Cold water tank overflowing – why?

Cold water tank overflowing? If there’s water dripping – or pouring from a pipe which points out of the house at loft level or from the eaves of the roof, then one of your cold water tanks is overflowing. This could be either the cold water storage cistern, which supplies the hot water cylinder and possibly the cold bathroom taps. Or, it could be the smaller feed and expansion cistern, which puts water into the central heating circuit. In this article, we’ll discuss what to do if the cold water storage cistern is overflowing.

Contents – cold water tank overflowing

Typical cause – a faulty ball valve

The most common reason for a cold water tank to overflow is a faulty ball valve. This valve controls the level of water in the cistern via a plastic float which sits on the surface of the water. As the water level in the cistern rises, the float pushes a washer onto the valve seating – a plastic nozzle inside the valve body. This stops the flow of water into the cistern.

Foreign objects, such as grit, dirt or limescale, or damage to either the valve seating or the washer may prevent a water tight seal. General wear and tear may split or chip the edges of the valve seating, and crack or split the washer. While these components are cheap to replace, it may be simpler to just replace the whole valve. Brand new Part 2 ball valves are cheap, reliable, and offer peace of mind that the problem is fixed.

The other cause – mixer valves

The cold water cistern can still overflow, even if the ball valve is working perfectly. This situation largely depends on the configuration of your plumbing system. Did you recently have a new mixer tap or shower with a mixer valve installed? If so, is it fed by the mains? This is very likely to be the cause of the problem. The high pressure mains water displaces the low pressure water from the hot water cylinder, pushing it back into the cold water cistern. If the cistern only overflows when a certain mixer tap or mixer shower is open, then this is almost certainly the cause. Alternatively, it may be caused by a pre-existing mixer tap or mixer shower developing an internal leak. The solution is to fit a non-return valve on the hot feed pipe to the tap or shower.