A flush is a volume of water released quickly, so a weak flush is nearly always a volume problem. Three things control it, and all three are adjustable or cleanable.
1. The fill level
If the cistern is not filling to its mark, the fill valve is stopping early. Most have an adjustment — a clip, a screw or a float arm — and the manufacturer’s instructions describe it for that valve.
2. The dual-flush setting
The reduced volume may simply be set too low for your pan. Adjusting it upwards is as legitimate as adjusting it down, and it is set on the flush valve during installation.
3. The rim holes
In hard water, the small holes under the rim scale up and the water no longer sweeps the bowl. That shows as a flush that fills but does not clear, and it is a descaling job rather than a valve one — within what the pan manufacturer permits.
What it is usually not
The pan itself, and the pressure of the supply. A cistern flushes with the water it holds, not with mains pressure — which is why a weak flush is a volume question.
Questions people actually ask
Does a higher cistern flush better?
It did on old high-level cisterns, and modern close-coupled pans are designed around the volume rather than the drop. Raising a modern cistern is not an option and would not be the fix.
Why does it need two flushes since I changed the valve?
Almost always the volume adjustment on the new valve, which is set at the factory for a generic cistern. It is designed to be set on installation, and the instructions say where.
Last reviewed 5 September 2026