Baxi Boiler Leaking Water

Baxi Boiler Leaking Water? Causes and Fixes – Housewarm

Last updated on February 17th, 2024

A 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 Baxi 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 Baxi Boiler Leaking Water From Underneath?

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

A 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.

Baxi Logo

Common Baxi Boiler Leaks

Certain boilers have certain recurring problems which typically get fixed by the boiler manufacturer when they make a new boiler or update a faulty part.

Here are some common recurring boiler leaks I come across:

Leaky Diverter Valve

A Baxi 80 or 105 boiler leaking on the left-hand side is a very common leak repair I come across.

On older Baxi boilers it’s very common for the diverter valve to leak out the front which can usually be repaired by:

  1. Stripping the diverter valve
  2. Replacing the hot water diaphragm
  3. Replacing the spindle nut
  4. Cleaning everything up
  5. Silicone greasing everything
Baxi diverter valve leaking
Baxi diverter valve leaking

Leaky Diverter Cartridge

On other Baxis, like the Duotec and Platinum boilers, they started using a diverter cartridge with a motor on top which also leak quite often.

The cartridge will leak into the motor which means, although an easy job, both parts need to be replaced.

Boiler Pipe Leaking

When you have a 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

Boiler Leaking Water From Overflow Pipe

A boiler blow-off (overflow) pipe leaking outside means the pressure relief valve (PRV) is letting water out.

Boiler Leaking Water From Overflow Pipe
Boiler blow-off 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 PRV.

You should make sure you find and fix any other problems before replacing it or the new PRV will just leak again. It’s most likely an expansion vessel problem.

Condensate Pipe Leaking

A 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 condensate pipe can cause it to leak sometimes but condensate wastewater is acidic and eats away at copper and brass. This is why they have to be all plastic.

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

Brass fitting on condensate pipe
Brass fitting used on condensate pipe

Filling Loop Leaking

A 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.

Flue Leaking Water

A 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 from the flue, which is why the flue’s internal pipe needs to be pointing slightly up and away from the boiler.

If the flue is pointing slightly down (or even straight), the condensate can drip out of the end of the flue terminal outside.

Room sealed boiler flue terminal
Boiler flue terminal

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 Safe registered engineer as soon as possible.

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 a local Gas Safe registered engineer to book an appointment or call the National Gas Emergency Service on 0800 111 999.


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

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

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

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.

    View all posts
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