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

Overflow pipe leaking? In this guide, we’ll tell you how to identify the problem and how it can be fixed. Alternatively, if you would prefer a qualified plumber to fix your leaking overflow, check out our guide to finding an emergency plumber.

A leaking overflow pipe means that water is entering a cistern when it shouldn’t. This is usually due to a faulty ball valve. 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 the toilet cistern is probably the culprit. If the leaking overflow pipe is at loft level or protrudes 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 do not know which is the problem cistern, then a visual inspection will tell you. If it’s not immediately obvious, take a look at the water level. If it is extremely close to the overflow, or higher than the level marked by residual limescale, then that may indicate the cistern in question. Remember that it may 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 may disguise a dripping overflow until the early morning, when a faulty ball valve has dripped all night into the cistern.

Check the ball valve. If the water is running continuously, is the arm down? Move it up and down – this may release it if it has got stuck. If the arm won’t rise when you lift it and release it, then the float is likely to have become waterlogged and will need to be replaced.

If the arm moves freely and the float is watertight, but the valve still does not shut off properly, the washer and/or nozzle may be worn, and will need to be replaced.

If you are absolutely certain that the valve shuts off normally and at the desired level, then the problem can only be 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. Mains pressure water is at a much higher pressure than gravity-fed water, such as the hot water from your hot water cylinder. Where mains pressure water and low-pressure water mix, the mains pressure water may displace the hot, and push it 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 is very likely that a mixer tap or mixer shower is the cause of the problem. Of course, this problem can also be caused by a pre-existing mixer tap or mixer shower developing a leak inside.

Another reason for a cistern to overflow – and indeed the only possible reason, if the ball valve is in working order and you do not have any water mixing outlets – is that the coil in the hot water cylinder has a hole in, most probably due to corrosion. In the event of a broken coil, it is the smaller feed and expansion cistern which is most likely to overflow, due to the lower water level trying to balance out with the level in the cold water storage cistern. The only solution to this problem is to replace the hot water cylinder.

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