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Published by Mo's Plumbing and Heating
How to Fix a Running Toilet
A toilet that trickles constantly is one of the few plumbing faults that is genuinely worth fixing yourself, because there are only three things inside a cistern that can cause it and the parts are cheap. The hard bit is not the repair. It is working out which of the three you have.
The Short Answer
Water is either passing from the cistern into the pan, which is the flush valve seal, or the cistern is overfilling and spilling into the overflow, which is the fill valve. Take the lid off and look at the water level. If it is sitting above the marked line or running into the overflow pipe, it is the fill valve. If the level is correct and water is still moving in the pan, it is the flush valve seal. That single observation splits the job in two.
The Three Parts Inside a Cistern
Lift the lid off and there is less in there than people expect. Almost every running toilet comes down to one of three components, and knowing what each does makes the diagnosis obvious rather than a guess.
The fill valve, sometimes called the inlet or ballvalve, lets water in and shuts it off at the right level. On an older cistern it is the arm with a float on the end; on a modern one it is usually a vertical plastic tower with a float collar sliding up it. When this fails it either does not shut off fully, so the cistern keeps filling, or it shuts off at the wrong height.
The flush valve is the outlet in the middle of the cistern, sitting on a rubber seal or a flapper. It lifts when you flush and drops back to seal. When the seal perishes or gets grit under it, water passes continuously from the cistern into the pan, which is the classic silent trickle down the back of the bowl.
The overflow is the safety route. On older cisterns it is a separate pipe running through the wall to the outside; on most modern ones it is built into the flush valve so surplus water simply spills into the pan rather than outside. That second design is why a modern running toilet often shows no external sign at all.
The Dye Test, Which Settles It in Ten Minutes
This is the single most useful thing in this guide and it costs nothing. It tells you definitively whether the flush valve is passing, which is the fault people most often misdiagnose.
- Take the cistern lid off and put it somewhere safe, flat and out of the way. They are heavy, they are ceramic, and they crack if they are leant against a wall.
- Note where the water level sits relative to the marked fill line inside the cistern, if there is one.
- Add a few drops of food colouring to the cistern water, or drop in something that will colour it. Stir it gently with your hand so it disperses.
- Do not flush. Leave it entirely alone for fifteen to twenty minutes.
- Look in the pan. Coloured water in the bowl means the flush valve seal is passing and that is your fault. Clear water in the pan means the seal is fine and the problem is the fill valve overfilling into the overflow.
If It Is the Flush Valve Seal
This is the commoner of the two and usually the cheaper fix. The seal is a rubber washer or a flapper that sits under the flush valve, and it fails in one of two ways: it perishes and hardens with age so it no longer conforms to the seat, or a piece of grit or limescale gets between it and the seat and holds it fractionally open.
Before buying anything, turn off the water at the isolator on the supply pipe, flush to empty the cistern, and lift the seal out to look at it. A surprising number of these are simply dirty. Wipe the seal and the seat it sits on with a cloth, check for grit, and refit. If the rubber is still soft and unmarked, that may genuinely be the whole repair.
If the rubber is hard, cracked, distorted or has a visible groove worn into it, it needs replacing. Take the old one with you to buy the new one, because there are a lot of sizes and they are not interchangeable. A photograph of the whole flush valve helps too, since the seal type depends on the valve design.
On some cisterns the seal is part of a sealed unit rather than a separate washer, in which case the whole flush valve comes out. That is a bigger job because it usually means disconnecting the cistern from the pan to reach the retaining nut underneath, and it is the point at which a lot of people reasonably stop.
If It Is the Fill Valve
Here the cistern is filling past its correct level and the surplus is going into the overflow. The fix is either an adjustment or a replacement, and try the adjustment first because it takes a minute.
The correct level is usually marked inside the cistern, and where it is not, aim for roughly twenty millimetres below the top of the overflow. On an older float arm, bending the arm very slightly downward lowers the shut-off level, though on modern plastic arms bending is a good way to snap one. On a modern tower valve there is a clip, screw or collar on the float that slides up and down the body to set the height.
If adjusting the level does not stop it, the valve is not shutting off fully and it needs replacing. Fill valves are inexpensive and most are a straightforward swap: isolate the water, empty the cistern, disconnect the supply, undo the back nut, fit the new one, set the level. The one thing worth checking before buying is whether yours is bottom entry or side entry, since the supply comes into the cistern in a different place on each and they are not interchangeable.
