Where this starts
A cistern is a small tank with one way in and one way out, and both are closed by a seal that has to work against standing water for years. When one stops closing, the tank never reaches its resting state and the supply keeps topping it up — which is the noise, and the bill.
THE DYE TEST IS THE WHOLE DIAGNOSIS and it costs almost nothing. Add food colouring or a dye tablet to the cistern, then leave it alone for fifteen minutes without flushing. If coloured water appears in the pan, the outlet seal is passing and water is running forward continuously. If the pan stays clear and the water level in the cistern has climbed to the overflow, the inlet valve is not shutting off. Those are different components and different work.
THE SOUND IS A USEFUL SECOND SIGNAL. A continuous soft hiss is water entering — the filling valve. An intermittent refill every few minutes with silence between is the classic passing outlet seal: the level drops slowly, the valve wakes up, tops it up, and shuts again. A trickle audible in the pan with nothing entering is the outlet alone.
TWO THINGS MAKE THIS MORE EXPENSIVE THAN IT LOOKS, and both are quiet. A passing outlet loses water continuously and, on a metered supply, does so at a rate that shows on a bill long before anybody notices the sound. And a cistern held permanently at its overflow level keeps the seal and the valve wet at the wrong height, which ages both.
One practical note before buying parts: cistern mechanisms are not standardised, and the component that fits is decided by the cistern, the flush type and often the manufacturer rather than by a size. Identifying which of the two components has failed is the useful half of the job; matching the part is the fiddly half.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The flush valve seal is passing
Most commonDye appears in the pan without flushing, often with an intermittent refill every few minutes and a faint trickle visible at the back of the pan.
How to check: The dye test. Colour in the pan with nothing flushed is the outlet, and no other reading is needed.
The filling valve is not shutting off
Most commonA continuous hiss of incoming water, a level that has risen to the overflow, and the pan staying clear in the dye test.
How to check: Where the water level sits. A level at or above the overflow line means the inlet never closed.
The float or level adjustment is set too high
CommonThe inlet valve closes correctly but only after the level has already reached the overflow, so the cistern is permanently spilling rather than permanently filling.
How to check: Whether the water sits above the marked line. A valve working correctly at the wrong setting behaves like a failed one.
Grit or debris holding a seal open
CommonA fault that comes and goes, often starting after work on the water supply or after a period with the water off, and sometimes cured by one flush.
How to check: Whether it is intermittent. A seal held open by a particle is not a worn seal, and replacing the part is not the fix.
A distorted or badly seated flush mechanism
CommonA push-button or dual-flush mechanism that does not return fully, sometimes with the button feeling soft, and the fault cured temporarily by pressing again.
How to check: Whether pressing and releasing the button stops it. A mechanism that is not returning is mechanical, not a seal.
Supply pressure higher than the valve is rated for
Less commonMore than one cistern in the building behaving the same way, often with noisy taps elsewhere and appliances filling fast.
How to check: Whether more than one toilet does it. One is a part; several is the supply.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates the outlet from the inlet definitively, which is the entire diagnosis, and it requires no tools and no dismantling.
- 2
A level at or above the overflow means the inlet is the problem or the setting is wrong; a level below it with the cistern still refilling means the outlet is.
- 3
Continuous is water entering; intermittent refills with silence between are the signature of a slowly passing outlet seal.
- 4
A mechanism that has not returned mimics a failed seal exactly, and it is the one cause that costs nothing to rule out.
- 5
Two cisterns failing the same way at the same time points at the supply rather than at the parts, and that is a single change instead of several repairs.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the cistern is part of a concealed or wall-hung frame, since access is through a panel and the parts are frame-specific rather than generic.
- Where more than one cistern in the building behaves the same way, which points at supply pressure and is a change to the incoming installation.
- Where the overflow discharges internally rather than outside and water has been escaping into the building fabric.
- Where the cistern or its connections are weeping as well as running, because a slow leak at a joint under a floor does damage a running cistern does not.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.