Boiler not firing up

Why is My Boiler Not Firing Up? The Most Common Reasons

Last updated on January 29th, 2024

Everyone will experience a boiler not firing up at some point.

Your boiler not working will probably ruin your day, especially on a cold winter’s night.

I have been a Gas Safe engineer and fixing boilers full-time for over a decade.

In this post, I’ll show you some of the many boiler problems I come across based on my good and bad experiences when I get called out to a boiler not working.

Common Reasons for a Boiler Not Firing Up

There are some very easy fixes for a boiler not working.

Here are some things you can troubleshoot and check yourself:

  • No power supply
  • No gas supply
  • Low boiler pressure
  • Thermostat problems
  • Frozen boiler condensate pipe

No Power Supply

When your boiler is not working and you have no lights on the boiler, there is a good chance you don’t have any electricity to the boiler.

Fused switched spur and fuse
Switched spur and fuse

Here are some things to check:

  1. The fuse in the switched spur next to the boiler (should be 3 amp)
  2. The circuit hasn’t tripped at the consumer unit (main fuse board for the house)
  3. The fuse in the boiler (usually 2 amp)
  4. You have money on a pre-payment electric meter

No Gas Supply

Having your boiler go to lockout when it tries to fire up could mean you don’t have any gas getting to the boiler.

Gas meter ecv turned off
Gas meter ECV turned off

Here are some possibilities:

  • Gas is turned off at the gas meter control valve
  • Gas credit has run out on a pre-payment gas meter

Low Boiler Pressure

Low boiler pressure is a very common reason for a central heating system and hot water to not work. You will need to repressurise the boiler to fix this problem.

Having your boiler pressure low is an easy fix but if your boiler pressure keeps dropping, that means you have a problem that needs to be found and fixed.

Thermostat Problems

A thermostat problem could be the reason your boiler is not working.

Hive room thermostat being set
Hive room thermostat

Here are some things you can check:

  1. The thermostat temperature is turned up
  2. The batteries are good (on wireless thermostats)
  3. Faulty thermostat

Frozen Boiler Condensate Pipe

A frozen condensate pipe will probably be the reason for your boiler not working if it’s below freezing outside.

When a boiler condensate pipe freezes, the boiler will go to lockout until it has been defrosted.

frozen condensate pipe
Frozen condensate pipe

Which Parts Cause a Boiler to Stop Working?

Heating But No Hot Water

Having no hot water from the boiler but the heating is working could be caused by many things.

Combi Boiler

A combi boiler not firing up for hot water could be for one of these reasons:

  • The plate heat exchanger is blocked
  • The diverter valve is stuck or damaged
  • The temperature sensor is broken
  • The flow/micro switch is broken

Heat Only or System Boiler

  • A 2 or 3-port valve is stuck or broken
  • The hot water cylinder thermostat is turned down or faulty
  • The programmer is not working properly

Hot Water But No Heating

When the central heating is not working but the hot water is, there could be many different reasons for this.

Here are some of those reasons:

Combi Boiler

  • The diverter valve is stuck or damaged
  • The room thermostat is turned down, broken, or needs the batteries replaced
  • The temperature sensor is faulty

Heat Only or System Boiler

  • 2 or 3-port valve is stuck or broken
  • The programmer is not working properly
  • The room thermostat is turned down, broken, or needs the batteries replaced
Thermostat Problem
Room thermostat

Pilot Light on Boiler Not Firing Up

A boiler pilot light is a constant small flame that is used to light the burner when the heating or hot water is called for.

Modern boilers don’t use pilot lights anymore, they use spark electrodes which fire the boiler up using electric sparks every time it’s needed.

If you have an old boiler with a pilot light and it’s not firing up, here are some potential reasons:

  1. Gas supply problem (blockage or no gas for example)
  2. Gas valve problem

If the pilot light is on but not firing up the boiler, this will usually indicate a faulty gas valve problem.

Boiler Clicking But Not Igniting

A boiler clicking but not igniting means you are getting electricity, and your external controls are working, so it has to be something further on in the firing-up process that’s causing the problem.

Here are some potential reasons why it’s clicking but not igniting:

  1. A gas supply problem
  2. The spark electrode is faulty


When your boiler is not working there are quite a few simple things you can check yourself and rule out without having to open the boiler up.

If you have a radiator not heating up then this will be a radiator problem and not the boiler.

When it comes to needing boiler parts to be fixed or replaced, like a gas valve, you should call a Gas Safe engineer to get it checked out properly and fixed.

Regular boiler servicing includes checking boiler parts and should be done every year to help prevent boiler breakdowns.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.


Why is my boiler clicking when firing up?

A boiler clicking when firing up means the spark electrode is doing it’s job by lighting the gas.

Old boilers used to have a pilot light which is a constant low flame ready to fire the burner up when you put the central heating on or call for hot water.

Modern boilers use an electric spark electrode which will click many times whenever you call for heat. The spark will light the gas coming through the burner and heat the heat exchanger.

It’s a much more efficient way of lighting the boiler, and a bit like using a lighter.

How do I know if my boiler fuse has blown?

A boiler typically has two fuses controlling the power supply to the boiler, one in the switched spur outside the boiler, and one inside the boiler.

Switched Spur

The switched spur is a white plate with a switch to isolate the electric supply to the boiler. It’s usually next to the boiler which should have a 3 amp fuse inside.

You can remove the fuse and use a multimeter on either end of the fuse to check if it’s blown. You are better off just replacing the fuse to find out if it’s blown.

Fuse Inside the Boiler

There are two ways to know if a boiler fuse (usually 2 amp) has blown:

• Look at the element inside the fuse to see if it’s not fully intact (only on clear fuses)

• Use a multimeter on the live and neutral terminals after the fuse to see if you’re getting 240v when the boiler is calling for heat

You should also check the circuit breaker at the main consumer unit (fuse board) for the house to see if it has tripped, or you might even have and old style fuse there which could have blown.

Will it damage my boiler if I turn it off?

No, it will not damage your boiler if you turn it off and it’s perfectly normal to turn it off.

What happens if a boiler pump stops working?

If a boiler pump stops working then the boiler will stop working and you will have to heating or hot water (unless your hot water is heated indirectly in a cylinder with electricity).

A boiler pump not working is not uncommon and replacing a central heating pump is a straight forward job for a Gas Safe registered engineer.

How do you manually reset a boiler?

To manually reset a boiler you need to press the reset button or turn the dial.

You can sometimes just press the button and release it but sometimes you need to keep the button pressed or dial held in position for a few seconds. Different boilers have different reset procedures, you’ll need to check the boilers manual to know for sure.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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