If you find your boiler isn’t firing up, there could be any number of causes but fortunately, there are many checks you can do and fix yourself first, before needing an engineer. However, if you don’t understand any of them it’s better to call a professional for safety reasons.
The following causes relate to a combi boiler not firing up but most will also relate to other types of boilers such as system and heat only.
Causes for a boiler not firing up:
No pilot light
The pilot light is a small constant gas flame that ignites your boiler. You can sometimes see it through the glass, mainly on older boilers. When the boiler won’t ignite and the pilot light has gone out, it is usually because the jet is blocked with debris. This requires a qualified engineer to clean. The boiler won’t ignite sometimes if the gas pressure is too low.
For old boilers that have pilot lights, check this is still on. You will need to get this on before the boiler ignites. Modern combi boilers have automatic ignition so you won’t need to do anything manually.
Pilot light on but boiler not firing up
If you can see that the pilot light is on through the small window, but the boiler won’t fire, the cause could possibly be due to the boiler being low on pressure or losing pressure; the thermostat is not operational; the pump could be leaking or broken, or valves are closed not allowing water into the system.
Boiler are fitted with various fail-safes that operate in certain conditions to prevent dangerous situations occurring or causing damage to the boiler. This is called a boiler lockout.
Wrong setting on room thermostat
When coming out of the summer and the cold period starts, you may feel cold even at 21⁰ room temperature, but the thermostat may not have been touched for months. So turn up the thermostat higher, say to 25⁰ C just to check the boiler fires up.
If the thermostat isn’t working, first check the batteries are in good working order and replace if necessary. Other than this, there might be a fault with the component inside or even faulty wiring.
Boiler has no power going to it
This is one of the first checks to make. Some things to check are:
Boiler fuse blown: Following a power outage and the electricity coming back on, there is sometimes a surge of power. This can blow the fuse at the spur which means your boiler will have no power going to it and won’t fire up. Replace the fuse with a new one and the boiler should power up again.
Check the thermostat has power going to it: Are the batteries in the room thermostat still working? If the screen is dim, change the batteries.
The RCD has tripped: This causes a power issue to the boiler. The level or floor the boiler is located in could have lost power. Check the electrical consumer unit for tripped switches.
Have you got a pay as you go meter?
This is just a reminder that the gas meter will need to be topped up with credit before any gas can come through to the boiler. It’s an easy thing to overlook at the end of summer when you’ve not needed the heating on.
Low boiler pressure
The most common cause of a combi boiler not firing up is low boiler pressure. A low reading on the gauge indicates system pressure loss which could be due a leak. If the gauge is below 1 bar and especially close to zero, you will need to repressurise it to between 1.2 and 1.5 bar (check the user manual for the recommended level for your appliance).
Low gas pressure
The amount of gas being supplied to the boiler is too low. If the gas pipe to the combi boiler is not large enough, or other gas appliances such as a gas hob, cooker or gas fire are on at the same time and attached to that piece of pipework, the boiler does not get adequate gas to work properly. You will need to get a larger diameter gas pipe installed.
Radiators need bleeding
Trapped air in the radiators can also cause the boiler to not fire up. Go round each radiator, starting with the top floor, and let any air out of them. The pressure can go down after doing this so check you have the right level on the gauge.
Central heating pump not running
For non pressurised systems, check that the pump is running by listening and feeling it for vibrations. In combi systems, the pump will be inside the boiler case so a gas engineer will have to investigate. The pump is one of the first components to start when there is a call for demand.
Blocked condensate pipe
The condensate pipe for a combi boiler can become blocked outside, especially in winter because of freezing conditions. As the condensate is discharged to an external drain, it can block the pipe in very cold weather which will cause the boiler to not fire.
You can clear any blockage on the external pipe by pouring warm water over the pipe, or attaching a hot water bottle until the blockage is removed.
Check out our full guide to unblocking a frozen condensate pipe on the link below:
Issues you will need to call a gas engineer for:
- No pilot light. Possibly blocked pilot injector unit
- Stuck or faulty circulating pump
- Incorrect gas pressures in the boiler
- Fan issues within the boiler
- Gas valve problems
- Internal leaks causing pressure loss
- Electrical fault within boiler
As you can see, there are many causes of a boiler not firing up, many of which you can check and fix yourself. But when it comes to checking the boiler, be sure to get a qualified gas engineer.
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*The information in this article should be used for general guidance only and not as financial or health advice. Full details are on the link in the footer to our disclaimer page. Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.
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