a more cost-effective option to avoid recurring repair expenses.
Repair Costs - When the cost of repairing the boiler exceeds 50% of the price of a new one, it’s usually more financially sensible to invest in a replacement rather than continuing with costly repairs.
Frequency of Breakdowns - If your boiler is breaking down more frequently, it’s often a sign of significant wear and tear and it is beyond economical repair. In such cases, replacing the boiler can help avoid mounting repair costs and the inconvenience of ongoing issues.
Energy Efficiency - Older boilers tend to be far less efficient than modern models, leading to higher running costs. Upgrading from a G-rated to an A-rated boiler could save you up to £385 per year in energy bills, making replacement a financially smart decision in the long run.
By carefully weighing these factors, you can determine whether replacing your boiler offers better value and peace of mind compared to continued repairs.
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If you’re aiming to get the best deal on a new boiler, consider getting a quote from us, here’s why:
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