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Why is my boiler leaking water?

Is your boiler leaking water? Read this troubleshooting guide to help
you diagnose the problem and get it fixed.

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Why is my boiler leaking water?

Is your boiler leaking water? The first step is to inspect the pressure gauge for any sign of excessive pressure. If the pressure is above 3 bar the pressure release valve (PRV) may be working to reduce pressure.

This is how the PRV is supposed to work. But if it’s not this there could be any number of reasons for a leak. Read below for more ideas.

Why is my boiler leaking from underneath?

If you find your boiler leaking water from the bottom, you must act quickly to stop the issue from getting worse. As noted above, check the pressure first. The pressure release valve (PRV) could be letting water out to reduce pressure.


If the boiler leaking is the result of the PRV you can try to reduce your boiler pressure. You can do this by bleeding a radiator until the pressure is abut 1 bar cold or 1.5 hot.


Sometimes, the nature of a boiler, getting hot and cooling down, can cause joints to contract and expand. This can cause joints to loosen and leak over time. They may need to be tightened. If you have a new boiler, check in with your boiler installation company. You may need a local Gas Safe heating engineer if it’s older.


As with joints, seals inside your boiler can perish or become damaged over time. Good manufacturer-led servicing will prevent this, as they will be properly checked. Again, you’ll need a Gas Safe heating engineer to open and repair your boiler.


Get in touch with a gas engineer to visually check how corroded your boiler is, as some corrosion may be internal. In this case, you’ll need a Gas Safe registered engineer to remove the front of any gas appliance and carry out internal checks.

Pipe and tap corrosion - Boiler leaking water? | Boilerhut

What if the corrosion isn’t repairable?

If a heating engineer has advised that your boiler is unrepairable you will need a new one. They may have given you a quote right then and there but it’s always worth getting a few quotes for emergency boiler replacement.

Leaking heat exchanger

Another reason your boiler may be leaking from the bottom is heat exchanger corrosion. Over time the heat exchanger goes through a lot of stress. They will be under warranty for manufacturing defaults if your boiler is still under warranty.

But, the heat exchanger is an expensive part of a boiler. In most cases, it isn’t cost-effective to replace the heat exchanger. It would be a safer option to have a new boiler installed and get a new 10 or 12-year warranty.

How do I check for a leaking heat exchanger?

Only a qualified gas engineer can diagnose a problem with the heat exchanger. It involves getting into the boiler and you will need to be Gas Safe to do this safely and correctly.

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Poor boiler installation

Sometimes boilers haven’t been installed correctly. In these cases it can’t be corrosion in the boiler.

If could be corrosion of older pipework in your home, but most of the time pipework into and out of the boiler will be new.

Don’t panic. This issue is easily corrected. Small water leaks are quite common installations, especially with a cheaper installation. Any reputable installer will return to correct any work and fix those leaks.

Why is my boiler leaking from the overflow pipe?

Pressure release valve issues

This issue typically affects combi or system boilers and is usually due to a problem with the pressure release valve (PRV). A relief valve or PRV is part of your boiler’s safety control. It is designed to limit pressure on the system. It does this by releasing water when the boiler operates above its working capacity.

Combi and system boilers work at an optimum pressure of 1.3-1.8 bar. However anything within 1-2 bar will usually work without issue. If you exceed this load the system may overflow with water. The PRV allows water out to stop components becoming damaged.

Sometimes, however, the PRV will open and then fail to close correctly. This means water will continue to leak out. This isn’t great, but it’s also better than something else leaking, like the heat exchanger.

Condensate pipe issues

Most boilers have separate condensate and pressure release valve pipes. The PRV pipe is designed to terminate and drip. The condensate pipe will terminate into a drain or drown pipe.

Usually, the condensate pipe will not show the cause of an issue. Viessmann boilers have their pressure release valve exit into the condensate pipe.

Why is there water on top of my boiler?

This may be due to the auto-air vent, which acts as another fail-safe component. When the boiler’s pressure is too high, the tension exists as air through the auto-air vent. This vent runs on pressurised valves, and these pipes can stick, resulting in water being released.

Typically, water drips from the top through the casing to pipes beneath the boiler. This will only apply to older boilers and would arise in a pressurised system, such as a combi and sealed system boiler.

Also, water leak ingress from a poorly installed flue can be condensate produced by the boiler or rainwater tracking down the flue. A Gas Aafe engineer would be required here to carry out any work on flues.

Is a leaking boiler dangerous?

A leaking boiler can become dangerous if the water is dripping over electrical systems. Although most electrical units have seals, it would be wise to turn the boiler off and call an engineer.

Will a leaking boiler lose pressure?

A boiler leakage will prompt the boiler to lose pressure. Any leak in the central heating system decreases pressure. So if the boiler is ok and you keep loosing pressure read our guide on if pressure is too low.

What should I do if my boiler is leaking water?

  • Complete visual checks to try and find the exact location
  • Bleed radiators if the pressure is too high
  • Call a Gas Safe heating engineer
  • Use towels or buckets to catch water to reduce damage to your home
  • If you are worried, turn the boiler off at the isolation switch

What’s next?


The important thing is to stop the leak. Call a Gas Safe engineer for any issues with the boiler. If it’s other pipework, a plumber should be able to help.


If the boiler is damaged beyond repair, or you’re sick of repairing it you are best off replacing it altogether.

We suggest getting two or three quotes for boiler installation. Get them from companies that are well-reviewed and work closely with a boiler manufacturer or two. This is because they will be able to offer you the best post-installation support.

If you want a quote from Boilerhut, simply answer a few questions and you’ll get an instant and free quote in less than 1 minute.

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