Overflow Pipe Leaking Causes And How To Fix It

James Elston
Written by James Elston
Updated on 5th December 2023
Posted on 4th December 2023
Overflow Pipe Leaking: Causes And How To Fix It

Why is my overflow pipe leaking & how do I fix it?

hot water

You don’t have to be a professional plumber to know that breaks in your water supply fixtures can leave your home flooded. That’s why overflow pipes are such an essential part of a home’s plumbing system. 

Leaks are clear indications that your overflow pipe is doing its job, however, there is always an underlying cause of the leak, and that’s what this guide is all about.

Keep reading if you want to learn the various causes of leaks in your overflow pipe, how to identify the source of the leak, and how to fix an overflow pipe leaking issue.

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What is an Overflow Pipe?

system boiler diagram with hot water tank

An overflow pipe extends to the outside from a cistern or water storage tank inside your home. Its primary function is to let excess water from your cold water storage tank flow outside your home instead of flooding and damaging your home.

The overflow pipe can also feed into the drain, in some cases, particularly when connected to a basin or bath. When connected to a copper cylinder, the water flows into an expansion tank.

The common plumbing systems that often have overflow pipes attached to them include:

  • Baths, sinks, and basins
  • Central heating water tank
  • Coldwater tank
  • Combi boiler
  • Toilet cistern
  • Copper cylinder

Causes of Leaks in an Overflow Pipe

overflowing background

While you want your overflow pipe to do its job, an overflow pipe leaking is a sign that something is wrong with your plumbing system. If water continues to leak from the overflow pipe for a long time, it can result in moisture damage. This can affect fixtures and appliances in the kitchen, bathroom, and other parts of your home.

You may be able to prevent flooding in your home, but it is difficult to rule out mould. For this reason, you should take steps to find the cause of the leak and take urgent remedial measures.

If your overflow pipe is leaking, the following are the possible culprits:

1. Worn Washers

Washers are the most common causes of leaks in an overflow pipe, and that’s not surprising considering their constant contact with water. Whether you have a low-quality washer or the washer has worn out over time, replacing a worn washer can stop the leak.

You should consider the cost of a boiler service as things like this could get picked up before they become a major problem. “The good news is that washers are usually quick and easy to replace provided the problem can be easily located” comments Ruban Selvanayagam of Property Solvers.

2. Stuck or Faulty Float Valve

Check if the float valve is flowing freely when the water is running. Try to release the float valve if it is stuck. If the valve moves but doesn’t rise, it might have a hole in it. Get a new one to replace the old one.

Replacing a broken float valve is not a complicated task, and it is a lot cheaper when done as soon as you detect a problem. If you let it linger for too long, it can lead to serious plumbing issues that can be significantly expensive.

3. Increased Water Pressure

If the water pressure is too much for your overflow pipe to handle, it can cause leaks. The water valve in the overflow pipe can come loose due to constant higher water pressure. Continuous increased pressure can also damage the pipe and cause leaks.

4. Broken Coil

A broken coil in a hot water cylinder can also cause an overflowing cistern. A punctured coil allows the hot water from the feed and expansion cistern (F&E) to mix with the water in the cold water storage cistern – two water bodies that should never mix in the first place.

The result is that the water pressure in both systems will try to equalise. Unfortunately, this is not possible since the water level in the F&E tank is usually lower than the water level in the cold cistern. Of course, this ends up in a continuous overflow in the F&E tank and resulting in a leak.

Top Tips on How to Identify and Fix a Leaking Overflow Pipe

Now that you know the likely causes of leaks in overflow pipes let’s learn how to identify where the leak is coming from. Of course, that’s the first step in fixing the problem.

1. Determine Which Cistern is Overflowing

If the dripping pipe protrudes from the eaves of your roof or is at loft level, the issue is likely with your central heating feed and expansion tank or cold water tank.

On the other hand, if the dripping pipe is on the first floor or ground level, your toilet cistern is likely overflowing.

If you’re not sure about where the leak is coming from, you may have to check the water level in each of the cisterns. The leak is likely coming from the water tank with a water level higher than it normally should be or nearest to the overflow level.

An easier way to be sure is to call a Gas Safe engineer to determine where the leak is coming from.

2. Turn Off the Water Feed and Drain the Water

The next step in fixing a dripping overflow pipe is to drain the water. The way to do this depends on where the problem is.

