Boiler Not Igniting: Causes & Fixes

If your boiler is not igniting or firing up, you’re likely left with no heating or hot water in your home – a frustrating and often cold experience, especially during a British winter. 

In this guide, I’ll walk you through every major reason why a boiler fails to ignite, explain exactly what’s happening inside your heating system, and give clear, step-by-step fixes you can safely try yourself — plus when it’s time to stop DIY and call a professional Gas Safe engineer.

Why Is My Boiler Not Igniting? The Most Common Causes

1. Low or No Gas Pressure to the Boiler

Low gas pressure is one of the top reasons a boiler won’t ignite across the UK.

What causes low gas pressure?

• The gas meter emergency control valve (ECV) has been knocked or turned off
• A problem with the gas supply pipework into the property
• An issue on the National Grid side (rare but happens in cold snaps)
• Frozen gas meter or condensate pipe in winter

How to fix low gas pressure yourself

1. Check the gas hob or cooker still works — if not, the whole house has no gas.
2. Locate your gas meter and make sure the lever on the emergency control valve is in line with the pipe (open position).
3. If the meter is outside and the pipe looks iced up, gently use warm (not boiling) water to thaw it.
4. If everything looks fine but still no gas, phone the National Gas Emergency Line on 0800 111 999.

2. Frozen Condensate Pipe (Very Common in Winter)

Almost all modern condensing boilers have an external white plastic condensate pipe that can freeze in sub-zero temperatures.

What causes a frozen condensate pipe?

The pipe carries acidic water from the boiler to the outside drain. When temperatures drop below 0 °C for several hours, the water inside freezes and blocks the pipe. The boiler detects the blockage via a pressure switch and locks out to protect itself.

How to fix a frozen condensate pipe

1. Locate the 22 mm or 32 mm white plastic pipe leaving the boiler (usually exits through an external wall).
2. Pour warm (not boiling) water over the frozen section until it thaws — you’ll often hear gurgling.
3. Reset the boiler once thawed.
4. Fit an insulation sleeve or trace heating cable afterwards to prevent reoccurrence.

3. Faulty Ignition Leads or Electrodes

The spark generator or ignition electrodes create the spark that lights the gas.

What causes ignition electrode problems?

• Carbon buildup or soot on the electrode tip
• Cracked ceramic insulator
• Loose or corroded ignition leads
• Electrode gap too wide or too narrow (should be 3-4 mm)

How to fix ignition electrode issues (only if you’re confident)

Gas Safe law allows homeowners to remove the boiler cover on most models. Clean the electrode gently with fine sandpaper if dirty, check the gap with a feeler gauge, and make sure leads are firmly pushed on. If the ceramic is cracked — call an engineer.

4. Blocked or Dirty Flame Detection Probe

The flame detection probe (ionisation probe) tells the gas valve it’s safe to keep the burner on.

What causes flame detection problems?

• Soot or corrosion on the probe tip
• Misaligned probe
• Faulty printed circuit board (PCB)

How to fix a dirty flame detection probe

Clean the metal tip carefully with fine emery cloth. Do NOT adjust its position unless you have the exact manufacturer specification.

5. Gas Valve or PCB Failure

The gas valve opens to let gas to the burner; the PCB is the “brain” of the boiler.

What causes gas valve or PCB issues?

• Age and wear (common after 10-12 years)
• Power surges
• Water ingress from leaks

How to fix gas valve or PCB problems

These are NOT DIY repairs. A Gas Safe engineer must diagnose with specialist tools and replace parts.

6. Air Pressure Switch (APS) Fault

The APS confirms the fan is running and the flue is clear before allowing ignition.

What causes APS problems?

• Blocked flue or air intake
• Fan failure
• Split or blocked APS tubes
• Faulty switch itself

How to fix an air pressure switch fault

Check external flue terminal for bird nests, leaves or snow. If clear, this is an engineer-only repair.

7. Incorrect Gas Pressure or Burner Pressure Settings

If the gas pressure at the burner is too low or too high, the boiler will lock out.

What causes incorrect burner pressure?

• Recent installation or service carried out incorrectly
• Gas valve modulator fault

How to fix incorrect gas pressure

Requires a Gas Safe engineer with a flue gas analyser to adjust safely.

How to Reset Your Boiler After Fixing the Issue

Most boilers have a reset button or require you to hold the flame button for 5-10 seconds. Always check your specific model manual (Worcester Bosch, Vaillant, Ideal, Baxi etc.) because the reset procedure differs. Only reset 2-3 times — persistent lockouts mean a deeper fault.

Preventing Boiler Ignition Problems in the Future

1. Book an annual boiler service with a Gas Safe registered engineer — this catches 90 % of ignition issues before they happen.
2. Insulate the condensate pipe with proper lagging or trace heating.
3. Fit a good-quality magnetic system filter (e.g., MagnaClean, Fernox TF1) to keep sludge away from electrodes.
4. Install a surge protector or voltage optimiser if you live in an area with unstable electricity.
5. Keep the area around the boiler and flue terminal clear.

When You Absolutely MUST Call a Gas Safe Engineer

You must call a Gas Safe registered engineer immediately if:

• You can smell gas
• The boiler is leaking water or making banging/ketting noises
• You see black soot marks around the boiler or flue
• The flame is yellow/orange instead of blue
• The fault keeps returning after reset
• You’re in any doubt whatsoever — it’s simply not worth the risk.

Working on gas appliances without Gas Safe registration is illegal in the UK and can invalidate your home insurance.

FAQs

Will my home insurance cover a boiler that won’t ignite?

Most standard buildings & contents policies do NOT cover boiler breakdown. You usually need specific boiler cover or home emergency add-on. Check your policy wording for “sudden mechanical breakdown”.

How long can I safely leave my boiler locked out before calling someone?

If it’s winter and you have no heating/hot water, treat it as urgent — vulnerable people or young children in the house means same-day call-out. In summer you can wait 24-48 hours for a non-emergency appointment.

My boiler shows no fault code at all — just completely dead. Could it still be an ignition problem?

Yes — a completely dead display usually points to frozen condensate (blocking the pressure switch), loss of power, blown internal fuse, or a failed PCB. Start by checking the fuse spur and reset button.

Is it normal to hear 5-6 clicking sounds when the boiler tries to light?

Up to 3-4 clicks is acceptable while the spark generator tries to light. More than 6 clicks usually means ignition electrodes or leads need attention.

Can a wireless thermostat battery cause the boiler not to fire up?

Yes — if the room thermostat or receiver batteries are flat, the boiler never receives a demand for heat, so it won’t attempt ignition at all. Replace batteries in both the thermostat and receiver unit first.

Stay warm, The Boilers2Go Team