Toilet keeps running: how to find the fault and fix it yourself

The fill valve, on the water supply, refills the cistern and shuts off when the float reaches the right water level. The flush valve, at the bottom, holds the water back with a seal and lifts when you press the button. Between them, the overflow pipe stops the cistern from overflowing. Nearly every fault comes from one of these parts, and in hard-water areas limescale is often to blame.

The toilet keeps running

Lift the lid and check the level. If it reaches the top of the overflow pipe and water runs into it, it's the float: set too high or faulty. If the level is lower and water still trickles into the bowl, the flush valve seal is leaking. To check, add a few drops of food colouring to the cistern without flushing: if the colour shows up in the bowl, the flush valve is leaking.

The cistern refills slowly, or not at all

Check the isolation valve is fully open. If it is, the fill valve filter is clogged with limescale, or the float is rubbing against the side and sticking.

The flush button does nothing

The cable or rod between the button and the flush valve has come loose. A cable clips back on by hand; a rod can usually be adjusted by screwing it in or out a few turns. If the button is broken, it's replaced on its own or with the lid, depending on the model.

Water leaking at the base of the toilet

Then the water is coming from the cistern-to-pan seal, the supply connection or a cracked cistern. Put kitchen paper under each joint to find the drip. The repair means taking the cistern off.

1. Close the isolation valve — The small valve on the supply pipe, behind or beside the toilet. Turn it clockwise as far as it goes, without forcing it.
2. Empty the cistern — Flush, then sponge out the water left in the bottom.
3. Lift the lid — Lay it flat on a towel: ceramic breaks easily.
4. Take a photo — Before you take anything apart. It helps when refitting and when buying the right part.

Isolation valve seized? If it won't turn or drips when you turn it, don't force it: it can snap. Turn off the main stopcock for the home and call a plumber.

If the float is set too high, the fill valve shuts off too late: the water reaches the overflow and runs non-stop. On most modern fill valves, the float slides on a rod or is adjusted with a screw or thumbwheel. Lower it one notch, turn the water back on and let it fill: the level should stop below the top of the overflow pipe, often at a mark on the valve. On an old ball valve with a metal arm, you can gently bend the arm downwards.

How do I know if the fill valve has failed?

If you push the float down and water keeps coming in, or the float is cracked and full of water, replace the whole fill valve: undo the supply nut under the cistern, then the fixing nut, and fit the new valve with its washers, hand-tight. Before buying, check whether the supply comes in at the side or the bottom: the parts are different.

Limescale jams the float, clogs the fill valve filter and hardens the flush valve seal. With the water off and the cistern empty, pour in enough white vinegar to cover the bottom and the seal, and leave it overnight. Next day, scrub off the deposits with an old toothbrush, turn the water back on and flush a few times.

Don't mix products. Never add bleach or any other cleaner to the vinegar: some mixtures give off toxic fumes.

Water level's fine, but water still trickles into the bowl? On most current flush valves, the centre part lifts out from the top without taking the cistern off.

1. Lift out the centre part — Water off, cistern empty: give it a quarter turn or unclip it, depending on the model, and lift it out. The seal is at the base.
2. Swap the seal — Take the old one to the shop to match the diameter. Clean the seat at the bottom of the cistern while you're there.
3. Refit and test — Turn the water on, flush several times and check nothing is running into the bowl.

On a wall-hung toilet, the cistern is hidden in the wall, inside a support frame. You get to it by removing the flush plate and the frame behind it: the opening gives access to the fill and flush valves, and often the isolation valve, without breaking into the wall.

It's trickier than a visible cistern: the plate clips snap easily, you're working through a small opening, and the parts are specific to each manufacturer. If you're not comfortable, leave the plate on: a photo is often enough for us to identify the model.

A running toilet isn't an emergency unless it overflows: turn the water off between uses and book the repair during the day. See our toilet flush repair page, or water leak if there's water on the floor. Send us a photo of the open cistern or the flush plate on WhatsApp: we'll identify the part and give you the price before we come.

• The isolation valve is seized or drips when you turn it.
• You've adjusted the float and changed the seal, and it still runs.
• The flush valve is broken or won't lift out: the cistern has to come off.
• Water at the base of the toilet.
• It's a concealed cistern and you can't see how to get to the mechanism.

For every other water problem, see our plumber in Luxembourg page.

• Descale the cistern with white vinegar once or twice a year.
• Avoid in-cistern cleaning tablets: they can damage the seals.
• Close and reopen the isolation valve now and then so it doesn't seize.