Plumbing
Toilet That Keeps Running: The 4 Usual Culprits
A toilet that never quite shuts off is almost always one of four parts. Here is how to find out which one before you buy anything.
July 20, 2026 · 6 min read
A running toilet is easy to live with and expensive to ignore. It is not dramatic. Nothing floods. But water is moving through that tank continuously, and it shows up on the bill about six weeks later when you have already forgotten the sound.
The good news is that a toilet tank is one of the simplest mechanisms in your house. There are only a handful of moving parts, and a running toilet is almost always one of four of them. Here is how we sort it out.
First: figure out which direction the water is going
Before you touch a part, do this. Take the tank lid off and set it somewhere it will not get knocked over. Now look at the water level relative to the overflow tube — that is the open vertical pipe in the middle of the tank.
If water is spilling over the top of that tube, your fill valve is not shutting off and the tank is overfilling. If the water level sits below the tube but the tank keeps refilling every few minutes, water is escaping down through the flush valve into the bowl. Those are two completely different repairs, and this one look tells you which one you are dealing with.
If you cannot tell, put a few drops of food coloring in the tank and do not flush. Come back in fifteen minutes. Color in the bowl means the flapper or flush valve seat is leaking.
1. A worn or warped flapper
This is the most common cause by a wide margin. The flapper is the rubber seal at the bottom of the tank that lifts when you flush and drops back to seal. Rubber does not stay flexible forever. It hardens, warps, or develops a mineral crust on the sealing edge, and it stops seating flat.
Hard water is rough on flappers, and North Texas water is hard. Minerals build on both the flapper and the plastic seat it lands on, and once that seal is uneven, you get the slow trickle that makes the fill valve kick on every ten minutes all night.
Test it by pressing down gently on the flapper with a wooden spoon handle while the toilet is idle. If the running stops while you hold pressure, the flapper is not sealing. Also check the chain: too much slack lets the flapper land crooked, too little slack holds it slightly open. You want just a little sag with the flapper closed.
2. A fill valve that will not shut off
The fill valve is the tall assembly on the left side of the tank. It refills after a flush and shuts off when the float reaches its set height. When the valve wears out or gets grit in the seal, it keeps letting a little water through even after the float is up. The excess goes down the overflow tube, so you get a constant faint hiss and no visible problem in the bowl.
Sometimes it is not the valve at all — it is the float set too high, so the water level is above the overflow tube and just runs down it forever. Correct level is roughly an inch below the top of the overflow tube. Most modern valves adjust with a thumbscrew or a clip on the side; older ball floats adjust with a screw at the arm.
If the level is correct and it still hisses, the valve itself is done. They are not expensive parts, and a fill valve is a reasonable homeowner replacement if you are comfortable shutting off the supply and disconnecting a supply line.
3. A cracked or misplaced overflow tube
Less common, but we do see it. If the overflow tube is cracked, or if it was cut short during a past repair, water drains into it before the tank ever fills to the shutoff point. The fill valve never gets the signal to stop, so it runs indefinitely.
The other version of this: the small refill hose that clips onto the overflow tube has slipped down inside it. When that hose is submerged, it can siphon tank water into the bowl continuously. Fix is free — clip it back so it discharges above the water line.
4. A worn flush valve seat
If you have already replaced the flapper and the toilet still leaks into the bowl, look at the plastic seat the flapper lands on. Run a finger around it. If it feels pitted, rough, or scaled, a brand new flapper will not seal against it either.
Sometimes it cleans up with a scrub. Often the honest answer is that the flush valve needs to be replaced, which means pulling the tank off the bowl. That is a real repair, and it is the point where a lot of homeowners are better off calling. On an older toilet with a scaled seat and a tired fill valve, we will usually tell you what a full rebuild kit costs versus a new toilet, and let you weigh it.
What you can safely check yourself
- Take the lid off and check the water level against the overflow tube.
- Do the food coloring test to confirm whether water is leaking into the bowl.
- Check the flapper chain for too much or too little slack.
- Press down on the flapper and see if the running stops.
- Make sure the small refill hose clips above the overflow tube, not inside it.
- Adjust the float so the water sits about an inch below the top of the overflow tube.
Those six checks solve a large share of running toilets, and none of them require a tool more serious than a screwdriver. There is a shutoff valve on the wall behind the toilet — turn it clockwise before you disconnect anything.
When it is worth calling instead
- You replaced the flapper and it still leaks into the bowl.
- The shutoff valve behind the toilet is seized or weeps when you turn it.
- The toilet rocks, or you see water at the base after a flush.
- The tank refills on its own with the supply valve fully closed.
- Multiple toilets in the house started acting up around the same time.
That last one matters. If several fixtures changed behavior at once, the issue may not be the toilets at all — it can point to pressure problems feeding the whole house, and that is worth having someone look at properly rather than replacing four flappers.
As for cost: a flapper swap is about as inexpensive as plumbing repairs get. A fill valve is a bit more. A full flush valve replacement or a tank pull sits higher because the tank has to come off and the gaskets get replaced with it. What moves the number is the toilet's age, whether the shutoff valve is still serviceable, and whether the closet flange underneath is in good shape. We will tell you which of those applies before we start, not after.
Wilson Family Plumbing, Heating & Air is family-owned and based right here in Rockwall, and we work across Rockwall County and the wider DFW metro. Licensed TX Master Plumber TXM11693. If a toilet has been running for weeks and you would rather it just stopped, call (469) 338-5533. Someone answers, day or night.
Frequently asked
How much water does a running toilet actually waste?
It depends entirely on whether it is a slow trickle past the flapper or a fill valve running wide open, so we will not throw a number at you. What we can say is that even a leak too quiet to hear runs continuously, and it is one of the most common explanations we find for a water bill that jumped with no other change in the household.
Why does my toilet run for a few seconds every so often with nobody using it?
That is called phantom flushing, and it means the tank is slowly losing water into the bowl until the float drops far enough to trigger a refill. Almost always a flapper that is no longer sealing, or a flush valve seat that has scaled up from hard water. The food coloring test confirms it in fifteen minutes.
Should I rebuild an old toilet or replace it?
If the bowl and tank are sound and the porcelain is not cracked, a rebuild with new internals is usually the better spend. We lean toward replacement when the flush valve seat is badly pitted, the tank-to-bowl gaskets are failing, and the toilet is an older high-volume model anyway. We will give you both numbers and our honest read on which one we would choose in your house.