Is Your Boiler Not Firing Up?

It’s cold outside, you switch your boiler on and wait, but much to your dismay, it doesn’t fire up! Let's see if we can help.

It’s cold outside, you switch your boiler on and wait, but much to your dismay, it doesn’t fire up! 

This is hardly an uncommon situation, as around 4 to 6% of all boilers develop a fault each year

However, don’t panic; this isn’t always the sign of a catastrophic issue and resolving it may be a case of pushing a few buttons!

Here’s a guide to what to do if your boiler doesn’t fire up.

Why is your boiler not firing up?

For gas boilers, ‘not firing up’ means there’s no gas igniting inside the boiler. 

Gas boilers ignite gas in a combustion chamber, which then exchanges heat with water that’s pumped around the house. If the boiler doesn’t fire up, it won’t be able to exchange heat with the water. However, it’s worth noting that the boiler can still fire and fail to heat the water. Check out our article on other boiler faults to learn more.

finger pressing the ignite button on a white boiler
Above: Gas boiler

1: Boiler has no power

Boilers need electricity to fire up and run, so the boiler might stop working altogether if the power is cut to the boiler. Likewise, if your home has no power due to a powercut or tripped switch, your boiler will probably stop working.

If your home reaches the temperature set on the thermostat, the boiler will turn on and off until the temperature stabilises. If there’s a communication issue between the thermostat and boiler, the boiler might switch off and refuse to turn on. 

Finally, the boiler might switch off automatically due to an internal fault. Check your boiler for an error code. 

  • Ensure you haven’t accidentally switched the boiler off. 
  • If your mains power switches off, so will your boiler. 
  • The thermostat may be switching the boiler off due to an issue. 
  • Check for an error code. 

2: Gas supply to your home is interrupted

Rarely, the gas supply to your home may cease. This might happen if your pay-as-you-go metre has run out or if you’ve not paid your bills and had your energy cut off. 

Gas works in the road or nearby can also disrupt your gas supply, but the council or contractors should have notified you if this is the case. 

Another possibility is that the gas supply pipe is frozen or the valves blocked or damaged. This is unlikely and will need to be diagnosed by a heating engineer. 

  • Works in and around your area can disrupt the gas supply. 
  • Your energy supplier may have shut off your energy.
  • Rarely, there might be an issue with your gas supply pipes.
  • On rare occasions, the gas supply pipes and valves become blocked and damaged. 

3: The boiler timer isn’t working, or the boiler doesn’t respond to the thermostat

Here’s a classic ‘finger fault’ that causes people to think their boiler isn’t working properly. Boilers respond to a thermostat, which is installed somewhere inside the house. The thermostat communicates with the boiler via radio. 


The thermostat's purpose is to measure the interior air temperature and relay that to the boiler. The boiler will moderate the heat to keep the home up to temperature. So, if there’s an issue with the thermostat, the boiler may fail to switch on. 

a white Digital thermostat displaying temperature on a cream wall
Above: Modern thermostat

This is possible if the batteries in the thermostat wear out, or if a new thermostat isn’t set up correctly or is out of range of the boiler. 

Another common issue is when the thermostat is positioned near a direct heat source, like a fan, oil or electric heater. This will cause the air temperature around the thermostat to rise beyond the home's ambient temperature, and the thermostat will switch off prematurely. 

Furthermore, ensure your central heating timer matches the actual time. If there’s been a powercut, the timer will need to be adjusted. 

  • The thermostat communicates with the boiler. 
  • Check the thermostat batteries. 
  • If it’s a new thermostat, ensure it’s set up correctly. 
  • Remove direct heat sources from within close proximity of the thermostat. 

4: Air trapped in radiators

Air trapped in radiators will only cause your boiler to stop working in extreme cases. However, this is one of the most common central heating issues and is often easy to remedy by bleeding the radiators. 

Radiators should have a bleed valve usually positioned in the radiator's top right or left-hand corner. Shut off your central heating, allow the water to cool, and use your bleed key to release small amounts of air (not water) from the bleed valve. Repeat for all troublesome radiators and hope for an improvement!

Hand bleeding white radiator with a silver key, over a white mug
Above: Radiator bleed procedure

If there’s a considerable amount of air in your central heating system, there may be an issue with the feed in the loft. This can cause boiler pressure issues if left unchecked. 