  • If the problem is with the toilet cistern, you need to turn off the water feed using the isolation valve. Get a flathead screwdriver, locate the isolation valve on the water pipe leading to the cistern, and turn the screw about a quarter turn. Next, flush the toilet to drain the water.
  • For water tanks, shut off the water from the mains by turning the stopcock. Now, run the bathroom tops to drain the tank.

Once you’ve drained the water, it is time to fix the problem. The exact steps to do this will depend on where the leak is coming from, as discussed below.

Overflow from Coldwater Feed and Expansion Tank

The large tank in your loft is the cold water feed and expansion tank. This supplies water to the baths and sinks in your home. The overflow pipe for this tank is mostly available as plastic, but it is not uncommon to find lead and copper options. The pipe connects from the tank and runs to the outside via the eaves.

If this overflow pipe is leaking, it is possibly a problem with the water feed or the water is pushed back up the outlet pipes. Here’s what to do in either case.

1. The Water Feed is Not Switching Off

Check to see if the float valve is stuck. Something may be blocking it and causing water to flow constantly into the tank. Inspect the ball to see if it is punctured and filled with water. The float valve washer may be worn and need to be replaced.

2. Water Comes Back up the Outlet Pipes

If the overflow pipe leaks only when you use a mixer shower or tap, the problem is likely with excess water coming back up via the outlet pipes. If this is the case, it is best to call a professional to diagnose and fix the issue.

Overflow from Central Heating Feed and Expansion Tank

The smaller tank in your loft is the central heating feed and expansion tank. Its overflow pipe is usually plastic, lead, or copper and runs from the tank that supplies water to your heating system to the outside of your home through the eaves.

If you have leaks in the overflow pipe of your central heating feed and expansion, you probably have a problem with the copper or hot water cylinder or the water feed into the tank is not shutting off. Here’s what you can do in either case before starting to check radiators and turning them off.

1. Problem with the Copper Water Cylinder

A problem with the copper or hot water cylinder can cause water to leak in the overflow pipe of your central heating feed and expansion tank. If the float valve is not stuck and there are no issues with the washer, the copper cylinder is most likely faulty.

We’ll get into the details on correcting this problem in the latter part of this guide. For now, it is important to mention that the issue is best handled by a professional.

2. The Water Feed to the Tank is Not Shutting Off

If the water feed to the tank continuously drips and won’t shut off, the most likely cause is the float valve. Like leaks in the overflow pipe of the cold water feed expansion tank, check the float valve and washer. You might need to untangle the float valve, replace a damaged float valve, or get a new washer.

Toilet Overflow Pipe

The toilet overflow pipe is usually plastic, but it can also be lead or copper. It runs from the toilet cistern and typically comes out behind the toilet wall.

If the water supply to the toilet cistern is not switched off, it can cause a leak in your toilet overflow pipe. Here are a few things you can do to fix this problem:

  • Check the pressure of the water feed: If the pressure in the water supply to the tank is too high, the toilet overflow pipe will start to leak. If this is the case, use a flathead screwdriver to turn the isolation valve. This will stop the flow of water.
  • Make sure the float valve is not stuck: When you flush the toilet, a chain pulls the flapper valve so that it opens. Sometimes the float valve might get stuck in this chain and cause an overflow pipe leaking. If this is the case, simply untangle the float valve from the chain to fix the problem.
  • Replace damaged float valve: Any damage to the float valve can cause the toilet overflow pipe to leak. You will need to replace the plastic float ball if there are signs of visible damage, such as a puncture. Remember to turn off the water supply before you drain the cistern. It is usually a good idea to bring the old float valve along when purchasing a new one to make sure you buy the right size. Install the new float valve, and the water should stop leaking.
  • Change the position of the float: If the float ball is too high, it won’t be able to shut off the water feed to the cistern at the right water level. This can cause water to leak from the toilet overflow pipe. To correct this, adjust the screw at the top of the plastic arm or bend the arm a bit if it is a metal arm to lower the ball height.
  • Replace worn washer: If the washer is worn out, it will not properly shut off the water feed to the cistern, leading to leaks. If this is the case, shut off the water supply to the cistern before draining it. Remove the float valve to enable you to access and take out the worn washer. Consider visiting your local home improvement store with the old washer so that you can pick out the right size. Install the new washer, and the overflow pipe should stop leaking.

