The sound is unmistakable: a low, continuous hiss from the bathroom, long after you have flushed. The toilet is running. It has been running for three days. You have lifted the cistern lid, looked at the parts inside, understood nothing, and put the lid back on.
Here is what is happening — and how to fix it yourself.
Why the toilet keeps running
Inside the cistern, there are two main components: the fill valve (which refills the cistern after a flush) and the flush valve (which releases the water when you press the handle). A constantly running toilet almost always means one of these is not sealing properly.
The most common cause is a worn fill valve. Over time, the rubber seal inside the valve degrades and water trickles continuously into the overflow tube. You can confirm this by lifting the lid and looking: if water is running down the overflow pipe at the back of the cistern, the fill valve is the problem.
What you need
- Replacement fill valve (available at any hardware store for €8–15 — take a photo of your current one to match the height)
- Adjustable wrench or pliers
- Old towel or small bucket
- About 25 minutes
How to fix it
- Turn off the water supply to the toilet. The isolation valve is usually on the pipe behind or below the cistern — turn it clockwise until it stops.
- Flush the toilet to empty the cistern.
- Disconnect the water supply hose from the bottom of the cistern. Have a towel ready — a small amount of water will drip out.
- Unscrew the large locking nut at the base of the fill valve (the tall cylindrical component) from outside the cistern. The valve lifts out from the top.
- Insert the new fill valve. Adjust the height according to the instructions — the top should sit about 25mm below the overflow tube.
- Secure the locking nut from underneath. Hand-tight, then a quarter turn with pliers.
- Reconnect the water supply hose. Turn the isolation valve back on slowly.
- Let the cistern fill. Adjust the float arm if needed so the water stops filling about 25mm below the overflow tube.
- Flush and check. The cistern should fill, stop, and stay quiet.
That is the repair
The part costs less than €15. The repair takes under 30 minutes the first time. A plumber charges $80–150 for the same job.
This is one of eight repairs documented in She Fixed It — with photos, part names, and instructions that assume you can do it, not instructions that try to convince you that you can.