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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.

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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.

Before ordering, measuring the fixing centres and checking whether access beneath the pan is realistic will settle top-fix versus bottom-fix and the correct spacing in one go. Plumb2U Epsom stocks both top-fix and bottom-fix hinge sets and soft-close spares across the common fixing centres, at trade prices, so plumbers replacing seats across several jobs a week can order to the measurement rather than to a guess. As a plumbers merchants stocking both fixing types in depth, Plumb2U's seat spares range covers most of what a trade buyer will meet on a typical round.

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The first check is timing. A whistling or high-pitched noise that happens throughout the entire fill cycle, easing off only as the cistern nears full, usually points to wear inside the fill valve itself, most often a perished washer or a worn diaphragm that is no longer sealing smoothly against the valve seat. Water forced through a narrowed or roughened opening produces exactly this kind of noise, and it tends to get worse over time rather than appearing suddenly.