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.

Leaking overflow pipe explained

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. Is it extremely close to the overflow, or higher than the level marked by residual limescale? This may indicate 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.

Faulty ball valve

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 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.

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.

Overflow pipe leaking but ball valve is OK

If you’re sure the valve shuts off normally and at the desired level, 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 occur if a pre-existing mixer tap or mixer shower develops a leak.

Broken coil

Another reason for a cistern to overflow is that the coil in the hot water cylinder is broken. A broken coil is usually the result of internal corrosion.

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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