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One of the 4 toilets in the house runs water through the fill tube even though it has not been flushed. This happens when a large flow of water from another water user such as filling the washing machine or flushing one of other toilets. Once the large flow ends the water flow from the suspect toilet stops. The suspect toilet does not flow water when none of the other large water flow sources are flowing.

  • Hello, and welcome to Home Improvement. Sounds like the fill valve is reacting to pressure changes, and is likely to need replacement. – Daniel Griscom Feb 9 at 16:51
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There are several types (and many brands and models) of toilet fill valves, aka ballcock valves.

The type most susceptible to pressure fluctuations are those that utilize a "weir diaphragm" assembly because these rely on inlet pressure to keep the valve closed. An old stiff (or worn/soft/contaminated) diaphragm can exacerbate this susceptibility such that a pressure bobble can cause the valve to open.

You should open the valve to inspect the bonnet area and diaphragm for problems; cleaning/flushing all parts and perhaps replacing the diaphragm (replacements are commonly available, specific to each brand and model).

Another option would be to just replace the entire valve.

  • This is an excellent answer to the question. I might have put more weight on just replacing the whole valve unit as they are inexpensive and messing around to find proper replacement parts for the existing valve may very well consume more time than outright replacement. – Michael Karas Feb 9 at 19:11
  • @Michael karas, yes for you and I outright replacement makes sense. For a novice there can be complications that are difficult to resolve when removing/installing a new valve and when messing with the supply line, whereas popping off the top of the valve and dropping a new diaphragm kit is fairly simple. – Jimmy Fix-it Feb 9 at 21:38

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