Home Improvement

Overflow Pipe Leaking: Reasons and Solutions

It’s only natural for things around your home to break after prolonged use. This goes for your water supply fixtures, as well – water tanks, boilers, toilets – what happens when there’s a fault with either of them and they begin to overflow? Well, if it weren’t for overflow pipes, the answer would be a flooded room and a damaged floor.

An overflow pipe leaking is a sign of an overflow pipe doing its job. But there’s always a reason behind the leak. And we’re here to teach you how to identify the underlying issue and what you can do to solve it.

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So, if you:

  • Are having issues with a leaking overflow pipe;
  • Want to identify where the leak is coming from and fix it;
  • Are trying to find out what’s causing the leak,

Then keep on reading, because this post is just for you!

What is an overflow pipe?

Before we get into the issue itself, it’s important to understand what an overflow pipe is. It’s a pipe connected to a cistern or water tank in your home, most often leading to the outside. Its purpose is to let any excess water from the tank run outside instead of indoors, preventing damage to your home.

You can find an overflow pipe attached to the following items:

  • Toilet cistern, cold water tank, central heating water tank, combi boiler
    These overflow pipes lead to the exterior of your home.
  • Copper cylinder
    The overflow pipe on a copper cylinder (or hot water cylinder) is also referred to as an expansion/vent pipe. It runs from the cylinder into the cold water feed and expansion tank.
  • Sinks, baths, basins
    The overflow pipes on any kitchen or bathroom sinks and baths lead the water into the drain, rather than outdoors.

How to fix a leaking overflow pipe

Now that you know what an overflow pipe’s purpose is, let’s find out what to do if you find that it’s leaking.

Most of the time, if an overflow pipe is leaking, the culprit is a faulty float valve. This is a component found in cold water and central heating tanks and toilet cisterns. It’s made up of a plastic or metal arm and a plastic ball attached to the end. The ball floats on top, controlling the amount of water that flows into the tank or cistern, keeping it full and, at the same time, stopping it from overflowing.

If the float valve is faulty, the water running into the tank might not shut off completely. This results in an overflowing tank or cistern, hence the leaking overflow pipe.

So, what can you do to fix this issue?

Identify where the leak is coming from

First, you need to find out where the leak is coming from.

If the pipe is located close to the ground, it’s most likely the toilet cistern that’s leaking. If the dripping pipe is at loft level, coming out of the eaves of the roof, the fault lies in your cold water or central heating tanks.

To make sure you’ve located the leak, check the water level in each water tank and cistern. If the water level is too close to the overflow point or higher than it usually is, then that’s where the leak originates.

Shut off and drain the water

In order to do anything to fix the problem, you’ll need to drain the water first.

For a toilet cistern, turn off the water supply by using the isolation valve. You can find it on the water pipe leading into the cistern. Grab a flat head screwdriver and turn the screw clockwise a quarter turn. Drain the water by flushing the toilet.

For water tanks, turn the stopcock to shut off the mains water. Drain the tank by running the bathroom taps. You only need to drain it enough to clear the spot where you need to work.

Now you’re prepared. But the way to fix the issue differs, depending on the source of the leak.

Cold water tank overflow pipe

The leaking overflow pipe may be coming from your cold water feed and expansion tank – the large water tank in the loft – which supplies your sinks, bath, and copper cylinder. The pipe runs to the outside through the eaves of the roof and is made of either plastic, copper, or lead, with plastic being the more commonly used material.

There are two reasons why your cold water tank overflow pipe may be leaking:

  • The water feed isn’t stopping
    If water is constantly flowing into the tank, the likely culprit is the float valve. Check to see if it’s stuck. Lift it up and see if anything is blocking it from moving. If that’s not the case, check if the ball at the end is perforated and filled with water. In this case, you’ll need to replace it. If you still haven’t found the problem, the float valve washer is likely worn out and in need of replacing.
  • Water is coming back up the outlet pipes
    If the issue is not with the float valve, then there is probably excess water coming from the outlet pipes. These are the pipes that lead water from the tank to your taps and hot water cylinder. The reason is a faulty mixer tap that’s connected to the mains water. The cold mains water can push the hot water back up the pipes, as it’s at a higher pressure, resulting in an overflowing tank. The easiest way to diagnose this is to check whether the overflow pipe is only leaking when you’re using the mixer tap. If you find that this is the problem, you’ll need to contact a professional to deal with it. Check this plumbing cost guide for prices to get a rough idea of how much you’d possibly pay for pipework.

Central heating tank overflow pipe

The central heating feed and expansion tank is the smaller one in your loft. This tank supplies your heating system with water. Again, the overflow pipe leads to the outside through the eaves and is made of plastic, copper, or lead.

