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I have a 3-story house with two 10-year old toilet bowls. Both bowls have a somewhat weak flush, but still work fine, though often the bowl needs plunging.

Just recently, I started hearing a hammering noise in the basement pipes when flushing the toilets. Note, this is not after the flush is completed, but as soon as the flush starts. The banging noise will stop once the toilet tank has completely finished refilling itself.

I read on some other forum that the problem is not the same as water hammer. What is this problem exactly? What is the solution?

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  • Tracking this down is going to require the use of a highly specialized tool; another person. Go in the basement and have the other person flush until you can identify a general area or a pipe. As of now, this is an utterly wild goose chase. I'm glad you ruled out water hammer somehow because I hired gremlins with a small hammer to await your flushes and drive you crazy.
    – MonkeyZeus
    Sep 20, 2022 at 13:05
  • the solution stated on other forum was to snake the cleanout, but no explanation why. i don't have access to a cleanout, at least i don't think so.
    – blue_ego
    Sep 20, 2022 at 13:51
  • I'm confused on how you ruled out water hammer?
    – MonkeyZeus
    Sep 20, 2022 at 13:53
  • @MonkeyZeus i haven't, but b/c the sound starts at the beginning of the flush, it's not exactly similar to the description i am reading.
    – blue_ego
    Sep 20, 2022 at 14:23

3 Answers 3

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The problem is the fill valve in the tank has worn and water rushing through is getting disturbed. Those vibrations are being felt in the basement supply line. Change the fill valve.

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  • Comments are not for extended discussion; this conversation has been moved to chat.
    – BMitch
    Sep 23, 2022 at 12:32
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You describe a noise when the tank (what tank?) has been refilled after a flush. This is classic water hammer: noise when water in motion (here, filling the tank) is suddenly forced to change direction or stop flowing.

Most commonly, this occurs because of a fault in the toilet's fill valve, the one in the toilet tank that turns off the water when the toilet tank has refilled.

You changed one of the toilet's fill valves. Now change the other toilet's fill valve.

While there are possible causes, you must eliminate the most likely causes first.

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Check your water pipes for a pressure reducing valve (PRV). The PRV will typically have a bell shape with an adjustment nut on top and be installed just after the main water shutoff. I've seen these go bad with similar symptoms, pipes banging when water is run rather than on a quick shutoff. The high pressure in the home also causes other valves, like the toilet valve, to fail prematurely.

If you have one, and the water pressure in your home is over the typical 40-60psi range, replace the PRV.

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