Property How to fix a running toilet
Plumbing · Beginner
How to fix a running toilet
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A cistern that will not stay quiet is usually a fill valve set too high, a worn flush-valve seal, or, on many older UK toilets, a tired siphon diaphragm. The water either trickles into the pan all the time, or the cistern fills, dumps and fills again. You can often change the small parts yourself once the supply is isolated. Lifting the pan or opening a soil pipe is a different job.
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At a glance
Your call
Go or no?
Fancy giving it a go, or would you rather call in a plumber? There’s no wrong answer.
Right then. Here’s the kit, then the steps.
No problem. We’ll show you how to find a local plumber.
Shop the full kit
You can pick up the items below as you need them. Prices vary by shop and country, so we have not invented a figure here — check the live price before you buy. Shop links go to a product search on a local store for your country. Until a tracking tag is in place they are ordinary store links, not tracking URLs.
All-in-one kit
There is not usually a single boxed kit for this job. Buy the items below as you need them, and skip anything you already have.
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A one-page list of tools, prep and the order of work, emailed once this is live.
Before you start
Safety
Stop and get a pro: Isolate the cistern supply before you take anything apart. If you cannot turn the water off, if the cistern or the pan is cracked, or if water is leaking at the base of the pan or from the soil pipe, stop and call a plumber. Do not lift the pan or undo the soil connection. Leave any electrical bidet seat or nearby electrics alone. This is not gas work and not a job on the incoming mains, beyond closing a stopcock if you have to.
- Turn the isolation valve on the supply pipe off, or close the household stopcock, before you drain the cistern.
- Toilet water is not clean. Wear gloves, and wash your hands when you finish.
- Support a heavy ceramic cistern lid with both hands — it is easy to chip the pan if you drop it.
- Do not overtighten plastic cistern fittings. They crack, and then the cupboard (or the ceiling below) gets wet.
- If the cistern is concealed in a wall-hung frame and you cannot reach the valves, get a plumber rather than cutting the panel to pieces.
Prep that is worth doing
- Listen and look: is water running into the pan all the time (flush seal, siphon or a high water line), or does the cistern refill every so often without a flush (the fill valve is letting by, or the seal is weeping slowly)?
- Find the isolation valve on the flexible or copper supply to the cistern, usually on the left or right just below it.
- If you can, take a photo of the inside of the cistern before you undo anything, so a new valve goes back the same way.
Tools and materials
Check the live price at a shop you trust. Typical prices on this site, where they appear, were checked in the UK and may be out of date. Do not invent or assume a figure from this table. Shop links go to a product search on a local store for your country. Until a tracking tag is in place they are ordinary store links, not tracking URLs.
| Item | Why you need it | Typical price | Shop link |
|---|---|---|---|
| A replacement flush-valve seal, flapper, or siphon diaphragm, as fitted | This is the usual cause of a quiet, constant trickle into the pan. The part has to match what is already in the cistern. | check price | |
| A replacement fill valve or float-valve washer, if the cistern will not shut off | If the water line sits above the overflow and keeps filling, the fill valve is not closing. | check price | |
| An adjustable spanner and a screwdriver | For supply nuts and for the siphon or valve fixings, depending on the model. | check price | |
| Rubber gloves, a sponge and a small bucket or jug | To empty the cistern and to catch the last of the water. | check price | |
| A cloth | For a drip and for gripping plastic nuts without bruising them. | check price |
Shop this list:
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Step-by-step
Isolate the water, drain the cistern, then deal with the fill side or the flush side, depending on where the water is going.
Isolate the water and drain the cistern
Close the isolation valve or the stopcock. Flush the toilet to empty most of the cistern, then sponge or bail out what is left so you are not working in a full tank.
Lift the lid and see what you have
Close-coupled cisterns in the UK often have a siphon (a plastic bell you lift with the flush handle or button) or a more modern drop-valve / dual-flush valve in the middle, and a fill valve at the side. Many US toilets use a flapper on a flush valve and a ballcock or float-cup fill valve. You do not need to know every trade name — you do need to see which part is worn or stuck.
If water is trickling into the pan, check the flush seal
On a flapper or drop valve, lift the seal. If it is warped, slimy or split, replace it with the same type, seating it flat on the flush-valve rim. On an older UK siphon, the diaphragm (a flexible flap inside the siphon) is a common failure — you usually undo a large nut under the cistern (put a bucket underneath, and support the siphon) and swap the diaphragm for one of the same size. If you have to lift a close-coupled cistern off the pan and you are not comfortable doing it, stop and call a plumber.
If the cistern will not shut off, adjust or replace the fill valve
The water line should sit below the overflow or warning pipe. Bend or adjust the float arm, or set the clip on a float-cup valve, so it closes a little earlier. If the valve still lets by, or the washer is worn, replace the fill valve with one that fits the hole in your cistern (bottom inlet or side inlet). Hand-tighten, then a little more, and turn the water on slowly to check.
Turn the water back on and test several flushes
Watch the fill line, the overflow, and the joints you have touched. The cistern should fill and stop, and should stay quiet. A short refill after a flush is normal. A continuous hiss is not. Check for drips under the cistern.
If it still runs
The overflow may be letting water into the pan because the fill valve is still too high, or the new seal may not be sitting flat. Try once more. A cracked cistern, a leaking close-coupling gasket, or a leak at the pan outlet is a stop-and-get-a-pro moment.
General guidance only. Check our disclaimer.
Common mistakes
- Not isolating the supply. The cistern will keep filling as you work, and a loosened fill valve can spray the room.
- Overtightening the large plastic nut under a close-coupled cistern. It is a common way to crack the cistern.
- Fitting the wrong diaphragm or seal. Take the old one with you. Close enough often still leaks.
- Ignoring a leak at the base of the pan. That is not a cistern-seal job. Call a plumber.
- Letting the water line sit above the overflow. The toilet will run forever, quietly, and waste a lot of water.
Upload a photo to see the finished job
When the mock-up is live, you can upload a photo and see an impression of the finished job — not a quote or an exact product match.
FAQ
Is it all right to turn the water off at the stopcock?
Yes, if there is no isolation valve on the cistern supply. Remember that this may also turn off other taps until you open it again.
The handle is loose, but the toilet is not running.
The link between the handle and the siphon or flush valve may have come off. That is usually a clip or a small rod inside the cistern, and it is worth a look before you buy parts.
How do I know if I have a siphon or a drop valve?
A siphon is a boxed plastic unit the handle lifts; there is often no rubber flapper visible. A drop or flush valve sits in the middle of the cistern floor with a seal you can lift. Either can be fixed at home if you can get the matching part.
The cistern is inside the wall.
Stop if you cannot open a proper access panel and see the valves clearly. Concealed frames are easy to damage, and a plumber will have the right kit.
Would rather not DIY? Find a local plumber
If the cistern is cracked, the leak is at the base of the pan, or you would rather not work over a close-coupled toilet with a bucket, a plumber will sort it without a second trip for the wrong seal.
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