Toilet fill valves and flush valves: which part do you need?

Most cistern repairs come down to one of two parts: the fill valve that lets water in, or the flush valve that lets it out into the pan. Buying the wrong one is easy, because there are two kinds of each and several sizes. This guide is about identifying the part. If your toilet is running and you are not sure why, start with why your toilet keeps running and come back here once you know which side is at fault.

Fill valves: side entry or bottom entry

Look at where the water pipe meets the cistern.

• Side entry. The supply pipe comes in through a hole high on the side of the cistern. The valve sits near the top, with a float or float cup beside it. Common on older close-coupled and low level cisterns, and on loft storage cisterns (which are a different job; this guide is about toilets).
• Bottom entry. The pipe comes up into the base of the cistern, usually from underneath on the left or right. The valve is a tall tube standing on the cistern floor with a float that rides up and down it. Common in modern close-coupled and concealed cisterns. See toilet types and wall-hung toilets.

You cannot swap one type for the other: the hole in the cistern decides. When you buy a replacement, check:

1. Entry type. Side or bottom, as above.
2. Thread size. Most UK toilet fill valves have a 1/2in (often written 13mm or 12.7mm) threaded shank, but check the old one.
3. Height (bottom entry). The valve has to fit under the lid and set the water level below the overflow. Most are adjustable within a range given by the maker. Measure the inside height of your cistern before you buy.
4. Supply pressure. Valves are made for mains pressure, low pressure (a tank in the loft), or both. If your cistern is fed from a loft tank, check the valve is suitable. Our guide to loft cold water tanks covers the tank's own ballvalve.
5. Connection. The old tap connector or flexible hose usually reuses fine, but have a new fibre or rubber washer to hand.

Is it the whole valve or just the diaphragm?

Many fill valves fail because a small rubber diaphragm or seal inside the head is worn or has grit on it. The valve hisses and will not shut off fully, or fills very slowly. On many modern valves you can isolate the water, twist off the head, clean or replace the seal, and refit it. Replacement seals are usually brand specific, so you need the maker's part for that valve.

Replace the whole valve when the body is cracked, the float arm or cup is broken, the thread is damaged, or a new seal did not cure it.

Flush valves: siphon or drop valve

Now look at what happens when you flush.

• Siphon. A lever on the cistern lifts a plastic disc inside a bent tube. When you let go, water carries on flowing until the cistern is empty. Siphons are the traditional UK design. A sound siphon does not usually let water through while idle, so a siphon toilet that keeps running is normally overfilling: look at the fill valve.
• Drop valve (often dual flush). A push button on the lid lifts a valve off a rubber seal at the base of the cistern. If the seal is worn or dirty, water trickles into the pan all the time. This is the most common cause of a constant trickle in newer toilets.

Siphon: diaphragm or whole siphon?

If a siphon toilet takes several pumps of the handle to flush, the flexible diaphragm (flap valve) on the lifting disc is probably split or worn. On some siphons it can be replaced on its own, but this often means taking the siphon out of the cistern, which on a close-coupled toilet can mean separating the cistern from the pan. Many people replace the whole siphon at that point. Check:

• the outlet size (UK lever cisterns are commonly 1 1/2in, but check);
• the siphon height, which needs to suit the cistern depth and lever position;
• whether it is single or dual flush, and whether the lever is on the front or the side.

Drop valve: seal or whole valve?

Many drop valves twist or unclip out of their base so the seal at the bottom can be cleaned or replaced without removing the cistern. That fixes most trickles. Replacement seals are brand specific, so note the maker's name on the valve. A whole valve is needed if the body is cracked or the button mechanism is broken. Match the outlet diameter (2in, about 51mm, is common), the cistern type and the button.

Before you start

• Close the toilet's isolating valve, or the stopcock if there is none, then flush to empty the cistern and sponge out the rest.
• Put a towel down. The lid is heavy and ceramic: lay it somewhere flat where it will not be knocked.
• Take a photo of the old part in place and keep it until the new one is working.
• Close-coupled toilets have a large seal between cistern and pan. If you disturb it, use a new one.

When you turn the water back on, open the valve slowly and check under the cistern for drips for the next few hours. The plumbing tool kit lists what is useful to have to hand.

Cistern parts at Wickes

Match the part to your cistern before you buy. Check the type, thread, outlet size and height on the product page and in the maker's instructions.

We haven't tested this. Figures are as the maker states them (checked 13 November 2026).

We did not find a separate siphon diaphragm on its own at Wickes when we looked. If you only want the diaphragm, check a plumbers' merchant and take the old one with you.

How do I know if my fill valve is side entry or bottom entry?

Follow the water pipe. If it goes into the side of the cistern near the top, it is side entry. If it comes up through the base, it is bottom entry.

Can I replace a siphon with a push-button drop valve?

Sometimes, with a conversion kit, but the outlet size, cistern shape and lid all have to suit. For a straightforward repair, replace like with like.

Why does my new fill valve fill so slowly?

Check the isolating valve is fully open, the filter on the valve is clear, and the valve suits your supply pressure. A valve made for mains pressure can be slow on a low pressure supply from a loft tank.