Boiler Not Firing Up? Causes & Fixes

The HSE gas safety guidance sets out what householders should and should not attempt on a gas appliance. A boiler that will not fire is a broad symptom with a fairly short list of causes. Working through them in order finds it most of the time, and even when it does not, knowing which stage it fails at makes the callout shorter.

The key question early on: does it try to ignite and fail, or does nothing happen at all? Those point at different things.

Check 1: System Pressure

The most common cause, and the easiest to fix.

Look at the gauge on the boiler. It should read 1 to 1.5 bar when cold. Below about 0.8 bar most boilers lock out and refuse to fire as a protective measure.

If it is low, repressurise using the filling loop — our guide to repressurising a boiler covers it brand by brand. If it drops again within days, there is a leak, and our guide to a boiler losing pressure covers finding it.

Check 2: Is Anything Actually Asking for Heat?

Obvious, and worth ruling out before assuming a fault.

• Room thermostat set above the current room temperature
• Programmer or timer actually in an on period, and the clock correct — a lost clock after a power cut is a common cause
• Smart thermostat online and connected, with batteries good
• Every thermostatic radiator valve open, especially in the room with the thermostat

If the boiler fires for hot water but not heating, that points at the diverter valve or a motorised valve rather than the boiler itself.

Check 3: Gas Supply

• Is the gas meter's emergency control valve open? The handle should be in line with the pipe.
• Do other gas appliances work — a hob, for instance?
• If you are on a prepayment meter, is there credit?

If you smell gas at any point, stop. Turn the supply off at the meter, open windows, do not touch electrical switches, leave the property and call the National Gas Emergency Service on 0800 111 999.

Check 4: The Condensate Pipe

A very common winter cause. Condensing boilers produce acidic water that drains away through a plastic pipe, and if part of that pipe runs outside it can freeze. The boiler detects the blockage and shuts down.

The clue is timing — a boiler that fails on the first genuinely cold morning of the year, often with a gurgling sound before lockout.

Our guide to thawing a frozen condensate pipe covers the fix, which is usually warm water poured along the exposed section.

Check 5: Ignition

If the boiler attempts to fire — you hear the fan spin up and clicking — then stops and shows a fault, the ignition sequence is failing.

Common causes: a dirty or worn ignition electrode, a failed flame sensor, an air pressure switch fault, or a blocked flue.

Reset once. If it locks out again immediately, stop resetting. Repeated resets on a genuine ignition fault are not safe, and it needs a Gas Safe registered engineer.

Check 6: Power and Controls

• Is the fused spur switch by the boiler on?
• Check the fuse in that spur — usually 3 amp.
• Any tripped breaker at the consumer unit?
• If the display is completely blank, it is a power problem rather than a boiler fault.

Check 7: A Seized Pump

After a summer of standing idle, the pump can stick. The boiler tries to fire, detects no flow, and locks out — Vaillant F75 is the classic version of this.

Some pumps can be freed by removing the central screw and turning the shaft with a screwdriver, but that is a job for someone comfortable working on the boiler, and on a sealed unit it is engineer territory.

What Not to Do

• Do not keep pressing reset. Lockouts are protective. Repeatedly clearing one on a real fault can allow gas into a chamber that is not igniting it.
• Do not remove the boiler casing. That is gas work and it is illegal for anyone not Gas Safe registered.
• Do not ignore a gas smell. Ever.

Frequently Asked Questions

Why does my boiler fire for hot water but not heating?

Usually the diverter valve or a motorised valve failing to switch circuits. The boiler is fine; the water is not going where it should.

The display is on but nothing happens.

Power is present, so look at whether anything is calling for heat — thermostat, timer, TRVs — before assuming a boiler fault.

How many times can I safely reset?

Once. If it locks out again straight away, the fault is real and needs diagnosing rather than clearing.

It works but keeps stopping after a few minutes.

That is short cycling rather than a failure to fire — different cause, covered in our guide to a boiler that keeps turning off.

If It Still Will Not Fire

Once pressure, controls, gas and condensate are ruled out, the remaining causes are inside the boiler and that is Gas Safe work. We cover Milton Keynes and the surrounding area for boiler repair. Get in touch and tell us the fault code and whether it tries to ignite — that usually identifies it before we arrive.