- What is an overflow pipe?
- What to do if your overflow pipe is leaking
- Toilet overflow pipe leaking
- Cold water feed and expansion tank overflow pipe leak
- Overflow from central heating feed and expansion tank
- Overflow from a copper cylinder
- Boiler overflow pipe
- Sink overflow pipe
- Why is my overflow pipe dripping outside?
What is an overflow pipe?
An overflow pipe leads from a water storage tank or cistern inside your home to the outside. They’re a vital part of your home’s plumbing system. If the water in the tank or cistern fills up to a higher level than it should, the excess water flows down through the overflow pipe and runs outside, rather than leaking into your home.
Some of the most common components in your home that require an overflow pipe are:
- Toilet cistern
- Cold water feed and expansion tank
- Central heating feed and expansion tank
- Combination boiler
- Copper cylinder
- Sinks, basins and baths
Of these common components, the first four overflow to the outside of your house. The overflow pipe (also known as the expansion/vent pipe) on a copper cylinder flows into the cold-water feed and expansion tank, and the overflow on basins and baths feed into the drain.
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Why is my overflow pipe leaking?
When an overflow pipe is dripping or running with water, it may be due to a problem with a float valve. Float valves are found in toilet cisterns, cold water tanks, and central heating feed and expansion tanks. It’s a device that controls the water level, and consists of a metal or plastic arm with a floating plastic ball on the end.
When the water in the tank is used, the ball float drops with the water level. This movement turns the cold-water feed on so the tank refills. As the water level rises and the tank fills up, the ball rises to switch the water feed off again.
The maximum water level in the tank should be about one inch below the overflow pipe. If the float valve doesn’t operate properly, the cold-water feed may not fully switch off, causing the water level to continue to rise above the overflow.
If you’re unsure whether it’s an overflow pipe that’s leaking, follow our handy guide on how to diagnose leaky pipes. We can help you find the problem quickly and easily:
1. Identify which water tank or cistern is overflowing
If the leaking pipe is at ground or first-floor level, then it’s likely to be your bathroom overflow pipe dripping water from the toilet cistern. However, if it’s at loft level and protrudes from the eaves of the roof, then it could be coming from your cold-water or central heating feed and expansion tank.
Check the water level in each of these tanks if you’re still unsure. If the water level is close to the overflow – or higher than the ring of limescale that indicates its usual level – then you’ve found the culprit.
Top tip: If there’s only a small amount of water dripping into your cistern or tank, it may take hours for it to fill above the overflow level. Normal water usage during the day could keep the level low enough to disguise the leak, so you may only notice it early in the morning.
2. Check the float valve
If water is running continuously into the tank, the float valve could be stuck in an open position. Lift it up to release it and check nothing is stopping it from moving freely. If it’s moving with no problems, check the plastic ball isn’t punctured and filled with water, in which case, it will need to be replaced.
If neither of these are the problem, and the water still doesn’t shut off, the washer may be worn and in need of replacement.
3. Turn off the water feed and drain the water
If you’ve identified that there’s a problem with your float valve, you’ll need to replace it or replace the washer as mentioned above. First, you’ll need to stop the flow of new water into the tank and drain it so you can work.
- To turn the water feed off on a toilet cistern, use the isolation valve – this is a valve located on the water pipe that leads into the cistern
- Use a flat head screwdriver to turn the screw a quarter turn clockwise
- Flush the toilet to drain the water
- Stop the water feed into either of the water tanks by turning off the main water supply at the stopcock
- Drain the water by turning on your bathroom taps, just enough so that the water level is below the area you need to work on
Your next steps will depend on what’s leaking and why.
Toilet overflow pipe leaking
The toilet overflow pipe runs from your toilet cistern to the outside of your house. It will be made of either plastic, copper or lead and will usually lead out through the wall directly behind the toilet.
If your toilet overflow pipe is leaking outside, it’s because the water feed into the cistern isn’t switching off. There are several possible fixes for this:
- Check the float valve isn’t stuck (as described above): It could also be tangled up in the chain that pulls open the flapper valve when you flush.
- Adjust the position of the float: The height required for the ball to shut the water feed off may be set too high. If the arm is metal, try lowering the ball height by bending the arm down. If it’s plastic, you’ll find an adjustment screw at the top of the arm. Turn it slightly to move the position of the float.
- Check the water feed pressure: The overflow pipe could be leaking because the pressure of the water coming into the tank is too high. Use the isolation valve (see steps 1 and 2 above) to reduce the flow of water.
- Replace the washer in the float valve: If the water feed still isn’t completely shutting off once the cistern is full, try changing the washer. First, turn off the water supply and drain the cistern (as described in step 5 above). Take the float valve apart and remove the old washer. Take the old washer to a DIY store when you get the replacement to ensure you get the right size. Put the new washer in and put the float valve back in place.
- Replace the float valve: If the float valve is damaged or there’s a puncture in the plastic ball, it will need to be replaced. Again, shut the water supply off and drain the cistern (as described in step 5 above). Remove the float valve and take it to a DIY store when you get the replacement to ensure you get the right size. You can now install the new float valve.
