How to Fix a Running Toilet: Flapper, Fill Valve, Float, and Flush Valve Seat

A running toilet is almost always one of three parts: a flapper that no longer seals, a float set too high or a fill valve that will not shut off, or a flush valve seat that is rough, scaled, or cracked. Start with the dye test to find out which. A flapper or fill valve is a homeowner repair with a part from the hardware store; a flush valve seat usually means a rebuild kit or a plumber, and an old toilet that needs all three is often better replaced.

A running toilet is the leak people live with longest, because nothing gets wet. It just hisses, or refills on its own every few minutes, and the water bill goes up. It is also the leak that wastes the most water in a house, by a wide margin. Here is how a plumber finds the cause, how to fix the common ones yourself, and where the line is.

Why does a toilet keep running?

Water leaves the tank in only two ways: down through the flush valve into the bowl, or over the top of the overflow tube. A toilet that runs is losing water one of those two ways, and the fill valve is refilling to keep up.

A flapper that does not seal lets water seep past into the bowl. The tank level drops, the float drops with it, and the fill valve runs for a few seconds every few minutes. Flappers harden, warp, and scale up with age.

A float set too high lets the tank fill past the top of the overflow tube, so water pours down the tube into the bowl and the fill valve never stops. The water line should sit about an inch below the top of the overflow tube.

A fill valve that will not shut off runs regardless of the float. Fill valves wear internally and clog with sediment and scale.

A flush valve seat that is rough, pitted, or cracked will not seal even with a brand-new flapper. Scale from hard water is the usual reason, and a hairline crack in a plastic overflow tube does the same thing.

A chain that is too short holds the flapper slightly open; a chain that is too long gets caught under it. Both are a thirty-second fix.

How do you do the dye test?

Take the tank lid off and put several drops of food coloring or a dye tablet in the tank water. Do not flush. Wait fifteen to thirty minutes. If color shows up in the bowl, water is getting past the flapper or the flush valve seat and that is your leak. If the bowl stays clear but the toilet still runs, the problem is the float or the fill valve. Do the test with the water line visible, and note whether it is above the top of the overflow tube; that alone answers the float question.

How do you fix a flapper that leaks?

1. Shut off the stop valve behind the toilet and flush to empty the tank.
2. Unhook the chain from the flush lever and unhook the flapper ears from the pegs on the overflow tube, or slide off the ring if it mounts that way.
3. Wipe the flush valve seat with a cloth and run a finger around it. It should be smooth. Scale can be cleaned off gently with a nonabrasive pad; a rough or pitted seat is the next section.
4. Match the new flapper to the flush valve. Two-inch and three-inch flush valves take different flappers, and some brands, including many TOTO and Kohler models, use their own. Take the old one to the store or look up the toilet model number, which is usually stamped inside the tank.
5. Install the new flapper, hook the chain with about a half inch of slack, turn the water back on, and repeat the dye test.

How do you adjust the float or replace the fill valve?

If the water line is above the overflow tube, lower the float. On a modern fill valve that is a screw or clip on the float cup; on an older ballcock it is the arm holding the ball, which bends down gently. The target is about an inch below the top of the overflow tube, or at the marked line inside the tank.

If lifting the float by hand does not stop the water, the fill valve has failed. Replacing it:

1. Shut off the stop valve, flush, and sponge out the remaining water.
2. Disconnect the supply line from the bottom of the fill valve and unscrew the locknut under the tank.
3. Lift out the old valve and set the new one to the tank height per its instructions, with the top of the valve above the overflow tube by the amount the instructions state.
4. Install, connect the refill tube to the overflow tube with its clip so it does not push below the water line, reconnect the supply, turn the water on slowly, and set the float.

A universal fill valve fits nearly every two-piece toilet. Replace the supply line at the same time if it is more than a few years old or is not braided stainless.

What if the flush valve seat is the problem?

When a new flapper still leaks and the seat is pitted, scaled beyond cleaning, or the plastic overflow tube is cracked, the flush valve itself has to come out. That means removing the tank from the bowl: disconnecting the supply, unbolting the tank, lifting it off, replacing the flush valve and the tank-to-bowl gasket, and reseating the tank so it neither rocks nor cracks when the bolts are tightened. Rebuild kits exist, and a handy homeowner can do it, but this is the step where a cracked tank or a leaking tank-to-bowl seal becomes a real problem, and it is the step where most people call. On an older toilet, the cost of a full rebuild in parts and time is often close to a new water-saving toilet installed, and we say so when we quote it.

How much water does a running toilet waste?

The EPA’s WaterSense program estimates that a leaking toilet can waste about 200 gallons a day. That is the equivalent of flushing a modern toilet more than a hundred times a day for nothing, and it shows up on the next water and sewer bill. The same program estimates that household leaks in general waste nearly 10,000 gallons a year in the average home; a running toilet can do that by itself in under two months.

How does hard water affect toilet parts?

Scale from hard water is the reason flappers and seats fail early here. It roughens the flush valve seat so the flapper cannot seal, stiffens the fill valve so it hangs open, and clogs the small ports that rinse the bowl. Municipal water in Woodbury and the East Metro is hard to very hard on the USGS scale. If you are replacing toilet parts and faucet cartridges on a regular schedule, measure the hardness with a free basic water test; a properly sized softener addresses the cause. Our water softener installation page covers when that makes sense.

When should you call a plumber?

Call when the flush valve seat is cracked or will not clean up, when the tank needs to come off, when the toilet rocks or water appears at the base after a flush, which is the wax seal and possibly the flange, when the tank or bowl is cracked, when a stop valve behind the toilet will not close, or when the toilet is old enough that a rebuild costs more than replacing it. A.J. Alberts repairs and installs toilets across Woodbury and the East Metro, including TOTO and Kohler, and quotes the repair and the replacement in writing so you can compare. See faucet repair and fixture installation for what a visit covers. If a faucet is dripping too, our guide to how to fix a leaky faucet works the same way.

Toilet running in Woodbury, Lake Elmo, Cottage Grove, White Bear Lake, or anywhere in the East Metro? Call A.J. Alberts at 651-738-0580 or book online. Written price before the work, no commissioned salespeople.

Sources and Further Reading

• EPA WaterSense: Fix a Leak Week, toilet and household leak figures
• EPA WaterSense: residential toilets
• USGS: hardness of water