Glow-worm Boiler Leaking Water

Glow-worm Boiler Leaking Water: How to Fix Top Causes

Last updated on January 23rd, 2024

A Glow-worm boiler leaking water can cause your boiler to not fire up, rust or become dangerous.

I have been fitting and fixing boilers full-time for over a decade and have repaired many leaking Glow-worm boilers.

In this post, I’ll share some of what I have learned over the years from my on-the-job experiences.

Why is My Glow-worm Boiler Leaking Water From Underneath?

There are many places a Glow-worm boiler can leak water from, but it’s almost always leaking from underneath.

A Glow-worm boiler leaking from underneath doesn’t mean the leak is at the bottom of the boiler though. The leak could be at the top of the boiler inside but collects in the bottom of the boiler before dripping out underneath.

The best way to find out where leaks are coming from is to keep checking as high as possible to look for signs of water because where it’s dripping from is not usually where it’s leaking from.

The most common leaks on Glow-worm boilers I come across are from the plastic diverter valve.

Glow-worm Boiler Pipe Leaking

When you have a Glow-worm boiler pipe leaking water you will need to do one of these things:

  • Tighten the nut if it’s on a compression fitting
  • Resolder the fitting
  • Replace the section of pipe
  • Solder a patch fitting on

Glow-worm Boiler Leaking Water From Overflow Pipe

A Glow-worm boiler leaking water outside means the pressure relief valve is leaking out of the blow-off or overflow pipe.

The pressure relief valve leaking is usually a sign of other problems causing it to leak but it often means you will need to replace the pressure relief valve.

You should make sure you find and fix any other problems before replacing it or it will just leak again.

Glow-worm Boiler Condensate Pipe Leaking

A Glow-worm boiler condensate pipe leaking out of the end outside is normal but if it’s leaking from a fitting inside the house then this could potentially be dangerous.

A frozen boiler condensate pipe can cause it to leak sometimes but condensate water is acidic and eats away at copper and brass which is why they have to be plastic.

If you have a boiler condensate pipe leaking onto a copper pipe, especially gas, then this should be fixed as soon as possible.

Glow-worm Boiler Filling Loop Leaking

A Glow-worm boiler filling loop leaking usually means it needs to be replaced.

You can tighten the nuts up if it’s leaking from them but you will probably need to replace the boiler filling loop or filling loop valve.

Glow-worm Boiler Flue Leaking Water

A Glow-worm boiler flue leaking water is typically a sign it’s not fitted correctly.

A condensing boiler creates condensate wastewater which is created from the flue gases.

This wastewater needs to run back into the boiler condensate trap via the flue which is why the flue needs to be pointing slightly up and away from the boiler.

If the flue is pointing slightly down (or even straight), the condensate will drip out of the end of the boiler.

If the boiler flue is leaking from anywhere else, eg a flue connection, this will need to be looked at and fixed by a gas engineer as soon s possible.

Glow-worm Boiler Leaking Gas

If you can smell gas coming from your boiler then you will probably have a boiler gas leak which needs to be fixed ASAP!

You should turn off the gas at the gas meter if you can, and call your local Gas Safe registered engineer to book an appointment or call the National Gas Emergency Service on 0800 111 999.


An old Glow-worm boiler leaking water may be a sign the boiler is on its way out and needs to be replaced soon.

A boiler leaking brown water typically means you have a dirty heating system which is causing problems and a good sign you need to flush the heating system after fixing the leak.

If you have a boiler leaking from any type of compression connections, eg a leaking boiler valve on the flow or return, or a boiler drain valve, you should try tightening the nuts with an adjustable spanner.

Feel free to leave feedback or 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.


  • 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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