Diagnosing a Running Toilet: Fill Valve or Flush Valve?

A flush valve, by contrast, works as a straightforward seal at the base of the cistern that lifts to let water fall directly through into the pan under gravity, with no siphoning action involved, and shuts again once the float or timer mechanism allows it to reseat. Flush valves are what almost all dual-flush cisterns use, since a siphon's all-or-nothing action makes a genuine short flush difficult to achieve, whereas a valve can simply be held open for a shorter or longer period.

A cistern that never quite stops filling is one of the most common callouts a plumber gets, and the fix hinges on correctly identifying which of two components has failed: the fill valve or the flush valve. Get this wrong and you swap a part that was never the problem, then head back out a second time.

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 fitted at the supply if it persists.

Available space inside and around the cistern also narrows the choice. A shallow cistern with limited depth may not give a bottom-entry valve enough room to seat correctly, while a side-entry valve needs clear wall space at the right height with no other fittings in the way.

Plumb2U stocks side levers, lid-mounted levers and the link arms and clips that connect them to flush valves and siphons, priced for trade buyers ordering more than one at a time. For a plumber working through several lever jobs a week, having a trade supplies merchant that lists the fitting details clearly rather than a single generic photo makes the difference between one van visit and two. Plumb2U, trading from Epsom but supplying trade accounts UK-wide, carries a lever range that covers most of what turns up on a round.

A bottom-entry fill valve, by contrast, takes its supply vertically through the base of the cistern, in line with where a rising main or supply pipe comes straight up from the floor void or from beneath a low-level suite. This arrangement is generally quieter, since the incoming water enters below the surface of the tank rather than splashing in from the side, which is why many modern and replacement fill valves are built as bottom-entry as standard, often with a side-entry option built into the same body.