Why is My Boiler Leaking Water? [How to Fix] Re March 25, 2020

Why is My Boiler Leaking Water? [How to Fix]

Ariston Boiler

Why is My Boiler Leaking Water? Whether your boiler is leaking from the bottom or from a condensate pipe, it’s a worrying sight.

Seeing a water leak coming from underneath your boiler is one of the least welcome boiler problems. You worry it’s not going to be something you can fix yourself and it could mean there is something seriously wrong with your system.

Before you start panicking and packing all the towels in your airing cupboard around your boiler, or worse, closing the door on it and forgetting about it – we’re here to talk you through a few causes and possible solutions to water leaking from your boiler.

DIY disaster

Firstly, we’d like to advise all of you DIY mavericks out there that you really shouldn’t mess with your boiler. If you have an issue we recommend taking the following steps:

  • Switch off your water supply by locating your stop cock (usually under a metal cover outside your property) and turning it clockwise or righty-tighty as it’s more commonly known.

  • Turn off your heating. There will be a switch near your boiler.

  • Clean up any excess water and put something down to catch any further excess water. Now that you’ve turned the heating off the system will depressurise and the leak should stop eventually.

  • Call a Gas Safe registered engineer to take a look and diagnose the issue or call WarmZilla and we’ll be happy to help.

However, if you’re the kind of person that tears up the rulebook and does things their own way then there are some things you can try before calling an engineer but don’t say we didn’t warn you – because we just did.


So, the first fix for a leaky boiler is pretty cheap and easy to try. It’s to add a sealant to your system. Over time the pipes in your heating system can develop tiny pinholes and eventually this could lead to a leak. The sealant works by coating the inside of the pipe which magically stops water from escaping.

It’s worth trying as you can pick up a leading brand leak sealer like Fernox from Amazon for £20 (we aren’t on commission). You can add the sealant through the filling loop or one of the radiators in your system using an adapter supplied with the product.

This can be a temporary fix on larger leaks or if it’s just a tiny leak it will hopefully solve the issue without too much bother. It takes between 1 – 24 hours to know whether it’s worked so you can try this first before spending any serious money.


It may sound obvious, but before you call an engineer out to fix your leaking boiler, check your guarantee or warranty to see if you’re still covered by the manufacturer. Depending on where the leak is coming from they may send an engineer out to fix it and it won’t cost you anything.

Usually, the leak would need to be coming from the boiler itself for the manufacturer’s warranty to cover it. If the leak is coming from pipework then it’s less likely to be covered.

If you can’t find the documentation then you should be able to check on the manufacturer’s website by entering the serial number located on your boiler. 

We’ve covered boiler warranties/guarantees, what the difference is and what to do if you need to use it.

Home insurance

Another obvious one, but depending on how comprehensive your home insurance cover is, you might be covered by it for boiler leaks. Alternatively, if you have boiler insurance then now would be the time to call them and get them out.

Temperature control valve

If there is an issue with your temperature control valve or TCV as they’re known it means that the water temperature in your boiler is no longer regulated. If the temperature gets too high it can cause leaks within your system.

This isn’t an issue you can fix yourself, you will need to call a Gas Safe engineer out to look at the problem. Any issue involving taking the cover off your boiler needs to be fixed by a Gas Safe registered heating engineer.


Corrosion is the most likely cause of the issue when it comes to a leaky boiler. In particular, the pipes located underneath your boiler are the first place to check for leaks (or wag an accusatory finger at). Boilers often leak from underneath due to corrosion and it’s an easy place to check before you start ripping up floorboards in search for the leak.

Leaks can (and will) develop in a system, even in hardy copper pipes over a long enough period of time. Rust will occur and corrosion will ensue, this is normal. You can try our advice regarding a leak sealant or you can get an expert who will either replace some of the corroded pipework or solder it, depending on how bad the corrosion is.

If your boiler is newly installed and you have a leak, don’t panic, it doesn’t mean they’ve done a bad job, even the most conscientious engineers will get the occasional leak on an install. Just call them up and they should arrange a time with you in the near future to return and remedy the issue. Any boiler installed by WarmZilla will offer this service.

