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A cistern that won't fill means a toilet you can't flush — annoying, but almost always a cheap, quick fix rather than a new toilet. Here's what's going on.
The usual causes
- Faulty or blocked fill (inlet) valve — clogged with sediment or scale, or simply worn out. The most common cause.
- Float set wrong or stuck — tells the valve to shut off too early, or jams.
- Partially closed isolation valve — the small valve feeding the toilet may have been turned down, restricting flow.
- Low water pressure — common upstairs, it makes the cistern fill slowly.
Fills slowly vs not at all
Slow filling usually points to a clogged fill valve or a partly-closed isolation valve. Not filling at all? Check the isolation valve is fully open and the fill valve isn't seized.
The pressure angle
If pressure is low (see low pressure upstairs), the cistern struggles to fill — resolving the pressure, or fitting a pump, helps alongside checking the valve.
The fix
It's typically a cheap fill valve or a quick adjustment, not a replacement toilet. We swap the fill mechanism and check the supply in one visit — see toilet repairs or book a plumber.
Frequently Asked Questions
The usual causes are a faulty or blocked fill (inlet) valve, a float set wrong or stuck, a partially closed isolation valve feeding the toilet, or low water pressure. If it fills very slowly, suspect a blocked fill valve or low pressure; if not at all, check the isolation valve is open and the fill valve isn't seized.