Vaillant Boiler leaking water

Why is My Vaillant Boiler Leaking Water From Underneath?

Last updated on February 21st, 2024

A Vaillant 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 Vaillant boilers.

Why is My Vaillant Boiler Leaking Water From Underneath?

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

A boiler leaking from underneath doesn’t mean the leak is at the bottom of the boiler. 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 Vaillant boilers I come across are from the plastic diverter valve.

Boiler Pipe Leaking

When you have a pipe on the boiler 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

Leaking Water From Overflow Pipe

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

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 pressure relief valve.

You should find and fix any other problems before replacing it or the new PRV will leak again.

Condensate Pipe Leaking

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

A frozen condensate pipe can cause it to leak sometimes. Condensate wastewater is acidic and eats away at copper and brass which is why the pipes have to be plastic.

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

Filling Loop Leaking

A filling loop leaking usually means it needs to be replaced. You can tighten the nut up if it’s leaking from there 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 from the flue gases.

This wastewater needs to run back into the boiler condensate trap via the flue which is why the internal 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.

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

Vaillant Boiler Guides


An old Vaillant 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. This 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 connection with a nut, you should try tightening the nuts with an adjustable spanner.

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.


If I turn my boiler off will it stop leaking?

No, your boiler will most likely not stop leaking if you turn it off. If it’s leaking condensate water, then it will eventually stop as the boiler will not be producing any condensate when turned off.

Is a leaking boiler an emergency?

A leaking boiler can be an emergency but most of the time it’s not.

Depending on exactly where the leak on the boiler is coming from, a leaking boiler can sometimes cause the boiler to not fire up when the pressure drops which could be seen as an emergency in the winter.

If the condensate pipe is dripping onto a gas pipe, the acidic condensate wastewater will eat away at the copper and brass and cause a dangerous gas leak emergency.


  • 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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2 thoughts on “Why is My Vaillant Boiler Leaking Water From Underneath?”

  1. Hello, I have an ecotech plus vaillant boiler and I have found the pressure goes up into the red zone to approx 2.8 when the heating is on. Is this dangerous and how do I fix it? Would this also affect the hot water from bath shower mixer tap as hot water flow is like a trickle from the mixer bath shower tap. Do I need to call a plumber out? Please help!

    1. Steven Reid

      Hi Jaya,
      Yeah you should probably call a plumber. Sounds like you have a boiler pressure problem (possibly expansion vessel). This will not be the cause of a low flow rate hot water tap, it’ll probably be at least be two separate problems.

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