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This Old House Plumbing and Heating Expert, Richard Trethewey explains the cause and solution of a dripping toilet

Question:

One of my toilets makes a dripping sound like it’s leaking. I’ve checked under it and in the wall where the water comes out, but everything is dry. What now?
James Dickson, Murfreesboro, TN

Answer:

If there’s no evidence of any water outside your toilet, the dripping noise is probably coming from inside the tank. Try this simple test: Turn off the water supply to the tank, hold down the flush lever until the tank is empty, and listen for a drip. Dollars to doughnuts, you won’t hear a thing.

There are only two ways for water to drip out of a tank: around the flapper valve or through the overflow tube. So while the tank is empty, take off its lid and inspect those two items. If the tube has a crack that extends below the tank’s fill line—the mark on the tank showing what the water level should be when the tank is full—it will allow water to leak into the bowl. A cracked tube should be replaced. If that’s not it, take off the flapper and inspect its underside. Any discoloration could indicate that the flapper isn’t making a tight seal with the valve seat. Wipe off the flapper and the valve seat it fits into, and reinstall it.

Now turn the water supply back on and watch as the tank fills up. If the water reaches the rim of the overflow tube, water could be spilling over it. To fix this, adjust the float ball or float cup so that the fill valve shuts off the flow when the water reaches the fill line.

One more thing: If there’s a little float on the flapper’s chain, make sure the float sits at the fill line when the tank is full. A submerged float could lift the flapper valve slightly and prevent it from making a tight seal. I’m fairly sure that one of these fixes will put an end to the dripping.

from September 2015