A running or slow-filling toilet usually means a worn fill valve. Here is how the replacement works and when to call a professional.
A toilet that runs constantly, fills slowly, or hisses usually has a worn fill (inlet) valve. Replacing it is a common repair — here's how it works.
Signs You Need a New Fill Valve
- Water continuously trickling into the pan
- The cistern fills very slowly or overfills
- Hissing or whistling while filling
The Replacement Process
- Isolate the water at the isolation valve on the supply pipe and flush to empty the cistern.
- Disconnect the supply and remove the old fill valve (held by a nut underneath the cistern).
- Fit the new valve, set to the correct height for your cistern, and reconnect the supply.
- Adjust the water level to the marked line and check for leaks.
When to Call a Professional
If the isolation valve is seized, the cistern is a concealed/wall-hung unit, or you're not confident, call a plumber — concealed cisterns in particular need care to avoid damaging the access. We handle these as part of bathroom maintenance and renovations.
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Frequently Asked Questions
Why does my toilet keep running?
Most often a worn fill valve or flush valve/flapper. A failing fill valve lets water trickle in continuously; replacing it usually solves it.
Can I replace a fill valve on a concealed cistern?
It's done through the flush-plate access, but concealed cisterns need more care. If you're unsure, a plumber can do it quickly without risking damage.
