The second check is a knocking or banging sound, sometimes called water hammer, that happens specifically at the moment the fill valve shuts off rather than during the fill itself. This is far more often a mains pressure issue than a valve fault, caused by the sudden stop of a fast-moving column of water hitting the closed valve and sending a shock through the pipework. A fill valve replacement on its own will sometimes quieten this slightly, particularly if swapping to a bottom-entry valve which fills more gently, but it will not fully resolve a genuine water hammer problem, which usually needs an air-filled arrestor plumb2u.com fitted at the supply if it persists.

For more detail, see heating controls supplier.

A cistern that whistles, knocks or hammers while filling is one of the more annoying faults a customer will report, and it is also one of the quicker ones to run down with a short checklist before a plumber even gets to the van. Two causes account for the large majority of noisy-cistern callouts: a worn fill valve and excessive mains pressure, and telling them apart early saves time on site.

Two cisterns can sit on near-identical pans and still take completely different spare parts, and the reason comes down to how the cistern connects to the toilet below it. Close-coupled and low-level designs are the two most common configurations found on UK jobs, and confusing the two when ordering spares is one of the easier mistakes to make on a busy round.

A side-entry fill valve takes its supply through the side wall of the cistern, typically a few centimetres up from the base, with the water pipe running horizontally into the tank. This is the older and still very common arrangement in UK homes, particularly where the supply pipe rises through a wall cavity or runs along a skirting board before turning up into the cistern. Side-entry valves are usually the easier option where floor space beneath the cistern is tight, since there is no need for the pipe to come straight up from below.

A siphon uses the physics of siphoning rather than a simple valve seal to move water from the cistern into the pan. Lifting the lever raises a plastic or rubber diaphragm inside the siphon body, which pushes a slug of water up and over a bend in the siphon tube, starting a siphoning action that then draws the rest of the cistern's contents down through the tube by suction until the water level drops low enough to break the siphon and stop the flush. This is why older cisterns produce that distinctive extended flush sound rather than the quicker valve-open, valve-close action of a modern flush valve.