Quick answer
A hiss after flushing means the inlet valve is refilling slowly, usually because its filter screen is partly blocked or its diaphragm has worn. It often occurs alongside a running toilet, since the same worn valve that fills slowly frequently also fails to shut off cleanly. Cleaning the filter is a ten-minute job and resolves a good proportion of cases.
Common causes
- Partly blocked inlet filter screen restricting flow
- Worn inlet valve diaphragm
- Low supply pressure to that fixture
- Partly closed wall isolation valve throttling the fill
- Scale build-up inside the valve body
- Float set so the valve closes very gradually at the end of the fill
What you can do now
- 1Close the wall stop-cock, remove the inlet filter screen and clean it
- 2Check the stop-cock is fully open rather than partly turned
- 3Time the refill — much longer than about a minute indicates a restriction
- 4Check whether the toilet is also running between flushes, which means the valve needs replacing rather than cleaning
- 5Compare with another toilet in the house to gauge what normal looks like on your pressure
When to call a professional
Book it in the same visit as any other cistern work. On its own a hiss is minor, but the same worn valve usually starts leaking soon afterwards, and a plumber replacing the inlet valve and diaphragm together deals with both. Call sooner if the hiss is continuous rather than only after flushing — that is a valve that is not shutting off at all.
Typical cost in Johannesburg
Diaphragm R350
Indicative ranges only. We provide written, itemised quotes before any work.
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