Water running from a cold water cistern or WC cistern overflow pipe is usually a sign of a sticking or leaking ballvalve, or a ballvalve float positioned wrongly. It is important not to let an overflow run for long – not only does it waste water, but it could lead to damp problems.
Before trouble strikes
Know how to turn off the water supply to the cisterns. Ensure that the ballvalve can be dismantled – or at least removed from the cistern. Keep spare washers – sizes and types vary depending on the type of valve – and perhaps a spare float or ball. It is a good idea to dismantle and clean ballvalves occasionally.
In an emergency
Isolate the offending ballvalve. If the float is a copper ball, check for leaks by holding and shaking it – if it is heavy and splashing noises can be heard, it is leaking. Replace it by a new plastic one (these are much less likely to leak). A temporary repair can be made by emptying out the water, replacing the ball, and fitting a plastic bag round it securely tied to the arm.
If a leaking float is not the cause, the valve should be removed from the inlet pipe and dismantled. Corrosion may make dismantling difficult clean up the valve and apply penetrating oil as necessary. The most likely cause of a leak is a worn washer (or diaphragm in a diaphragm valve). This can be replaced. The valve seat or nozzle may also have worn – with some types of p valve these can also be replaced: low-pressure valves have a larger opening in the nozzle than high-pressure ones, so make sure you choose the right sort.
On the other hand, a running overflow may be due simply to dirt or grit in the valve causing it to stick – make sure the nozzle and other waterways are clear, and clean all parts thoroughly before reassembling the valve. A worn split pin may even be partly to blame for a leak.
With the valve back in place in the cistern and the water turned on again, check that the level of the water in the cistern is correct about 25mm to 50mm below the overflow outlet (or no higher than the water line level in the cistern if there is one). If the level is not right, gently bend the ball arm – upwards to increase the level, downwards to decrease it. With some diaphragm valves, the float is moved along the arm to alter the water level.
