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

An overflow pipe leaking or dripping is due to water entering a cistern when it shouldn’t, either via the ball valve or via the outlet pipes at the bottom. In most cases, it’s due to a faulty ball valve.

Overflow pipe leaking – which cistern?

The first thing to do is to identify which of the cisterns in your house is overflowing. If the overflow pipe is at ground floor or first floor level, then a toilet cistern is probably the culprit.

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.

In all cases, if you don’t know which is the problem cistern, simply looking inside will 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 tell you the cistern in question. 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 a 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 isn’t stuck and won’t rise by itself, the float is probably waterlogged. You can easily replace it by screwing a new one on.

Does the arm move freely? Is the float watertight? If the answer to these questions is 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? This means that 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 mains-pressure water and low-pressure water mix, for example, in a mixer tap, the mains pressure cold water may displace the hot water. It pushes back through the hot water cylinder and back into the cold water storage cistern.

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. This is due to the water level in the F&E cistern trying to reach the same level as the water in the cold water storage cistern. The only solution to this problem is to replace the hot water cylinder.

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