Sink Overflow Pipe

bathroom sink overflowing

Whether it’s your kitchen sink or bathroom basin, the overflow pipe is usually the small opening at the back of the sink right under the rim. This opening connects to a pipe that leads down the drain, preventing the sink from overflowing in case the plug is in, and the tap is left running.

Typically, an overflowing sink doesn’t drip outside your home. Instead, the problem is right in the sink and not with an overflow pipe protruding outside the house. This makes it even easier to fix the problem. Simply pull the plug out and let the sink drain.

Boiler Overflow Pipe

The pressure relief valve in a boiler is a pipe that’s similar to a regular overflow pipe in your home plumbing system. The pipe prevents a build-up of pressure in the system by releasing water when the system is under high pressure.

This valve extends from the boiler to outside the house. It only leaks or releases water if there is an issue with the system. The problem might be due to a loss of charge in the expansion vessel or the expansion vessel failing. It is also possible that the pressure relief valve is installed wrongly, thereby leading to the boiler overflow pipe leaking.

This type of overflow pipe leak is better handled by a certified Gas Safe Registered engineer. Attempting to fix this problem by yourself can compound the issue, especially if you confuse the boiler condensate pipe with the pressure relief valve.

The condensate pipe is usually plastic and may run internally into a waste pipe or externally like a regular overflow pipe. On the other hand, the pipe of the pressure relief valve is copper and runs from the boiler to outside the house. The condenser pipe drips when you are running hot water in your central heating system. Sometimes this can leak if you had a frozen condensate pipe in the winter.

All of these can easily confuse a non-professional, which is why it is best to call in an expert as soon as you notice a problem with your boiler overflow pipe.

By the way, if water leaks from your boiler or you can’t explain why there’s a pool of water underneath your boiler, you might want to take a look at this guide on boiler leaks to know the likely causes and how to fix the problem.

Overflow from the Copper Cylinder

The overflow pipe of the hot water or copper cylinder connects to the cold water tank and allows for the expansion of the cylinder as the water is heated.

If you notice the copper cylinder overflow pipe leaking, the usual culprits are the copper cylinder coil or your thermostat setting.

Let’s briefly discuss each below.

1. Problem with the Copper Cylinder Coil

The copper cylinder gets both hot and cold water but is compartmentalised separately. Hot water is supplied from the boiler to the cylinder via a coil, and the rest of the main cylinder gets cold water from the tank.

If the coil should become damaged or perforated, hot water from the coil will leak into the cold water and mix with it in the main cylinder. This causes the central heating tank to overflow.

The best way to fix this problem is to replace the copper cylinder coil. As you may have guessed, this is not your typical DIY repair. It is usually best to call in a certified professional to rectify the problem.

2. Thermostat Temperature Is Set Too High

hand adjust the dial controlling water temperature on a white boiler

Don’t worry if the overflow pipe on the copper cylinder drips when you heat water; this is normal.

However, if there’s a steady stream of water from the pipe, the water in the tank is too hot, making it overflow. Thankfully, this problem is not difficult to fix. Simply turn down your thermostat to prevent the water from boiling over.

Key Takeaways

An overflow pipe leaking outside your home is usually traceable to a central heating feed and expansion tank or your toilet cistern. A faulty boiler could also be the culprit in some cases.

Although the overflow pipe of the kitchen sink or bathroom basin could also leak, the water doesn’t flow directly outside your home. It goes down your drain instead.

Tracing and fixing these issues might sound a little out of your league if you don’t like DIY home repairs. Thankfully, you don’t have to do any plumbing jobs you’re not cut out to do.

Don’t hesitate to contact us if you need help with your plumbing system, central heating system, or a new boiler. We’ll be glad to connect you with Gas Safe registered engineers to handle any issue you may have in this regard.

Written by
James Elston
James Elston Director Of Boiler Central
Posted on: 4th December 2023

James Elston is our resident boiler replacement and heating expert here at Boiler Central. With over 20 years experience in the boiler installation industry, James ensures that he knows everything there is about our Gas Safe boiler installations, energy saving and home heating solutions. This can be from simply procuring the latest best combi boilers, to reviewing and ensuring that Boiler Central maintains the highest standards across our boiler installation company.

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