The two possible culprits for a leaking overflow pipe from a central heating tank are:

  • The water feed isn’t stopping
    Just like with the cold water tank, the float valve is to blame here. Check to see if it’s stuck, or if the ball, washer, or the whole valve are in need of replacing.
  • Faulty hot water cylinder
    If the float valve is fully operational, the other possible reason is that there’s an issue with the hot water (or copper) cylinder. This can be caused by two things which we will cover next.

Copper cylinder overflow pipe

The overflow pipe of a copper (or hot water) cylinder leads into the cold water tank instead of the outside. This lets the water in the cylinder expand when heated.

If you find the hot water cylinder overflow pipe leaking, there are two possible causes:

  • Your thermostat is set to a higher temperature
    While it’s not unusual for the overflow pipe to drip when the water is heating, a stream of water indicates that the tank is overflowing due to the water being too hot. This is an easy fix – just turn your thermostat down a notch.
  • Damaged copper cylinder coil
    Hot water cylinders have a coil inside, which gets hot water from the boiler. The rest of the cylinder gets cold water from the tank. If this coil is perforated, the hot water inside it will leak into the main cylinder, mixing with the cold water. When this occurs, the central heating tank often overflows. Unfortunately, you can only solve this issue by replacing the copper cylinder. This is best left to a professional plumber.

Boiler overflow pipe

Technically, this isn’t exactly an overflow pipe. Combination (or combi) boilers have one much like it, though – the pressure relief valve. If the boiler builds up too much pressure, this valve releases some water to lower it. The pipe is made of copper and leads to the outside of the house.

You might confuse the pressure relief valve with the condensate pipe. However, they differ, and not only in their purpose. The condensate pipe is plastic and sometimes leads to a waste pipe instead of outside.

So, why is your boiler overflow pipe leaking?

  • There is too much pressure inside the boiler, possibly due to a failing expansion vessel.
  • The pressure relief valve isn’t seated right.
  • The boiler is way too old and has leaks not only from this valve but other parts as well. Check our post on boiler leaking water for more information.

As boiler repairs can be dangerous, we recommend you call a Gas Safe registered boiler engineer to carry them out and ensure the safety of your home and family.

Find out what the expert boiler engineers at Fantastic Services can do for you!

Toilet overflow pipe

Locating a toilet overflow pipe is easy – it runs to the outside and is located directly behind your toilet. As with the water tanks, the pipe is made of plastic, copper, or lead.

If your toilet overflow pipe is leaking, it means the water supply to the cistern isn’t stopping. There are a couple of things you can do:

  • Check the float valve
    As described above, the float valve might be at fault here. It might simply be set too high. Try lowering its position – if the arm is metal, just bend it down slightly; if it’s plastic, find the adjustment screw at the top and turn it. If that doesn’t do the trick, go through the checks we mentioned earlier to see if it’s faulty and if its parts (or the whole thing) need replacing.
  • Check the water supply pressure
    The water could be coming into the cistern at too high a pressure, causing the overflow pipe to leak. To lower the pressure, turn the isolation valve located on the water supply pipe.

Bath or sink overflow pipe

Unlike the rest of the pipes we mentioned in this post, bath and sink overflow pipes don’t leak outside. You might recognise these as the small openings at the back of a basin below the rim. These openings are connected to pipes running directly into the drain. Their purpose is to drain the basin if you leave the water running with the plug in.

To stop the sink from overflowing, just take out the plug and let it drain. However, if it’s filling up with water without the plug being in place, then you’re probably dealing with a blocked drain.

You may also like:
Cleaning Guides
How to Unclog a Kitchen Sink Drain with Standing Water

Don’t want to tackle a leaking overflow pipe on your own?

If you’re not a fan of DIY home repairs, or you’re dealing with an issue that requires professional help, look no further than Fantastic Services! The experienced and certified plumbers we work with will make sure any problems are expertly resolved. And, should you need boiler repair, the Gas Safe registered engineers can handle it all, ensuring the safety of you, your family, and your home. So, book your service today using our simple booking form!

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  • Overflow pipes can be found on cold water and central heating tanks, hot water cylinders, toilet cisterns, combi boilers, sinks and baths.
  • Most overflow pipes lead to the outside of the house.
  • Their purpose is to prevent the tanks or basins from overflowing and damaging your home.
  • Before you start fixing a leaking overflow pipe, you need to identify where the leak is coming from and shut off the water supply to that item.
  • The reason for a leaking overflow pipe is most commonly a faulty float valve.
  • Some issues that can cause an overflow pipe to leak require professional help.

And there you have it – hopefully, you’ve found the solution to your overflow problem. And, if you’re dealing with even more leaks, read our guide on how to find a leaking pipe and find out what you can do.


Was this post helpful to you? How did you handle your leaking overflow pipe? Let us know in the comments below!

Image source: Shutterstock / Peter Baxter

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