How do I know if my toilet fill valve needs replacing?

The cistern fills very slowly, will not stop filling, runs into the overflow even after adjusting the float, or hisses long after flushing. If cleaning the valve's filter and adjusting the float does not help, replace it.

Do I need to turn the water off to change a fill valve?

Yes. Use the isolating valve on the pipe to the cistern. If there is not one, turn off the main stopcock and open a cold tap downstairs to drain the pressure.

Why is my toilet still running after replacing the fill valve?

Either the float is set too high so water reaches the overflow, or the flush valve seal is leaking into the pan and the fill valve keeps topping up. Lower the float first, then check the flush valve.

Are toilet fill valves universal?

Many are sold as universal, but they still come as side-entry or bottom-entry, and some only suit mains or tank pressure. Check the entry position, pressure, shank length and height before buying.

How do I stop water running into the toilet overflow?

Lower the float so the water stops 20 to 25mm below the top of the overflow. If it still rises after that, the valve is not shutting off and needs cleaning or replacing.

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How to replace a toilet fill valve and adjust the float

To replace a toilet fill valve, turn off the isolating valve on the pipe feeding the cistern, flush and sponge the cistern dry, undo the tap connector under the cistern, then undo the large plastic backnut holding the valve in place and lift the old valve out. The new valve goes in the same hole with its washer inside the cistern, the backnut is tightened by hand plus a little more, and the supply is reconnected. Turn the water back on slowly, check for drips, then adjust the float so the water stops at the level marked in the cistern. On an ordinary close-coupled toilet with good access it takes about an hour. Before you buy a valve, check whether the supply comes in from the side or the bottom and whether it is fed from the mains or a tank.

Loyal Plumbers · Updated 28 September 2026

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Is it the fill valve at all?

The fill valve (also called a ballcock, float valve or inlet valve) lets water into the cistern after a flush and shuts off when the float rises. It is responsible for three common faults: a cistern that fills very slowly, a cistern that will not stop filling and runs into the overflow, and a hissing or dribbling sound long after flushing.

Water trickling into the pan with the cistern full is usually a different part, the flush valve seal at the bottom of the cistern. If the fill valve keeps topping up because water is escaping into the pan, replacing the fill valve will not fix it. Our guide to a toilet that keeps refilling explains how to tell the two apart, and the guide to a cistern that is slow to fill covers the supply side.

Sometimes the valve only needs adjusting or cleaning. Grit from street works or old pipework can lodge in the small inlet and slow it right down. Many modern valves have a removable top or filter that can be rinsed in a minute. Try that and the float adjustment below before replacing anything.

Choosing the right replacement

Fill valves are sold to suit how the water enters the cistern and the pressure it arrives at. Getting either wrong is the most common reason a DIY replacement does not work.

What to check before buying a fill valve
Check	Why it matters	How to tell
Side entry or bottom entry	The valve must match where the supply pipe comes in	Look at where the pipe joins the cistern: side wall or underneath
Mains or tank-fed pressure	Some valves only work on one, others have an adjustable or swappable seat	Mains-fed cisterns usually refill briskly; tank-fed ones from a loft tank are slower
Thread size and shank length	Most UK cisterns take a standard half-inch shank, but shank length varies	Measure the old valve's shank below the backnut
Height inside the cistern	Tall valves can foul the lid or flush mechanism in slim cisterns	Measure the cistern's inside depth
Approval	Fittings on the water supply must comply with the Water Supply (Water Fittings) Regulations	Buy a valve marked as WRAS approved or otherwise shown to comply

What you need

Leave the old parts in place until you have the new valve in your hand and have checked it matches.

The new fill valve, with its washer and backnut
A flat-head screwdriver for the isolating valve
An adjustable spanner or pair of grips for the tap connector
A sponge, an old towel and a small bowl
A torch for looking under the cistern

Step by step: removing the old valve

Work carefully under the cistern. Plastic backnuts and old tap connectors can be brittle, and ceramic cisterns crack if a nut is levered against them.

