A running toilet is one of two five-euro valves, and a pencil mark tells you which. Mark the water level inside the cistern, turn off the supply at the isolating valve, and come back in twenty minutes.
If the level has dropped
The flush valve is leaking — its seal has hardened and water is trickling past it into the pan. This is the commoner of the two, and the fix is a new seal or a new flush valve.
If the level has not dropped
The fill valve is at fault: it is not shutting off, and the excess is going down the overflow into the pan. The fix is a new fill valve, or sometimes just an adjustment of its float.
Why it is worth ten minutes
A toilet that runs continuously wastes a genuinely large volume of water, and it is the sort of fault people live with for years because it makes no noise. Both parts are stocked, both are sold to fit a range of cisterns, and neither needs the toilet removed.
What you need before ordering
The cistern opening diameter for a flush valve, whether the fill valve enters at the bottom or the side, and its thread. A photograph of the open cistern with a tape across it answers all three.
Questions people actually ask
Can I replace just the seal instead of the whole valve?
Often yes, and some makers sell the seal separately for exactly this. Whether yours is available as a part is the difference between a two-euro fix and a fifteen-euro one.
Do I need to drain the cistern first?
Yes — close the isolating valve, flush to empty it, and sponge out what remains. Everything above the water line then comes apart dry, which is the whole reason the job is straightforward.
Last reviewed 5 September 2026