Overflow pipe leaking: why is it leaking & how to fix it

An overflow pipe leaking or dripping is most likely due to a faulty cistern ball valve. You can fix this by replacing the valve or some of its internal components.

A dripping overflow pipe can also indicate a fault somewhere else on your plumbing system. This guide should help you diagnose the problem. On the other hand, if your pipework itself is leaking, check out this article on how to fix leaking copper pipe.

Overflow pipe leaking – finding the source

The first thing to do is identify what is actually overflowing. If the overflow pipe is plastic and at ground-floor or first-floor level, then a toilet cistern is probably the culprit.

If the pipe is metal, it probably belongs to your boiler or an unvented hot water cylinder. In both cases, the pipe takes water away from the pressure release valve.

There may be two pipes sticking outside from your boiler. The other is the condensate pipe, which drains away waste water from the condensing process. It’s normal for water to come out of this pipe as part of the boiler’s normal operation. (However, it’s wrong if the water simply drips out onto the ground. This pipe should terminate in a gully below the leaf guard. Ideally, it should be connected directly to the soil pipe.)

If the leaking overflow pipe is at loft level or sticks out from the eaves of the roof, then the source is either the cold water storage cistern, or the smaller feed and expansion cistern that supplies the boiler, if you have one.

Which cistern is overflowing?

If you don’t know which overflow pipe belongs to which cistern, simply looking inside each cistern should tell you. If it’s not immediately obvious, take a look at the water level. Is it extremely close to the overflow, or higher than the level where there’s limescale residue? This may give you the answer.

Remember, it can take hours for a small leak to raise the water level in a cistern enough for it to overflow. Normal water use in the home during the day can disguise a dripping overflow until the early morning. The cistern then overflows when the faulty ball valve has dripped into the cistern all night.

Faulty ball valve

Check the ball valve. If the water is running continuously, is the valve arm down? Move it up and down – this may release it if it has got stuck. If the arm moves freely but won’t rise by itself, the float probably has a hole in it. You can easily replace it by screwing a new one on.

Does the arm move freely? Is the float watertight? If yes, but the valve still doesn’t shut off properly, the washer and/or nozzle may be worn.

Overflow pipe leaking but the ball valve is OK

Does the valve shut off normally and at the correct level? If so, then the cause is water entering the cistern through the outlet pipes at the bottom.

The cause of this fault relates to one of either two aspects of your plumbing system: a mixer tap or shower, or an indirect hot water cylinder.

A mixer tap or shower

Mains pressure water is at a much higher pressure than gravity-fed water, like the water from your hot water cylinder. If the two bodies of water mix, the cold water will displace the hot. It will push back through the hot water cylinder and back into the cold water storage cistern.

This is very likely to happen if a mixer tap or shower has a faulty or absent non-return valve. and there isn’t such a valve on the hot supply pipe.

Think back – did the cistern only start to overflow after the recent installation of a device with a mixer valve? Or does it overflow when you have a bath or a shower? If so, then it’s very likely that a mixer tap or mixer shower is the cause of the problem. Of course, this problem can also occur if an existing mixer tap or mixer shower develops a leak.

Broken coil

Another reason for an overflowing cistern is a broken coil in the hot water cylinder. In this case, it’s most likely the smaller feed and expansion cistern that overflows.

Hot water from the boiler flows through the coil in order to heat the water in the hot water cylinder. The two bodies of water should never mix.

If the coil is punctured, the two water systems – the domestic hot water and your central heating – effectively become one. And consequently, their water levels try to equalise.

The end result is usually the water level in the F&E cistern trying to reach the same water level in the cold water storage cistern. But of course, since the water level in the cold cistern is generally much higher than the one in the F&E tank, higher even than its overflow, the F&E tank overflows continuously.

Of course, this problem can also make the cold water storage cistern overflow if the water level in the F&E cistern is higher by design. For example, if the F&E tank is mounted on a wooden plinth higher than the cold water cistern.

In the event of a broken coil, the only solution is to replace the hot water cylinder.

Overflowing feed and expansion cistern

If your feed and expansion cistern is overflowing and the ball valve is OK, then the first thing to consider is the water level.

There must be enough free space in the cistern so that when the heating is on, the water can expand by 4% of its original volume. If this space isn’t there when the system is cold, then an overflow is very likely.

You can adjust the water level by undoing the nut which secures the float in place and changing the float’s position. If there isn’t a mechanism to do this and the float is simply screwed onto the end of the metal arm, you can carefully bend it. You’ll need both hands for this so that the force is exerted directly onto the arm and not on the valve or its fitting.

The water level must be low enough to cover the outlet. However, if it’s too low, air can get drawn into the system when the pump is on. This must not happen.

Overflowing hot water cylinder

If you suspect your unvented hot water cylinder is the culprit, check its tundish. This is usually (but not always) close to the cylinder in the airing cupboard. It consists of a plastic, bell-shaped fitting with a visible gap between the two pipes. You may see water flowing, or dripping, or evidence that it has done.

A dripping tundish may indicate cylinder overheating, a faulty or inadequately pressurised expansion vessel, a problem with the pressure release valve, a faulty inlet control valve, or backflow from a mixer tap.

If your unvented cylinder is overflowing, it’s best to call a qualified engineer. That’s because it’s illegal for unqualified individuals to carry out work on unvented hot water cylinders.

Overflowing toilet

Are there drips on the ground below the toilet cistern? This is more likely due to condensation forming on the cistern than a problem with the cistern itself.

You can prevent this from happening by reducing the humidity in the bathroom. When showering or taking a bath, don’t forget to open a window or use the extractor fan.

On the other hand, if your toilet won’t stop filling or you see water constantly running into the pan, there may very well be a problem with the ball valve. Check out this article for more info on this topic.

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