Do I need a bottom entry or side entry toilet fill valve?

Fill valves are cheap and near-universal within their type, so the only real decision is which type you need — and that is determined entirely by where the hole in your cistern is, not by preference. Getting it wrong means an unusable part and a trip back to the shop, so the check takes two minutes and is worth doing before ordering.

Step by step

1. Look at where the supply pipe arrives.Trace the small pipe from the isolating valve. If it turns upwards and disappears into the underside of the cistern, you have a bottom entry valve. If it runs up the wall and into the side of the cistern near the top, you have side entry. That is the whole decision.
2. Check whether the cistern actually has both holes.Some cisterns are moulded or drilled with both a base hole and a side hole, with one blanked off. If yours has a spare blanked hole, you have the option to change type — but you also have to move the supply pipe, which turns a fifteen-minute job into a plumbing job.
3. Measure the tail length available.The valve's threaded tail has to pass through the cistern wall or base and take a backnut, with room for the supply connection below or beside it. In a tight concealed cistern or a close-coupled installation with a low pan, there may be very little room. Measure before buying.
4. Check the internal height.The valve has to fit under the lid with its float able to rise fully. Most modern valves are adjustable in height, and short-body versions exist for shallow cisterns. If the cistern is slimline or concealed, look specifically for a compact valve.
5. Match the connection size.UK toilet supplies are almost always half-inch BSP. Check whether the existing connection is a compression fitting, a flexible tap connector or a push-fit, and buy accordingly — a flexible connector makes fitting much easier in a tight space.
6. Prefer a bottom entry valve where you have the choice.Bottom entry valves discharge below the water line, which makes them significantly quieter than a side entry valve dropping water from above. In a bathroom next to a bedroom this is a real difference.
7. Buy a valve with a serviceable filter.The single most common fill valve fault is a blocked inlet filter, and a valve you can take the head off and rinse without draining anything is worth a little more. Check the description says the filter is accessible.
8. Consider the water level adjustment method.Modern valves adjust by a screw or a sliding clip on the float stem, which is precise and holds its setting. Older ballvalve designs adjust by bending an arm, which is crude and drifts. If you are replacing an old ballvalve, this alone is worth the change.

Common mistakes

• Ordering by the picture on the box rather than checking where the pipe enters — Bottom and side entry valves look broadly similar in a photograph and are not interchangeable without moving the supply.
• Buying a full-height valve for a concealed cistern — There is often no room under the lid or the access panel, and the valve cannot be adjusted once fitted.
• Reusing the old backnut and washer on a new valve — The washers supplied with the new valve are matched to it. Old ones have taken a set and are the usual reason a new valve weeps at the base.

Alternatives

• Delayed-action fill valve: Waits until the flush valve has closed before starting to fill. Slightly quieter overall and avoids the fill fighting the flush on some cisterns. Uncommon and worth having if it is offered.
• Compact or short-body valve: For slimline, concealed and low-level cisterns where a standard valve will not fit under the lid or clear the flush valve.
• Complete inlet and outlet kit: Where the flush valve is also worn, a matched pair replaces everything internal in one go and often costs little more than the two parts separately.

Questions people ask