Main Boiler Not Firing Up

Main Boiler Not Firing Up? A Troubleshooting Help Guide

Last updated on October 21st, 2023

Everyone will experience a boiler not firing up at some point. Your Main boiler not working will probably ruin your day, especially on a cold winter’s night.

I have been fitting 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 experiences when I get called to a Main boiler not firing up.

Main combi boiler heating

Common Reasons for a Main Boiler Not Firing Up

There are some very easy fixes for a Main boiler not firing up.

Here are some things you can troubleshoot and check yourself:

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

No Power Supply

When your Main 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.

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 a Main boiler (usually 2 amp)
  4. You have money on a pre-payment electric meter

No Gas Supply

Gas meter ecv turned off
Gas meter ECV turned off

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

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 Main Boiler Pressure

Low boiler pressure is a very common reason for a Main boiler not firing up. You will need to repressurise the boiler to fix this problem.

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

Thermostat Problems

Thermostat Problem
Room thermostat

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

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 Condensate Pipe

A frozen boiler condensate pipe will probably be the reason for your boiler not firing up 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 Main 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 room thermostat is turned down, broken, or needs the batteries replaced
  • The programmer is not working properly

Pilot Light on Main 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 Main 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.

Main Boiler Clicking But Not Igniting

A Main 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

Main Boiler Guides


When your Main 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.

Any boiler part replacements and testing should be done by a Gas Safe registered engineer.

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 find it helpful.


  • 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.

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