A WC cistern that leaks

Introduction

A cistern leaks in two ways: a steady trickle into the pan, or water on the floor. It is not a blocked pan. Drops on the outside of a cold cistern, with a dry pan, are condensation, not a leak.

How to tell it apart

A trickle into the pan without a flush: the flush valve or its washer is not closing.

The water inside reaches the overflow: the level is too high and the float is not stopping the inlet.

Drops only on the cold tank, pan dry: condensation. You do not “repair” the cistern to dry it. See window condensation and, if you want the cold-glass comparison, the dew point.

A concealed cistern: the mechanism is not in view. From the plate, read the brand and the model. If the WC sends no water and the pan fills, open a WC that will not flush.

Likely causes

• A flush washer or valve that does not close, with a trickle into the pan.
• A level that is too high: water leaves by the internal overflow.
• A float that does not shut the inlet.
• Condensation on a cold cistern, mistaken for a leak.
• Water behind the plate of a concealed cistern: a dehumidifier does not remove it.

What to do now (€0)

1. Close the isolating valve on the feed, if it is there and it turns. If it will not turn, stop.
2. Dry the floor. If it wets again with the isolator shut, the water is not coming from this cistern.
3. A visible cistern: a few drops of food colouring in the water, without flushing. If the colour shows in the pan, the flush valve or washer is not holding.
4. Look at the level. If water enters the overflow, the float is letting too much in. Do not bend the arm unless it is the part for this model.
5. Concealed: note brand and model from the plate, then close it. Do not strip the frame.

A small DIY step

1. The small step is to close the isolator, run the dye test and photograph the model. Without the matching part it is not a repair.
2. The concealed plate is there so you can read the name, not so you can swap the mechanism at random.
3. There is no universal spare. On this track the quiz shows no cards, and none are added.
4. Do not aim a dehumidifier at water behind the plate.

Mistakes to avoid

• Stripping a concealed cistern at random.
• Mistaking condensation on the outside for a leak into the pan.
• Flushing during the dye test: the colour goes down anyway and the test no longer tells you anything.
• Leaving the isolator open while water reaches the floor.
• Believing a dehumidifier dries a leak inside the wall.

When to call a professional

A plumber for water on the floor, for a concealed cistern, and for a visible one if you do not have the spare for that model.

Float or washer only with the part for that model, after the test. If the part does not match, a plumber.

Safety

Water on the floor near sockets or appliances: cut the power at the main before you touch anything nearby, and only if you can reach the switch without walking in the water. Do not open boards, sockets or appliances while they are live: an electrician.

Condensation is not mould. If mould around a real leak is widespread or comes back, or someone in the house has breathing trouble or is frail: a technician and, for health, a doctor.

Questions people ask