When installing a new boiler the increased pressure can cause water leaks elsewhere in the system, this is fairly common and is not the fault of the installer and is not the result of the work they have undertaken. Bear this in mind when getting a new boiler installed.

General wear and tear

We all suffer from a bit of wear and tear over time, your boiler is no different. As it gets older, more and more problems will occur. If your boiler is over 10 years old then it may be time to consider a boiler replacement.

WarmZilla offers new boiler packages from as little as £1489 inc VAT, smart thermostat, system filter, installation, and a system cleanse. Get a boiler quote in minutes and choose to pay via finance and spread the payments monthly or pay for your boiler outright.

There are plenty of advantages to replacing your old boiler – saving you money on repairs, saving you money on heating bills, and saving the environment. 

High Pressure

Some people thrive under pressure, all boilers struggle under pressure. They like a nice steady 1.5bar on the pressure gauge located on the front of your boiler. Too little pressure or too much pressure and they’ll turn off and display an error code that will mean nothing to you, unless you speak boiler, like us.

Your boiler is fitted with a pressure release valve, so if the pressure gets too high it can release it, rather than building it up until it explodes. If your pressure release valve has malfunctioned then your boiler will need to release pressure somewhere, so it will release water and leak.

You can check the pressure by taking a look at the front of your boiler and looking at the pressure gauge, the dial going from green to red. Anything over 2bar is too much pressure and you will need to release some.

We’ve written an in-depth article about boiler pressure, too much, too little, how to decrease or increase pressure – it’s all in there.


You may be wondering how seals could have caused your boiler to leak, you haven’t even seen one since last year’s visit to Bristol Zoo, why would they want to tamper with your heating system? Because they like it cold, that’s why.

We’re talking about the seals on your boiler pump. Over time they can deteriorate and perish, which will lead to leaks inside your boiler. You can replace the seals individually but there’s often not much cost between buying the seals and just buying a new pump. So you may as well just get the whole pump replaced if that’s what your issue is.

Heat exchanger

You won’t be able to diagnose a broken heat exchanger, you have to be trained in boiler from birth. If someone tells you that your heat exchanger has cracked they are pretty much signing the death warrant on your boiler.

Heat exchanger replacements cost hundreds of pounds and if it’s gone, there’s a good chance you’ll be experiencing other problems even if you do get it fixed. We recommend getting a new boiler if you get a broken heat exchanger diagnosed.

If you’re wondering what boilers are good, bad, or just ugly, you can check our best boilers of 2021 article, where we run all boilers through their paces and announce winners in each category. It’s a bit like boiler Olympics, only with less television coverage.

Loose joints

I know what you’re thinking, ‘loose joints’ doesn’t sound so bad. Nice and limber. Loose joints are good for you but bad for boilers. We’re referring to the joins and bends on pipes.

Over time, with water expanding upon heating and contracting upon cooling, joints can become loosened and if they get too loose they will leak water. The good news is that it’s a relatively cheap fix, the bad news is that if the leak isn’t located under the boiler but somewhere under your floorboards it can be disruptive to locate.

You may be able to temporarily fix the leak by tightening the joint if it’s on a radiator, winding plumbers tape (PTFE tape) around the affected area, or using a leak sealant spray. A temporary measure can give you time to arrange for a professional to come in and fix it properly.

Leak error codes

Worcester Bosch boiler error codes for water leak:
A1, E9, CE207, HO7

Ideal boiler error codes for water leak:
F1, L1, FD

Vaillant boiler error codes for water leak:
F.22, F.24, F.13, F.73, S.41, S.53

Baxi boiler error codes for water leak
117, 118, 125, E78, H.02 – 06


A boiler leak is unlikely to be dangerous but it’s something we recommend getting a professional in to take a look at. If untended, it can cause further damage to your boiler and potentially your home.

We don’t recommend trying to fix a leak yourself but to call in a Gas Safe registered engineer. If the fix sounds expensive then we recommend getting a quote from WarmZilla as we are the cheapest on the market. We even have a ‘Best Price Guarantee’ – if you find a cheaper like-for-like quote, we’ll beat it by £50.

We recommend getting an annual boiler service to ensure that any boiler issues are picked up early and dealt with rather than being allowed to fester and become worse (and more expensive).

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