There is a particular sound a running toilet makes. Not the obvious rush of a flush, but a thin trickle underneath everything else in the house, the kind you stop hearing after a week because your brain files it as normal.
That is the problem with this fault. It is quiet, it is not urgent, and nothing bad appears to be happening. Meanwhile clean water is going down the drain continuously, and it shows up on the bill before it shows up anywhere else.
The good news is that a running toilet is one of the few plumbing faults where the diagnosis is genuinely straightforward, and most of the repairs are within reach of anyone willing to take the lid off the tank.
What is actually happening in there
Worth understanding the mechanism before poking at it, because it makes the fault obvious rather than mysterious.
When you flush, a lever lifts a rubber seal at the bottom of the tank. That seal is the flapper. Lifting it lets the tank empty into the bowl. The flapper then drops back down and seals the opening.
With the tank now empty, a float drops, which opens the fill valve. Water runs in. As the tank fills, the float rises, and at a set height it shuts the fill valve off.
There is also an overflow tube, a vertical open pipe in the middle of the tank. Its job is to stop the tank overflowing onto your floor if the fill valve fails to shut off. Anything above a certain level simply runs down that tube into the bowl.
So a toilet runs continuously for one of two reasons. Either water is escaping past the flapper into the bowl, so the tank never stays full and the fill valve keeps topping it up. Or the fill valve is not shutting off, and the excess is going down the overflow tube.
Those two causes look identical from outside the tank. Telling them apart takes about a minute.
The test that tells you which one it is
Take the lid off the tank and set it somewhere safe. It is porcelain and it does not survive a drop.
Look at the top of the overflow tube.
If water is running into the overflow tube, the tank is overfilling. The fault is on the fill side, and it is either the float setting or the fill valve itself.
If the overflow tube is dry but the toilet is still running, water is leaking past the flapper. The tank is draining slowly into the bowl and the fill valve is doing its job by replacing it.
If you cannot tell, there is an old trick that settles it. Put a few drops of food colouring into the tank and leave it alone for twenty minutes without flushing. If colour appears in the bowl, the flapper is leaking. If the bowl stays clear, the flapper is fine and the problem is on the fill side.
If water is going into the overflow tube
The float is set too high. This is the simplest possible fix. The fill valve is shutting off, but only after the water has already risen above the overflow tube.
The water level should sit roughly two and a half centimetres below the top of the overflow tube. Most tanks have a line marked on them.
How you lower it depends on the age of the toilet. Older ones have a float ball on a metal arm, and you bend the arm downward slightly. Newer ones have a cylindrical float that slides on the fill valve body, adjusted with a screw or a clip. Either way, lower it a little, flush, and check where the water settles.
The fill valve is not shutting off. If the float is set correctly and water still runs into the overflow tube, the valve itself has failed. Sediment and mineral deposits build up inside them and eventually they stop sealing.
Fill valves are inexpensive and a common homeowner replacement. You shut off the supply at the wall, flush to empty the tank, sponge out the last of the water, disconnect the supply line, and swap the unit. It is genuinely a job most people can do.
The one thing to watch is the shut-off valve at the wall. On older installations these seize, and if it will not close you cannot do the job without shutting off the water to the whole house. Try it before you commit to anything.
If the flapper is leaking
This is the more common of the two by some distance.
The flapper is worn. Rubber does not last forever in constantly wet conditions. It hardens, warps slightly, and stops making a clean seal. Lift it out and look at it. If the underside feels stiff rather than soft, or you can see the seat has cut a groove into it, it is finished.
Flappers are cheap and take a couple of minutes to change. The one point worth care is buying the right one. They are not all the same shape, and a flapper that almost fits will leak just as badly as the old one.
The chain is wrong. Very common and very easy to miss.
Too short, and it holds the flapper slightly open all the time. Too long, and it can slip under the flapper as it drops, propping it open. Tangled, and it does either.
The chain should have a small amount of slack when the flapper is seated. Not taut, not looping so far that it can get caught. If yours has extra length hanging, clip the surplus or hook it further along.
The flush valve seat is damaged. This is the one that catches people out after they have already replaced the flapper twice.
The flapper seals against a plastic or brass rim at the bottom of the tank. If that rim is pitted, scaled or scratched, a brand new flapper will still leak because the surface it seals against is no longer flat.
Run a finger around it. It should feel smooth. If it feels rough or gritty, cleaning it carefully sometimes helps. If it is genuinely damaged, replacing the flush valve means removing the tank from the bowl, and at that point it is a different job. That is a reasonable moment to bring in a plumber for the toilet repair rather than take a working toilet apart on a Saturday.
When it is not worth repairing
Sometimes the honest answer is that the toilet has had its run.
If you are replacing parts every year, if the tank or bowl is cracked anywhere, if the toilet rocks when you sit on it, or if it is old enough that matching parts are getting hard to find, the repairs stop being economical. A toilet that needs a new flapper, a new fill valve and a new flush valve is most of the way to the cost of a modern unit that will use noticeably less water per flush.
That is a judgement call rather than a rule, and it depends on the toilet.
The related fault that looks similar
A running toilet and a toilet that will not fill are opposite problems with overlapping symptoms, and people mix them up constantly.
If your issue is that the tank empties and then struggles to refill, or refills very slowly, that is a different diagnosis with different causes. We covered it separately in why a toilet tank will not fill after a flush.
Why not to leave it
Nothing dramatic happens if you ignore a running toilet. That is precisely why they get ignored for months.
What does happen is continuous water use, quietly, around the clock. On a metered supply that is money going down the drain in the most literal sense. On a septic system it is worse, because the extra volume puts load on a system that was sized for normal household use.
There is also the slow damage. Constant flow across the flush valve seat wears it further, which is how a five minute flapper change turns into a full tank removal a year later.
Take the lid off, do the food colouring test, and you will usually know what you are dealing with inside half an hour.
If the shut-off valve at the wall will not turn, if the tank needs to come off, or if you have replaced the obvious parts and it is still running, we handle toilet repairs and replacements across Pickering, Ajax and Whitby. Call 647-248-7755 any hour.