Cold water feed and expansion tank overflow pipe
The cold water feed and expansion tank overflow pipe runs from the large water tank in your loft to the outside of your house. It will be made of either plastic, copper or lead and usually leads out through the eaves and soffits. The tank supplies your bath and basin taps and your hot water cylinder with water.
If your cold water feed and expansion tank overflow pipe is leaking, there are two possible causes:
1. The water feed into the tank isn’t switching off
Look inside the tank. If you can see water constantly running or dripping from the water feed, check if the float valve is stuck. As and if needed, adjust the position of the float, replace the washer or replace the float valve.
2. Water is being pushed back up the outlet pipes
If your float valve is working fine, and the cold water feed to the tank switches off when it should, then excess water can only be coming back up through the outlet pipes. These are the pipes that feed water into your taps and water cylinder.
This problem is caused by a faulty mixer shower or tap that’s plumbed into the mains water feed. When you use the mixer, the high-pressure cold mains water pushes the low-pressure hot water back up the pipes into the cold water tank, causing the tank to overfill. If this is the case, you may notice that the overflow pipe only leaks when you use the faulty tap or shower.
For this problem, it’s best to call out a professional plumber who can diagnose the problem and fix it for you.
Overflow from central heating feed and expansion tank
The central heating feed and expansion tank overflow pipe runs from the small water tank in your loft to the outside of your house. It will be made of either plastic, copper or lead and usually leads out through the eaves and soffits. The tank supplies your heating system and allows water to expand safely within the system when it’s heated.
If your central heating feed and expansion tank overflow pipe is leaking, there are two possible causes:
1. The water feed into the tank isn’t switching off
When you look inside the tank, can you see water constantly running or dripping from the water feed? If so, ensure that the float valve isn’t stuck or doesn’t need adjusting. Alternatively it may be that the washer or whole float valve needs replacing.
2. There is a problem with your hot water cylinder
If your float valve is working fine, and the cold water feed to the tank switches off when it should, then the only other cause is a problem with your hot water cylinder. Read the next section to find out more.
Overflow from a copper cylinder
The overflow (also called the expansion/vent pipe) from a copper cylinder runs into the cold water feed and expansion tank in your loft. This pipe allows for the water in the cylinder to expand when it’s heated.
There are two possible reasons for your copper cylinder to be causing an overflow pipe to leak:
1. The temperature on your thermostat is too high
It’s normal for the overflow pipe on a copper cylinder to drip when your water is heating. But if you find it’s running with water and causing your tank to overflow, then your water could be getting too hot. To fix this, simply adjust the temperature on your thermostat down a little.
2. The coil inside the copper cylinder is perforated
Inside the copper cylinder, there’s a coil that’s supplied with hot water from the boiler. The rest of the copper cylinder is supplied with cold water from the cold water storage tank.
An overflow problem occurs when there is a perforation in the coil, and the water inside it leaks out into the main cylinder. When this happens, the domestic water (that supplies your cold taps and toilets) and the central heating water (that supplies your hot taps, boiler and radiators) mix.
Gravity tries to equalise the water level between the cold water feed and expansion tank and the central heating feed and expansion tank – and since the central heating tank has a lower water level to start with, it’s usually this tank that fills above its normal level and overflows.
The only solution for this problem is to replace the copper cylinder. It’s best to call out a professional plumber who can carry out the work for you.
Boiler overflow pipe
Combination boilers have a pipe that’s similar to an overflow pipe, called a pressure relief valve. This valve releases water if the system builds up too much pressure. The pipe is copper and runs from the boiler to the outside of your house, and only releases water if there’s a problem.
Be careful not to confuse the pressure release valve with the condense pipe. The condense pipe is plastic and is designed to drip when your central heating is turned on or when you’re running hot water. You may only see one external pipe, as sometimes the condense pipe is run internally into a waste pipe.
Your boiler’s overflow pipe may leak water if:
- The boiler has built up too much pressure due to the expansion vessel failing or losing its charge
- The pressure relief valve is not seated correctly, causing water to pass by it
For safety reasons, it’s best to call out a Gas Safe registered engineer who can come and fix this problem for you.
To protect your boiler from problems in future, follow these seven steps to help avoid a boiler breakdown.
Sink overflow pipe
The overflow on a sink is a small opening that’s found on the back of the sink, just below the rim and is connected to a pipe that leads down into the drain. These overflows are found in bathroom basins, baths and kitchen sinks, and help to stop your sink overflowing if you leave the tap running when the plug is in.
Unlike other overflows, you will see the problem in the sink itself rather than notice a dripping overflow pipe outside your house.
The simple solution to an overflowing sink is to pull the plug out and allow the bowl to drain.
Overflow pipe dripping outside
In summary, if you see an overflow pipe leaking outside, the likely cause is an overflowing toilet cistern, cold water or central heating feed and expansion tank. The issue could also be your boiler overflow pipe dripping due to a faulty combination boiler.
But remember, if your cold water or central heating feed and expansion tank is overflowing, it could be your copper cylinder that’s the cause, since these are connected.
If a basin, bath, or kitchen sink overflow pipe is leaking, you won’t see this outside as these pipes lead directly down the drain.