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Last updated: 1st March, 2023


Leaking Overflow Pipe - Causes and Fixes

Leaking Overflow Pipe - Causes and Fixes

Key takeaways

  • An overflow pipe removes excess water from a home's heating system to avoid flooding.
  • Increased water pressure and component failure can lead to a leaking condensate pipe.
  • Any leak should be repaired by a qualified heating engineer.

Any time a boiler or pipework is leaking, it’s a bad sign, and an overflow pipe is no different. The necessary precautions must be taken to avoid further and potentially serious damage.

In this blog we’re going to be exploring the issue of a leaking overflow pipe, including what are the potential causes, and how to fix them.

What Is An Overflow Pipe?

An overflow pipe’s general purpose is to transport excess water from a home's water storage tank (or cistern) to outside the home, avoiding flooding.

An overflow pipe can feed into the drain or in some cases an expansion tank (copper cylinder).

The common plumbing systems that feature an overflow pipe:

  • Combi boiler

  • Central heating water tank

  • Coldwater tank

  • Toilet cistern

  • Copper cylinder

  • Baths, sinks, and basins

Leak Causes

There are several potential causes of an overflow pipe leak, these can include:

Increased water pressure

When water pressure is too high for an overflow pipe to handle it can cause leaks or the water valve can become loose.


Faulty (or jammed) float valve

Ballcocks or float valves control the filling of a cistern or tank, firstly you must assess if the float valve is flowing freely when the water is running, if stuck, attempt to carefully release. If the valve moves but does not rise with the water, it could have a hole in it.

Washer deterioration

The plumbing washers may be worn due to their constant contact with water, or some boilers have lower quality components than others which may not last as long.

Broken Coil

A broken coil in the hot water cylinder can also cause an overflowing cistern, this can allow hot water from the feed and expansion tank to mix with the cold water in the storage cistern.

Hallmarks of a Leak

Apart from the obvious signs of a leak - the expelling of water from a source that should not do so, keep on the lookout for a boiler pressure gauge reading unusually low, i.e anything below 1.5 bars.

Fault (error) codes are another indicator of low pressure due to leaks or component failure. Cracks and corrosion in the boiler casing or pipework are also classic signs of a leak.

Time to call an Engineer

While possible to remedy smaller issues, leaks can be serious especially if they’re left or not properly fixed, compounding the tissue.

It’s prudent in the event of a leaking overflow pipe to contact a Gas Safe registered engineer to come and fully analyse the situation and carry out any repairs necessary.


Having your boiler annually serviced by an expert engineer can act as proactive preventive maintenance, detecting faults and allowing necessary repairs.


Upgrade Your Boiler

Modern, efficient boilers have been expertly designed for longevity and have been meticulously crafted to ensure the longest possible usage.

Advancement in modern technology means newer boilers are far more robust and durable than older generation boilers, not to mention far greater in their conversion of energy into heat, potentially saving you money.

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