Turn off the isolating valve on the pipe below or beside the cistern. A screwdriver-slot valve is off when the slot is across the pipe. If there is no isolating valve, turn off the main stopcock.
Take the lid off the cistern, flush, hold the handle down to empty as much as possible, then sponge out the rest into the bowl.
Put the towel under the cistern. Undo the tap connector nut where the supply pipe meets the bottom of the valve shank. A little water will come out.
Undo the large plastic backnut on the outside of the cistern that holds the valve in place. Hand-tight is usually enough to undo it; use grips gently if not.
Lift the old valve out from inside the cistern, noting how it sat and which way the float faced.

Step by step: fitting the new valve

Clean the area around the hole inside and outside the cistern so the new washer seals on a clean surface.

Set the valve height if it is adjustable, following the maker's instructions, so the top sits below the lid and above the overflow.
Fit the sealing washer on the valve's shank inside the cistern, push the shank through the hole, and do up the backnut underneath. Hand-tight plus a quarter turn is normally right. Overtightening can crack the cistern or distort the washer.
Make sure the float can move up and down freely without touching the cistern wall, the flush valve or the lever arm.
Reconnect the tap connector to the shank, using a new fibre washer if the old one is squashed or split. Tighten it firmly, but not with force.
Turn the isolating valve on slowly. Watch the backnut and the connector for drips while the cistern fills, and dry them with tissue so any drip shows up.

If the old tap connector is corroded, or the isolating valve is seized or drips from its spindle, replace those at the same time. A new valve on a weeping connector will still leave a wet floor.

How to adjust the float

The water should stop at the level marked inside the cistern, often a line on the overflow or flush valve. If there is no mark, set it roughly 20 to 25mm below the top of the overflow pipe. Too high and water trickles into the overflow; too low and the flush is weak.

How you adjust it depends on the valve type.

Vertical float valve (float cup around the valve body): turn the adjusting screw or slide the clip on the float rod. Up raises the water level, down lowers it.
Traditional arm valve with a round float: turn the adjusting screw near the pivot, or move the float along a threaded arm. Do not bend an old brass arm; it can snap or crack the valve body.
Compact or concealed valves: follow the maker's instructions, as some use a dial on top.
After each adjustment, flush and let the cistern refill fully before judging the level.

When to call a plumber instead

Concealed cisterns behind panels or in wall frames, wall-hung toilets and back-to-wall units can make this a much bigger job, because the valve can only be reached through the flush plate opening or by removing boxing. Seized isolating valves, corroded pipework and cisterns that were fitted without an isolating valve are also common reasons to stop.

If you would rather have it done, Loyal Plumbers replaces fill valves, flush valves and whole toilets across London and everywhere inside the M25, 24 hours a day, with 10 vans on the road. Our call-out charge is £60. Call 020 4652 8585 or send an enquiry with a photo of the inside of the cistern and a note of whether the pipe comes in from the side or underneath.

Common questions

How do I know if my toilet fill valve needs replacing?: The cistern fills very slowly, will not stop filling, runs into the overflow even after adjusting the float, or hisses long after flushing. If cleaning the valve's filter and adjusting the float does not help, replace it.
Do I need to turn the water off to change a fill valve?: Yes. Use the isolating valve on the pipe to the cistern. If there is not one, turn off the main stopcock and open a cold tap downstairs to drain the pressure.
Why is my toilet still running after replacing the fill valve?: Either the float is set too high so water reaches the overflow, or the flush valve seal is leaking into the pan and the fill valve keeps topping up. Lower the float first, then check the flush valve.
Are toilet fill valves universal?: Many are sold as universal, but they still come as side-entry or bottom-entry, and some only suit mains or tank pressure. Check the entry position, pressure, shank length and height before buying.
How do I stop water running into the toilet overflow?: Lower the float so the water stops 20 to 25mm below the top of the overflow. If it still rises after that, the valve is not shutting off and needs cleaning or replacing.

Sources and further reading

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