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Last updated: 11th April, 2024


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.


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Frequently Asked Questions

Why is Water Dripping from My Boiler Overflow Pipe?

Water dripping from the boiler overflow pipe, often referred to as the pressure relief or safety valve, can indicate several issues:
Excessive Pressure: The most common reason is too much pressure inside the boiler, causing the valve to open to relieve it.
Faulty Valve: The valve itself may be faulty, not closing properly even when the pressure is normal.
Expansion Vessel Issues: If the expansion vessel, which absorbs excess water pressure, isn’t functioning properly, it can lead to pressure issues and cause dripping.
Overheating: Some boilers release water if they're overheating as a safety measure.

How Do I Stop My Overflow Pipe from Dripping?

To stop the overflow pipe from dripping, you should:
Check the Pressure Gauge: Ensure the boiler pressure is within the recommended range (usually between 1 to 1.5 bar). Adjust if necessary.
Inspect the Valve: If the pressure is correct, the valve might be faulty and need replacing.
Check the Expansion Vessel: Have a professional inspect the expansion vessel and recharge or replace it if needed.
Seek Professional Help: Because boiler systems can be complex, it’s best to contact a qualified heating engineer to diagnose and fix the issue safely.

Is a Dripping Overflow Pipe an Emergency?

A dripping overflow pipe can indicate a potential issue with your boiler system that shouldn't be ignored. While it may not always be an immediate emergency, it's important to address it promptly to prevent further damage to the boiler or your property due to leaks.

Is It Normal for Water to Come Out of Overflow Pipe?

Occasional drips may occur, especially during or after the boiler heats up due to expansion within the system. However, consistent or excessive dripping is not normal and indicates a problem that needs investigation.

What Is the Pipe Dripping Water Outside House?

The pipe dripping water outside your house could be an overflow pipe from a plumbing fixture, water heater, or boiler. It's often a safety feature designed to release excess water or pressure. Consistent dripping from this pipe suggests a potential issue within the system.

Who Is Responsible for Water Leaks Outside the House?

Responsibility for water leaks outside the house can depend on where the leak is located:
On Your Property: If the leak is on your property (e.g., from an external overflow pipe or outdoor plumbing), it's typically the homeowner's responsibility.
Public or Shared Systems: For leaks in public water lines or shared external plumbing, the local water utility or homeowners’ association may be responsible. It’s best to report these leaks to them for assessment.

Why Is My Boiler Leaking from the Pipe Outside?

If your boiler is leaking from the pipe outside, it could be due to:
Excessive Pressure: High pressure inside the boiler can force water out through the pressure relief valve.
Faulty Pressure Relief Valve: A valve that doesn’t close properly can leak water even when pressure is normal.
System Overheating: The boiler may release water to prevent overheating.
Expansion Vessel Failure: A malfunctioning expansion vessel may not absorb excess pressure correctly, leading to leaks.
Addressing boiler leaks promptly by consulting with a certified professional is crucial to maintaining the system’s safety and efficiency.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.