Cold Water Tank Overflow Running in London: Causes and How to Fix It

What the Overflow Pipe Does

A cold water storage cistern is fitted with an overflow pipe — a 22 mm or 28 mm pipe that exits the cistern approximately 25 mm below the underside of the lid and runs to discharge at the external wall of the property, typically at high level above a gutter. The overflow is a safety device. Under normal operation, the water level in the cistern is controlled by the ballcock (inlet float valve), which shuts off the mains supply when the water level reaches the set point — always well below the overflow outlet. Water should never enter the overflow pipe under normal conditions. If water is running from the overflow outlet, the ballcock has failed to shut off the mains supply at the correct level and the cistern is overfilling.

Why the Ballcock Fails

The ballcock is a mechanical valve with a rubber seat washer that is pressed closed by the rising water level acting on a plastic float connected to the valve by a float arm. In London hard water areas, the rubber seat washer is subject to accelerated wear from the calcium and magnesium in the water supply — the same scale that accumulates on kettle elements and shower heads. The seat washer hardens, becomes misshapen, or develops a tear over time, and when this happens the valve no longer closes fully at the set water level. Water continues to flow past the hardened or damaged washer, the cistern fills beyond the normal level, and eventually discharges through the overflow.

A second failure mode is a float that has cracked or filled with water. The float is a sealed plastic ball on the end of the float arm. If the float develops a crack — from age, UV exposure, or mechanical damage — water enters the float, increasing its weight and reducing its buoyancy. The float sits lower in the water than it should, and the ballcock does not reach its shutoff position until the water level is higher than normal. In severe cases, the float sinks entirely and the valve never closes, leading to continuous overflow.

Diagnosing the Cause

Diagnosing a loft cistern overflow in a London property requires accessing the loft and inspecting the cistern. Remove the lid and observe the water level. If the water level is above the normal set point (approximately 25 mm below the overflow outlet), the ballcock is not closing correctly. Lift the float arm manually to fully close the valve — if water flow stops immediately, the valve mechanism is intact and the float may be waterlogged or the set point needs adjustment. If water continues to flow even with the float arm held up, the valve seat is worn and the valve assembly needs replacement.

Check the float for water ingress by removing it from the arm and shaking it — a waterlogged float will feel heavy and may drip water when shaken. If the float is sound but the valve leaks with the arm held up, order a replacement ballcock assembly. Modern float valves use a diaphragm rather than a washer — the Torbeck valve and the Fluidmaster 400A are the most commonly fitted replacements in London properties and are significantly more durable in hard water conditions than older rubber washer valves.

The Repair

Replacing a ballcock in a London loft cistern involves isolating the mains cold water supply at the internal stop tap or a service valve on the rising main. The cistern is drained below the inlet valve level by opening a cold tap at a lower level. The ballcock assembly is unscrewed from the cistern wall — typically a 3/4 inch BSP thread or a 1/2 inch thread depending on the cistern age. The new valve is fitted in reverse order, the water supply is restored, and the float level is adjusted so the cistern fills to approximately 25 mm below the overflow outlet. The repair takes approximately 30 to 60 minutes. Contact Prestige Engineers for ballcock replacement and cistern repairs across all London boroughs.