Toilet keeps running — why, and how to fix it
A toilet that runs continuously is wasting somewhere between 200 and 400 litres a day. On a water meter that's real money, and it's usually a cheap part.
First: which side is it?
Take the cistern lid off and look at where the water is going.
- Water trickling down into the pan → the flush valve at the bottom of the cistern isn't sealing.
- Water running into the overflow, or the level sitting above the marked line → the fill valve isn't shutting off.
- Cistern refills on its own every so often with nobody flushing → a slow flush-valve leak. Same cause as the first one, just smaller.
The dye test
If you can't see an obvious trickle, put a few drops of food colouring in the cistern and don't flush for half an hour. Colour in the pan means the flush valve is leaking. No colour means look at the fill valve.
The three causes
1. Worn flush valve seal
The rubber seal at the bottom of the cistern hardens with age and stops seating properly. Commonest cause by a distance. The seal is a few pounds; the whole valve isn't much more. Straightforward for a confident DIYer — isolate the water, flush, sponge the cistern dry, and lift the valve out.
2. Fill valve not shutting off
The valve that refills the cistern isn't closing, so water keeps coming in and escapes down the internal overflow. Sometimes it's just the float set too high — adjust it so the water sits about 25mm below the overflow. If adjusting it doesn't hold, replace the valve.
3. Grit or limescale on the seat
Hard water area, so this is more common here than in most of the country. A bit of scale on the valve seat holds it a fraction open. Clean the seat and the seal; if the plastic is pitted, replace it.
What to do first, before anything else
Turn the isolation valve behind the toilet a quarter turn to stop the water. The toilet still flushes once, and you've stopped the waste while you decide.
When to call someone
If the isolation valve is seized or absent, if the cistern is concealed behind a wall or in a unit, if it's a close-coupled cistern that has to come off the pan, or if you've replaced a part and it still runs. Also if there's water on the floor — that's a different fault and worth looking at properly.
Toilet repair is usually a single visit.