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Quick answer
A boiler that will not ignite is usually either not getting gas, not getting a spark, or not detecting the flame once it lights. The causes householders can check are an interrupted gas supply (including a prepayment meter out of credit), a frozen or blocked condensate pipe and low system pressure. The causes that need a Gas Safe engineer include a worn ignition electrode, a faulty flame sensor, a failing gas valve, a fan or air pressure fault, or a problem with the control board.
Key takeaways
- Ignition needs three things: gas, a spark and flame detection. A fault in any one stops the boiler.
- Check other gas appliances and your meter first; a supply problem is common and easy to rule out.
- In cold weather, a frozen condensate pipe often shows up as an ignition fault.
- Worn electrodes and flame sensors are routine repair parts and are often picked up at an annual service.
- Repeated ignition lockouts need an engineer, not repeated resets.
How a modern boiler lights
When the boiler receives a call for heat or hot water, the fan starts to clear the combustion chamber and a pressure switch or sensor confirms air is flowing. The gas valve opens and an ignition electrode sparks to light the burner. A flame sensing probe then confirms the flame is present. If the flame is not detected within a few seconds, the gas valve closes and, after a set number of attempts, the boiler locks out with an ignition fault code.
Causes you can check yourself
Gas supply. If the hob will not light either, the problem is upstream of the boiler. Check the meter, the emergency control valve by the meter and, on prepayment, your credit and any emergency credit. See prepayment meter and boiler not working.
Frozen condensate pipe. The condensate pipe carries acidic water from the boiler to a drain. When it freezes, water backs up and the boiler shuts down, often with an ignition or flame fault. Our guide to thawing a frozen condensate pipe walks you through it.
Low pressure. Many boilers will not attempt ignition below around 0.5 to 1 bar. Check the gauge and repressurise if needed.
Power and settings. Make sure the boiler is receiving a demand: timer on, thermostat set above room temperature and the hot water or heating switched on.
Causes that need an engineer
Ignition electrode. The electrode wears and can become misaligned or coated in deposits, giving a weak spark or none at all. You may hear clicking without the burner lighting.
Flame sensor. If the burner lights briefly and then goes out, the flame may not be detected. A dirty or failing sensor is a common cause.
Gas valve. A gas valve that fails to open, or opens incorrectly, stops the boiler lighting. This is a gas-carrying component and must only be tested and replaced by a Gas Safe engineer.
Fan and air pressure. If the fan is slow or the air pressure switch does not confirm airflow, the boiler will not proceed to ignition. A blocked flue or condensate trap can cause the same symptom.
PCB. The control board co-ordinates the whole sequence. Faults here are less common but do happen. See our guide to a boiler PCB fault.
Brand-specific ignition codes
Different manufacturers label ignition faults differently. For example, Vaillant uses F28 and F29, Worcester Bosch uses EA 227, Baxi uses E133 and Ideal uses L2 and F2. We have separate guides for Vaillant F28 and F29, Worcester Bosch EA 227, Baxi E133 and Ideal L2 and F2.
Why servicing reduces ignition faults
An annual service includes checking the burner, electrodes and combustion, and cleaning where the manufacturer requires it. Many ignition problems develop slowly, so a service often catches a worn electrode or a partly blocked condensate trap before it leaves you without heating. Book an annual boiler service if yours is due.
Book with Gasmaster
Boiler refusing to light? Gasmaster Plumbing & Heating, Gas Safe registered (220932), diagnoses and repairs ignition faults across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service or book through our boiler service booking page.
Frequently asked questions
Why is my boiler clicking but not lighting?
Clicking is usually the ignition electrode sparking. If it clicks without lighting, the likely causes are no gas reaching the burner or a spark that is too weak or misplaced.
Can I relight my boiler myself?
Modern boilers have automatic ignition; you can press reset once or twice. Older boilers with a pilot light can be relit by following the manufacturer’s instructions on the appliance. If it will not stay lit, call an engineer.
Is a boiler that won’t ignite dangerous?
A boiler that fails to ignite is shutting itself down safely. The danger would come from tampering with it. If you smell gas, call 0800 111 999.
How much does an ignition repair cost?
It depends on the part. Electrodes are among the cheaper repairs; gas valves and PCBs cost more. Our post on boiler repair costs explains what affects the price.
Sources
- Gas Safe Register (gassaferegister.co.uk)
- Health and Safety Executive, gas safety guidance (hse.gov.uk)
- Manufacturer installation and servicing manuals (Vaillant, Worcester Bosch, Baxi, Ideal)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
