Where this starts
A flush is a quantity of water delivered fast enough to carry the contents through the trap and into the drain. It fails in one of three places: not enough water leaves the cistern, the water leaves but is delivered badly, or the pan and the drain cannot take it.
THE BUCKET TEST SPLITS IT IN ONE GO. Tip five or six litres in quickly and directly. A clean clear means everything downstream of the cistern works and the water simply is not arriving properly. A sluggish or incomplete clear with a full bucket means the restriction is in the pan's trap, the branch or the drain, and every part in the cistern could be perfect.
IF IT IS THE CISTERN, three things account for nearly all of it. The water LEVEL is set too low, so less than the design volume is available — the level should be at the marked line and is very often well below it after a repair. The flush VALVE or siphon is not opening fully or is closing early, which on a dual flush is frequently a mechanism that has stopped travelling properly. Or the cistern is not refilling to the line between uses, which looks like a weak flush and is really a slow fill.
IF IT IS THE PAN, the classic and invisible cause is the RIM. A pan flushes partly through the rim channel, and in a hard water area that channel scales up over years until most of the water is bypassing it — so the bowl is not being washed and the siphon is not being primed, even though the same volume left the cistern. Looking up under the rim with a mirror shows it immediately.
ONE THING WORTH RULING OUT EARLY because it is common and free: many modern dual-flush mechanisms have an adjustable full-flush volume that has been set to the minimum, either at the factory or by somebody being efficient. The flush is then working exactly as configured and is simply too small for the pan it is fitted to.
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.
Cistern water level set too low
Most commonA visibly low level in the cistern well below the marked line, often after a valve was replaced or adjusted, with the flush otherwise behaving normally.
How to check: Where the level sits against the mark inside the cistern. Below it means the flush is short of water before it starts.
The rim channel is scaled up
Most commonWater entering the bowl from only part of the rim, a bowl that is not washed all round, and a flush that runs down one side — usually in a hard water area.
How to check: Look up under the rim with a mirror. Scale blocking the channel diverts the water away from where it does its work.
A flush valve or siphon not fully operating
CommonThe handle or button feeling soft or short in travel, the flush stopping early, or needing to be held down to work — commonly on a dual-flush mechanism.
How to check: Whether holding the control down makes it better. A mechanism not travelling fully delivers only part of the cistern.
A restriction in the pan's trap or the branch
CommonThe bucket test failing to clear, slow drainage, gurgling, and sometimes the level rising before it falls.
How to check: The bucket test. A full fast bucket that does not clear means the cistern was never the problem.
The full-flush volume set to its minimum
CommonA modern dual-flush mechanism working smoothly but delivering a small volume on the full setting, often in a recently fitted or replaced cistern.
How to check: Whether the mechanism has an adjustment and where it is set. A correctly working flush that is simply too small looks identical to a faulty one.
The cistern refilling too slowly
Less commonThe first flush of the day fine and the second weak, with the cistern still filling when it is used again, often alongside poor flow elsewhere.
How to check: How long the cistern takes to reach the line. A slow fill reads as a weak flush to anybody in a hurry.
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 everything upstream of the pan from everything downstream in one action, and half the causes on this page become irrelevant depending on the answer.
- 2
A level below the line means the flush is short of water before anything else happens, and it is the most common and most easily corrected cause.
- 3
A scaled rim channel diverts the flush away from where it washes the bowl and primes the trap, and it is invisible from any normal angle.
- 4
A mechanism that is not completing its travel delivers part of the cistern, and that points at the flush valve rather than at the water.
- 5
A modern dual flush set to its minimum works perfectly and flushes badly, which is a setting rather than a fault and costs nothing to correct.
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 bucket test does not clear the pan, since the restriction is in the trap, the branch or the drain and may need rodding or inspection.
- Where more than one fitting in the house is draining slowly, which points at a shared branch or the drain rather than at the WC.
- Where the cistern is concealed in a frame or a wall, because access is through a panel and the parts are frame-specific.
- Where the pan is old and the rim is heavily scaled, since descaling a rim channel is limited and a pan that has lost its channel is usually replaced.
- Where there is any sign of water escaping at the pan connector or the cistern joints, which is damage to the floor rather than a flushing question.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.