Toilet keeps running? Causes and solution

How does a cistern work?

Every cistern contains two components that do the work together:

• The fill valve with the float lets water into the cistern. As the water rises, the float goes up and the valve shuts off the supply.
• The flush mechanism (flush valve) at the bottom lets the water flow into the bowl in one go when flushing. Afterwards a rubber seal closes the opening again. The flush mechanism also has an overflow tube: if the water rises too high, it runs down it into the bowl, so the cistern doesn't overflow.

With a toilet that keeps running, one of those two isn't closing properly.

First, check: which type of cistern do you have?

With a separate or raised cistern (on the pan or high on the wall) you remove the lid. With a wall-hung toilet with concealed cistern the cistern is behind the wall. You access it by carefully removing the operating plate with the flush buttons; behind it is an opening through which you can reach the fill valve and flush mechanism. Check the manual or the manufacturer's website beforehand to see how your plate comes off.

Step-by-step guide: why does the toilet keep running?

1. Turn off the supply. Under or next to the cistern there is a small tap (the isolation valve). With a concealed cistern it is often behind the flush plate.
2. Check the water level. Turn the tap back on and let the cistern fill. If the water flows over the edge of the overflow pipe, the fill valve isn't closing in time. Then it's the float or the fill valve.
3. Is the water below the overflow and yet a trickle is running into the bowl? Then the seal of the flush mechanism is leaking. The water then slowly drains away and the cistern keeps refilling a little.
4. Check the flush button or pull rod. If the button is sticking, the pull rod keeps catching or the cable is too tight, the flush mechanism stays slightly open.

A simple test: put a few drops of colouring (for example some food colouring) in the cistern and don't flush for a while. If the water in the bowl colours, the flush mechanism is leaking.

The solutions

Float set too high

On many fill valves you can adjust the height of the float, with a screw, a clip or by sliding the float along a rod. Set the float slightly lower, so that the water stays well below the rim of the overflow.

Fill valve doesn't shut off

Limescale, sand or a worn diaphragm cause the valve to keep letting water through. Sometimes it helps to open the valve and flush it clean. If the diaphragm is worn or the valve is old, a new fill valve is the best solution. Take the old one to the shop or note down the make and model.

Flush mechanism leaks

The rubber seal ring at the bottom of the flush mechanism becomes hard over time or gets fouled with limescale. Clean the ring and the seat; if the rubber is hard or deformed, replace it. With some systems you replace the ring separately, with others the entire internal mechanism.

Cistern leaking on the outside

If you see water on the floor or down the pan, it's a different story:

• Is it dripping at the water connection, then the coupling of the stop valve or the flexible hose is leaking.
• With a close-coupled cistern the seal between cistern and bowl may leak, or the rubbers under the fixing bolts.
• Condensation: if the cistern is wet on the outside in warm, humid weather, it may be condensation. That's not a leak.
• With a concealed cistern you only notice a leak when moisture comes through the wall or the ceiling below. In that case, read damp patch on the ceiling.

Repair or replace?

Parts of a cistern can usually be replaced individually. Replacing the internals makes sense if the parts are old and calcified, if they are no longer available separately, or if you keep having to repair them. If a concealed cistern behind the wall is leaking, or if the toilet itself needs to be replaced, leave that to a tradesman.

Having a toilet repaired: costs and what the technician does

A running toilet is almost always a worn part in the cistern: the float, the fill valve or the seal rubber of the flush tank. We replace the faulty part or the complete internal mechanism, so everything is matched again. With a concealed cistern we work through the opening behind the control plate; the wall doesn't need to be opened for this.

If the cistern is leaking on the outside, or there's water around the base of the bowl, we look at the connection to the drain and the water pipe. This way we prevent a small leak from damaging the floor or your neighbours' ceiling.

If the cistern or the pan is very old, replacement may be more sensible. We will tell you honestly what makes sense.