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How to fix a leaking overflow pipe

Wondering why your overflow pipe is leaking? This guide will help you identify the cause of the leak and how to fix it.

First things first, you may be wondering, what is an overflow pipe for? Well, if it wasn’t for overflow pipes, things like water tanks, boilers, and toilets would end up overfilling and therefore flooding rooms. Overflow pipes are there to remove any excess water and ‘send’ it to the drainage system.

Weirdly enough, a leaking overflow pipe is a sign of the pipe doing its job. However, there is always a reason behind a leak, and it will need to be rectified, or it could lead to bigger issues.

This guide is here to help you identify your specific leak and find the solution to fix it. However, some issues that can cause an overflow pipe to leak require skilled, professional help.

Don’t want to tackle the problem on your own? Call in an experienced, trustworthy plumber who will be pleased to assist.

Why is my overflow pipe leaking?

An overflow pipe is fitted to the following water supply fixtures to avoid them overflowing:

  • Toilet cisterns, cold water tanks, central heating water tanks and combi boilers all have overflow pipes that are connected to the exterior of the house
  • A hot water cylinder has an overflow pipe that’s also known as an expansion or vent pipe. This pipe runs from the cylinder into the cold water feed and expansion tank
  • Sinks, baths, and basins all have overflow pipes that lead the water into the drain rather than outdoors

Reasons for leaking overflow pipe

When you discover a dripping or running overflow pipe outside, the most common problem is the float valve, which is found in toilet cisterns, cold water tanks and central heating feed and expansion tanks. You know the thing we mean – that strange-looking plastic arm and ball that goes up and down with the level of the water (hence, why it’s called a float valve).

When the water in the tank is used, the level of the water goes down, and so does the float valve. The movement from this then turns the cold water feed on so that the tank refills. As the water level rises, the ball also rises to switch the water feed off again when the tank is full. However, if the float valve isn’t working properly, the cold water feed might not switch off and could cause the water level to continue to rise above the overflow pipe. Voila! You have your leak!

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How to fix a leaking overflow pipe

Whether it’s a cracked pipe or you have boiler leaking water from your overflow pipe, keep reading to find out how to fix a leaking overflow pipe.

How do I stop my overflow pipe from leaking?

Step one

Firstly, you need to identify what tank or cistern is overflowing by working out what the pipe is attached to. If it’s at ground or first-floor level, then it’s probably going to be your bathroom overflow pipe dripping. This could be coming from the bath, basin or toilet cistern. However, if the leaking overflow pipe is coming from the loft, then it could be your cold water or central heating feed that is leaking.

If you’re still unsure and need help solving the puzzle, call in a plumber. They will be able to check the water level in each of these places and find the culprit.

Step two

If you’ve found the culprit and water is running continuously into the tank, check that the float valve isn’t being held down by an obstruction. If it’s moving with no problems, the plastic ball could be punctured and filled with water. It will need to be replaced if so.

Still haven’t found the problem, and the water is still running? The washer on the valve could be worn and also need replacing. We recommend speaking to an experienced plumber for help.

Step three

A problem with your float valve? Then you’ll need to replace it or replace the washer, as mentioned above. But first, you’ll need to stop the flow of new water into the problematic tank and drain it so you can work.

How to stop the flow of new water in different tanks:

Toilet cistern: Use the isolation valve to turn the water feed off. The valve is located on the water pipe that leads into the cistern. You will need a flathead screwdriver to turn the screw, then flush the toilet to drain the water.

Water tank: Stop the water feed by turning the mains water off at the stopcock. Then drain it by turning on your bathroom taps.

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How to fix a leaking overflow pipe from the toilet

If you’ve noticed that you have water leaking from your toilet overflow pipe, below are some tips for how you can fix the problem.

Tips on fixing a leaking toilet overflow pipe

The toilet overflow pipe is pretty obvious as it’s directly behind the toilet and runs outside. If it’s leaking or dripping, it means the water supply to the toilet cistern isn’t turning off. So, here’s what you can do:

  • Check the float valve isn’t obstructed
  • Adjust the position of the float, as it might be set too high to turn the water off
  • Check the water supply pressure isn’t too high, causing the overflow pipe to leak. Lower it by turning the isolation valve on the water supply pipe
  • Replace the washer in the float valve
  • Replace the float valve itself if it’s punctured

If this all sounds beyond the realm of your DIY skills, a plumber will be happy to help. 

Is a leaking overflow pipe dangerous?

The leaking pipe itself isn’t dangerous, it’s what it leaks onto. For example, leaking water that’s left can cause damage to walls and foundations.

On the other hand, if the leak is coming from an overflow pipe from a gas boiler, this could be dangerous. You will need to call a Gas Safe boiler engineer immediately to carry out the repairs safely.

Combi boiler leaking water from overflow pipe

Combi boilers have an overflow pipe that leads to the outside of the house and consists of a pressure relief valve. This valve will release water when the system is under too much pressure.

Boiler leaking water from overflow pipe

The boiler pressure relief valve only flows when there is a problem in the system. If you see this valve leaking, you should immediately take heed and call a plumber.


What is the pipe dripping water outside my house?

The pipe dripping water outside your house is an overflow pipe that’s most likely connected to an overflowing toilet cistern, cold water or central heating feed and expansion tank. It could also be your boiler overflow pipe dripping due to a faulty combi boiler. If you have any concerns, call in a plumber as soon as you can.

Why is hot water coming out of my overflow pipe?

This is likely to be coming from your boiler or water heater. A water heater only has a life span of 8-15 years and can be blighted by rust, sediment build-up and corrosion that can eat away the bottom of the tank and cause a leak into the overflow.

To stop the leak, shut off the water supply, turn off the heating, and stay well away from any hot water that may scald. Then call a reliable plumber immediately for help.

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What others think of this article:


Plumber is coming and everything you listed I wrote down. Overflow pipe outside is making a pool of water, tried to locate toilet valve to shut off the water, but couldn't find it. Water on floor, and dampness on wall. Thanks for your advice.

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