Running toilet cistern: causes and repair

In short: A toilet cistern that runs constantly or does not fill properly wastes a lot of water and can often be fixed with simple steps. Usually the cause is a limescaled seal, a badly adjusted float or a faulty fill valve.

In more detail: At its core, a cistern has two components: the fill valve, which lets water in, and the flush valve, which opens when you flush. With concealed in-wall cisterns, the mechanism is reached through the opening behind the flush plate.

In detail

• Turn off the water first: Before any work, close the isolating valve on the supply pipe and flush once so that the cistern is empty.
• Spotting a leak: A thin, constant trickle into the bowl or a faint hissing sound indicates that water is running through continuously.
• Flush valve seal: If the rubber seal on the flush valve is limescaled, dirty or perished, it no longer closes; clean it or replace it with a matching spare part.
• Adjusting the float: If the water level is too high, water runs away through the overflow pipe; adjust the float so that the fill valve closes earlier.
• Fill valve: If the cistern fills very slowly or does not stop filling, the fill valve is often limescaled or faulty; a small filter at the connection may also be blocked.
• Descaling: Limescale deposits can often be dissolved with a vinegar or citric acid solution; follow the manufacturer’s guidance on materials.
• Spare parts: Many valves and seals are manufacturer-specific; note down the model or take the old part with you when buying.
• Tenants: In rented flats, agree minor repairs with the landlord and report damage, particularly with in-wall systems.
• Professionals: If there is water inside the wall, leaking connections or any uncertainty, it is better to call a plumber, as water damage can be expensive (wasserhahn-tropft-reparieren, abfluss-verstopft-was-tun).