An overflow is an important part of a cistern, and serves two purposes in the event of a fault: to take away excess water in order to prevent structural damage to a property, and to indicate to the homeowner that there is a problem – in most cases, a faulty ball valve. An overflow pipe which exits a property on the ground floor or from the first floor is likely the overflow for a toilet cistern; an overflow pipe which exits at the level of the loft or protrudes from under the eaves of the roof is likely the overflow for a cold water storage cistern or a feed and expansion cistern.

There are two types of overflow generally found in household plumbing: side entry, and bottom entry. Bottom entry overflows tend to be found in toilet cisterns. The overflow in a cold water storage cistern is almost always side entry, and water regulations require that the overflow dips into the surface of the water in order to prevent cold drafts from entering the cistern, and that there is a filter screen to prevent insects from getting inside. These items are available as part of a Byelaw 30 kit. The water level in the cistern must be at least 25mm below the overflow when the cistern is full and the ball valve is closed. The overflow should be at least 19mm in diameter, and should be capable of evacuating all of the excess water under maximum fault conditions, such as in complete failure of the ball valve.

As previously stated, the most common cause of an overflowing cistern is a faulty ball valve. However, there are circumstances in which a cold water storage cistern or feed and expansion cistern can overflow, even with the ball valve in working order. A common reason for the feed and expansion cistern to overflow in this way is a perforation in the hot water cylinder’s coil. This effectively merges the domestic water system with the central heating circuit, and gravity tries to equalise the water level between the two cisterns. As the feed and expansion cistern is generally the cistern with the lower water level, the water level will rise in order to level off with the cold cistern, but the overflow prevents this from happening. The only way to resolve the issue is to replace the hot water cylinder.

A common reason for a cold water storage cistern with a working ball valve to overflow is a mixer valve, such as a shower or mixer tap, which is plumbed into the mains. When the tap or shower is open, the higher pressure mains water displaces the hot water and pushes it back the way it came, effectively filling up the cistern from its outlet.