  • If your radiators aren’t working well, consider bleeding them. 
  • Bleeding radiators is simple, but shut off the heating first. 
  • A small amount of water may come out, but try and avoid releasing too much.
  • Repeat for each problematic radiator. 

5: Frozen condensate pipe

Conventional condensing boilers often have a condensate pipe installed outside. In extremely low temperatures, the pipe can freeze, causing the boiler to shut down automatically. If this occurs, your boiler may fire up for a short time and close down. 

Condensate pipe on a brick wall
Above: Condensate pipe

There will likely be an error code if your boiler displays them. To remedy this, you can defrost the pipe yourself. 

  • Check for a pipe extending outside from underneath the boiler.
  • In low temperatures, this pipe can become frozen. 
  • Check your boiler for error messages. 
  • You can defrost the pipe yourself using hot water. 

6: Faulty diverter valve

While this won’t necessarily prevent the boiler from firing up, a faulty diverter valve typically causes the hot water temperature to drop. Moreover, you might receive hot water but poor heating, or vice versa. 


Diverter valves become worn over time and eventually need replacing. Again, this is a job for a qualified heating engineer. 

  • A faulty diverter valve can cause a drop in hot water or radiator temperature. 
  • If your water gets warm only when your central heating is turned on, it could be the diverter valve. 
  • Diverter valves are relatively easy to replace. 
  • Call a heating engineer. 

7: Pilot light on boiler not firing up 

Some older conventional boilers feature pilot lights as opposed to electrical firing systems. If the pilot light is off, the boiler won’t fire up. 

Pilot lights can wear out over time, but can also be extinguished by strong winds or drafts. This is common when the boiler is installed in a windy garage. 

If you suspect your pilot light of blowing out, you should contact a heating engineer ASAP. However, this is easy to fix if the light simply needs to be reignited. 

  • Not all boilers have a pilot light. If yours does, you should be able to see a flickering flame through a window on the front cover. 
  • The flame can burn out due to worn parts. 
  • Strong winds or drafts can blow out the light. 
  • If you suspect that the pilot light is out, contact a heating engineer ASAP. 

8: Internal boiler fault 

If your boiler fails to fire up at all, there might be an internal fault. Worn parts, leaks and damaged circuitry can cause the boiler to shut down automatically. 

In these situations, your boiler should display an error code (if they have a digital display). Compare the error code to the manufacturer’s guidance to diagnose the fault. 

  • All manner of internal boiler faults can cause the boiler to switch off. 
  • New boilers have advanced automatic fail safes. 
  • Check your boiler for error messages. 
  • Contact our heating engineers if unsure. 

Boiler still not firing up? Try these

Here are the top four steps to fixing a boiler that isn’t firing up:

1: Reprogram the boiler and timer

If your power has been interrupted, you might need to reprogram the boiler and timer. Ensure the timer is set appropriately for when you want your heating to switch on/off and ensure the time itself is in sync with the actual time. 

Pressing the set button on a digital thermostat on a cream wall
Above: Timer and thermostat unit.

Check your boiler settings and ensure both hot water and central heating are switched on. 

2: Check the pressure gauge

Make sure the pressure is around 1.5 bar. This does vary slightly between manufacturers, so check the manual if you’re unsure. Either way, the low pressure and high-pressure zones will be marked on the dial, so you should be able to tell if boiler pressure is a problem. 

3: Check the thermostat 

Thermostats need to be set up properly to ‘talk’ to the boiler. Many are battery-operated - check the batteries if you suspect that your thermostat isn’t working. 

Also, ensure there are no direct heat sources near the thermostat - this will throw off its temperature sensor and prevent your boiler from working properly. 

4: Bleed the radiators

Bleeding radiators are somewhat of a cover-all for basic radiator faults. Airlocks in the radiator prevent them from heating properly - you can bleed them using a bleed key (see below). Check out our guide to radiator faults for more on that topic.

Fingers holding a silver radiator key about to bleed a white radiator
Above: Typical bleed key and valve.

Summary: Boiler not firing up and how to fix it

If your boiler isn’t firing or isn’t working properly, don’t panic! But, do attempt to resolve the fault as soon as possible. 

Book our heating engineers if you can’t figure out what’s wrong. Fiddling about with your boiler isn’t advised under any circumstances, and it may even be illegal! Get ahead of any problems with booking your boiler service to avoid having these issues.

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