Toilet Keeps Running? Here's What's Actually Happening
The sound is water leaving your house
A toilet that keeps running is not idling. It is moving fresh water from the supply line, through the tank, into the bowl and out to the sewer, continuously, at a rate that can reach several hundred gallons a day. That is the reason a running toilet shows up on a water bill long before anyone in the house finds it irritating enough to mention.
The good news is that the cause is almost always one of three cheap parts, and the diagnosis takes about two minutes.
Test one: the dye test
Take the tank lid off and drop a few drops of food colouring into the tank water. Do not flush. Come back in fifteen minutes and look in the bowl. If there is colour in the bowl, water is passing from the tank into the bowl without a flush, and your flapper or flush valve seat is the problem.
If the bowl is still clear but you can hear water, the fill valve is running instead, and the water is going down the overflow tube rather than past the flapper.
What each result means
Colour in the bowl points at the flapper. Rubber flappers harden, warp and stop sealing, usually after three to five years, and chlorine tablets in the tank speed that up considerably. The seat the flapper closes against can also pit with mineral deposits, in which case a new flapper alone will not seal.
No colour but a constantly refilling tank points at the fill valve. Either it is not shutting off at the set level, or the float has been knocked out of adjustment and the water level has risen above the overflow tube, so the tank is quietly draining into it forever.
A third possibility, less common but worth knowing: the flush handle chain is too short and is holding the flapper fractionally open. Look at the chain slack before you buy anything.
When it is not really the toilet
If you replace the flapper and it starts running again within weeks, look at your water pressure. High incoming pressure hammers the fill valve and shortens the life of every soft part in the tank. Anything much above 80 psi is worth fixing at the pressure reducing valve rather than replacing toilet internals twice a year.
If the bowl also empties slowly, or gurgles when a nearby tub drains, the toilet is not the problem at all. That is a venting or drainage issue further down the line, and it needs a drain camera rather than a trip to the hardware store.
The honest repair-or-replace line
Internals are inexpensive and worth renewing on any sound bowl, whatever the age of the toilet. Replace the whole unit when the china is cracked, when the closet flange underneath has rotted and the toilet rocks, or when it is an old high-volume model that uses three or four times the water of a current one on every single flush.
If your toilet has been running for months, it is worth checking the rest of the house at the same time. A running toilet is often the loudest of several small losses.
If any of this describes what is happening at your address right now, call (888) 809-6756. We answer 24 hours a day across South Carolina and will tell you what to expect before anyone comes out.