If you've found your boiler leaking water, the usual causes are a loose internal seal, a worn pump, a pressure relief valve letting water out because pressure's too high, or, in cold weather, a frozen condensate pipe backing up and overflowing. Where the water's coming from on the boiler, and whether it's warm or cold, both help narrow down the cause.
This page covers what to check yourself, what needs an engineer, and roughly what it costs to fix.
Why is my boiler leaking water from underneath?
Water pooling underneath usually comes from a leaking seal, a failed pressure relief valve, or a worn pump inside the boiler. Older boilers are more prone to this as seals and internal parts wear with age and use. A boiler leaking water from underneath isn't something to leave, since water can reach electrical components.
Turn the boiler off and, if you can do so safely, the water supply to it, then book a Gas Safe registered engineer to find the source. Several different parts can cause water in the same spot, so it's not usually a job to diagnose yourself.
Is it expensive to fix a leaking boiler?
It depends what's causing the leak. Boiler repairs generally cost £120 to £750, averaging around £300, and a straightforward seal or valve replacement sits toward the lower end. Older boilers with internal corrosion, where several parts have worn together, can cost more because more than one thing needs fixing.
An engineer can only give you an accurate price once they've traced the actual source, since a leak that looks the same from the outside can have quite different causes and costs behind it.
My combi boiler overflow pipe is leaking outside. What could be causing this and how can I fix it?
The overflow (or condensate) pipe is designed to drip occasionally, that's normal. A pipe that's leaking or dripping steadily usually means the pressure relief valve has released water because the system pressure's too high, or the condensate pipe is partly frozen and backing up.
In cold weather, a frozen condensate pipe is a common cause. You can thaw it yourself by pouring warm (not boiling) water over the frozen section, or holding a hot water bottle against it, then resetting the boiler once it's clear. Never use boiling water or a naked flame on the pipe, since it can crack. If thawing doesn't fix it, or you can't safely reach the pipe, it's an engineer's job.
Can I still use my boiler if it's leaking?
It depends on the leak. A small, external drip from the condensate pipe in freezing weather is usually safe to leave until you can thaw it or get it looked at. A leak from inside the boiler casing, or anywhere near electrics, is not something to keep running, since water and electrical components don't mix safely.
If you're unsure which you've got, the safer choice is to turn the boiler off and book an engineer rather than keep using it and hope it's the harmless kind.
Questions people ask
Can a boiler just lose pressure without a leak?
Yes. Bleeding a radiator releases a small amount of water each time, which is normal and not a leak. Pressure can also drop slowly through a faulty pressure relief valve without any visible water. A pressure drop with no sign of water anywhere is still worth investigating, since it usually means something inside the system needs attention.
My Worcester boiler overflow pipe is dripping. What could be causing this and how can I fix it?
A dripping overflow pipe on any brand, including Worcester, usually means the pressure relief valve has released water because pressure's too high, or the pipe's partly frozen in cold weather. Check your pressure gauge first: if it's reading high, bleeding a radiator slightly can bring it down. If pressure looks normal, a frozen pipe is more likely the cause.
How to find a water leak that isn't visible?
If you suspect a leak but can't see water, check for damp patches, a musty smell, or pressure that keeps dropping without explanation. These are signs water's escaping somewhere in the pipework rather than at the boiler itself. Tracing a hidden leak, especially one under flooring or inside a wall, is a job for a Gas Safe registered engineer with the right equipment.
What's the average lifespan of a boiler?
Most boilers last 10 to 15 years with regular servicing. Leaks become more common toward the end of that range, as seals and internal components wear. If your boiler's older and leaking, it's worth asking the engineer whether a repair still makes sense compared with the cost of a newer, more efficient boiler.
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