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How to fix a leaky boiler

Boiler Internal

What to do if your boiler is leaking

If you’ve discovered that your boiler is leaking there could be a few reasons why. Our quick guidance will help you to diagnose the problem and get to the cause of the leak.

1. Find where the leak is coming from
When water is leaking from your boiler, it is best to try and determine what is leaking and when, so you can inform your chosen engineer. First, take a look at all the pipes below the boiler; there should be seven.

Try to determine whether the leak is coming from one of the six copper pipes, or the plastic condensate pipe. If it’s coming from the condensate pipe, it may just be a loose connection and should be a simple enough fix.

2. Check when the leak is occurring
First, turn the boiler off and check what happens when it is cold. Is the leak still happening? Now, turn the boiler on and check when it is hot and running. A lot of the time, you’ll find that when your boiler is on, the pipework expands and there is no leak.

When your boiler starts to cool down, you will often notice the leak again. If you have this information prepared for your engineer, it can save a lot of time, as they will now be aware it is one of the hot pipes to the boiler.

However, if it is still leaking when the boiler is hot, it is likely the cold mains pipe.

3. See if the boiler is losing pressure
If your boiler is leaking AND losing pressure, it may be leaking from a pipe which passes to the outside of the building. This is the PRV pressure relief valve. A leak means the valve needs replaced ASAP, which should be a straightforward fix.

The alternative is that there’s a slightly bigger problem that will require a replacement pressure vessel.

Time to call in the professionals
Whenever water is leaking, it can be very stressful. I’d recommend carrying out all of the above checks and then calling in a heating engineer as soon as possible. This way, you can let him know the situation and he will be able to fix it relatively quickly.

Once I had a customer trace a leak back to the plastic condensate pipe, which helped us greatly as we knew what we were going to see when we got there. It turns out that when the kitchen fitter had built a cupboard around the boiler, the box was too tight, putting stress on the pipe.

I have also experienced other cases of older boilers where there has been a small drip from the valve connections, and the fix was really just a matter of changing washers. The leak could have gotten a lot worse if the customers had tried to tighten it themselves.

TIP: Never tighten leaking boiler connections yourself. Some of these connections have fibre washers that are not meant to be over tightened. If you try this, a small leak may suddenly get worse.

Safety advice

Although most of the time a leaky boiler is not directly dangerous, you must make sure you deal with the problem as swiftly as possible as water damage can be costly and there’s a chance it could affect your neighbours.

Most boilers have sensors which mean the boiler will not fire if there is insufficient water pressure. In other words, the boiler can’t dry fire and cause damage to the boiler

However, you must never leave a boiler leaking whilst continuing to top up the pressure. This will cause sludge to build up and the system will lose its protective inhibitor.

TIP: Always know where to turn your hot and cold water supplies off. This is a good safety measure if water is close to the electrics.


If you’re stressing over your leaky boiler, try not to worry. This blog includes a handy list of things to check before calling in this professionals. Knowing where the problem originates can save your engineer a lot of time, but never try to fix a leak yourself; you could end up making things worse.

If you have a leaky boiler:

  • Find where the leak is coming from
  • Check when the leak is occurring
  • See if the boiler is losing pressure

Remember, never keep topping up the pressure on a leaky boiler, as this could cause a sludge build-up, causing the system to lose its protective inhibitor.

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If you need one of our highly experienced gas safe registered engineers to check or repair your boiler you can contact us anytime to arrange a convenient time to visit.

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Disclaimer: The information on this web page is presented for general guidance purposes only and is supplied without liability. Such information is provided in good faith. Whilst every care has been taken in its preparation, no responsibility is accepted for any errors or omissions which it may contain. The information on this may be updated or changed at any time without warning.

How to fix a leaky boiler

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