Where a valve has been replaced and the toilet still overfills, look at the water pressure. A fill valve struggling against unusually high pressure can chatter or fail to seat, and there are valves specifically rated for higher pressures.
Before You Start
Find the isolator first. There should be a small valve on the supply pipe just below the cistern, usually a slot that turns with a flat screwdriver. Turning it a quarter turn so the slot sits across the pipe shuts the water to that toilet alone, which means you are not turning off the whole flat to change a washer.
If there is no isolator, that is worth knowing before you take anything apart rather than after. On an older installation you may have to use the main stopcock, and if you are going to be at this for an hour that affects everyone in the property.
Have a towel down and a bowl to hand. There is always more water left in a cistern than the flush suggests, and it arrives on the floor at the moment you disconnect something.
And take a photograph of the inside of the cistern before you remove anything. It is the cheapest insurance there is when you are standing in a merchant trying to describe a part you last saw forty minutes ago.
When It Is Not Worth Repairing
Most running toilets are worth fixing, and the parts cost very little against a replacement. There are a few cases where that flips.
A cracked cistern is the obvious one. A hairline crack that weeps will not be reliably repaired and it will get worse, and the cistern is often no longer available separately for an older suite. Similarly, a cistern where the flush valve has been replaced twice and the fault keeps returning usually has a worn seat that no new seal will seat on properly.
Older suites where parts are obsolete are the other case. A close-coupled toilet from the 1980s in a discontinued colour is not going to yield a matching cistern, and at that point the honest advice is to keep it running with whatever parts still fit, or to plan a replacement rather than pay repeatedly for repairs to something with no spares left.
The other thing that shifts the maths is a toilet that is loose on the floor or leaking at the pan connector. If the unit has to come off anyway to fix that, replacing it while it is off is much less work than doing both jobs separately.
Why Running Toilets Get Left Longer in Scotland
There is a genuine reason these go unfixed here for months, and it is not carelessness. Most Scottish households do not have a water meter. Domestic water and waste water charges are collected alongside council tax rather than by volume used, so a toilet that has been trickling since March does not show up as a rising bill the way it would for a metered household further south.
That removes the usual alarm. Down south a running toilet announces itself on the next bill and gets dealt with; here the first sign is often just the noise, and people stop hearing it after a fortnight. So the practical advice is different in Scotland: you have to go and look rather than wait to be told, and the dye test above is the way to check a suspicion cheaply. Worth doing on a spare or downstairs toilet nobody uses daily, since those are where a trickle runs unnoticed longest.
The other local factor is what the overflow does in a tenement. Older Glasgow cisterns commonly have a real overflow pipe running through the external wall, and on an upper floor that discharges down a gable or into the back court where you will never see it from inside the flat. A neighbour noticing a persistent drip down the wall is sometimes the first anyone hears of it. If you have an external overflow, its outlet is worth locating once so you know where to look.
And the older stock throws up cisterns that a modern guide does not describe. High-level cisterns with a long flush pipe still exist in original tenement bathrooms and in properties that have kept a period suite, and their internals are a different arrangement with different parts. If yours is mounted near the ceiling with a chain, the principles above hold but the components will not match the pictures on a merchant's shelf, and taking the old part with you stops being advice and becomes essential.
When to Call Someone Out
Worth calling when the cistern has to come off the pan, when the fault has come back after a seal replacement, when there is no isolator and the whole flat has to be shut down to work on it, or when the cistern or pan is cracked. Also worth it if the toilet is loose on the floor or damp is showing at the base, because that is a different fault from the running and it matters more. Mo or Kyle handle most of this work, and if a toilet is genuinely past economic repair we will say so rather than fitting a third seal to a worn seat.
Toilet Installation in GlasgowCommon Questions
Establish which of two faults you have before touching anything. Take the lid off and look at the water level: sitting above the fill line or running into the overflow means the fill valve, and a correct level with water still moving in the pan means the flush valve seal. The dye test described above confirms it in about fifteen minutes and costs nothing.
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How to Fix a Running Toilet With a Button Flush
Take the button out of the lid first, because you cannot get the lid off with it in place on most designs.
Why Does My Toilet Keep Running?
Listen to the pattern rather than the noise.
Why Won't My Toilet Flush?
Lift the cistern lid and flush.
How Do You Fix a Toilet Seat?
Most loose seats are fixed by tightening the two fixings underneath the back of the pan, from below, by hand or with a spanner.
How to Fix a Leaking Toilet
Dry the floor completely, then work out where the water reappears from.
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