I live in high rise condo with identical units on each floor. Only when my neighbor directly above me flushes his toilet does my commode run briefly. My toilet functions correctly. Our building engineer has no answer for this.

  • Varying water pressure? What happens if you suddenly turn the bathroom faucets on or off? – Daniel Griscom May 16 '16 at 16:58

It's normal. Your "out pipe" and his share a connection. As the water rushes past your connection, suction is created, and a little water goes "out" your toilet. I would get one of those "refresh" kits that replace the flapper, just to be sure and call it done.

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  • I agree that if the flapper is not sealing this could cause the problem with the vacuum created from a flush on a upper floor. – Ed Beal May 16 '16 at 17:50
  • How is this possible? The flapper holds water in the tank. If water suctioned it would be from the bowl. Lowering the water in the bowl, can't cause suction at the flapper. Essentially the bowl overflows into try he drain. @EdBeal – Tyson May 16 '16 at 18:14
  • @Tyson correct. – Harper - Reinstate Monica May 16 '16 at 18:33
  • The water falling from above makes a negative pressure on the vent stack and pulls a little by the flapper, I have seen this in multi story buildings and a new flapper was all that was needed to fix it. – Ed Beal May 16 '16 at 18:52
  • There's an air gap between the water in the bowl and the water in the tank. I could see water being sucked from the bowl, but how could that create suction on the flapper? @EdBeal – Tyson May 16 '16 at 18:59

Coteyr has the right idea, wrong pipe. The water supply pipe is constricted somewhere below both your apartments. Probably from scale buildup.

His toilet's demand is causing a dramatic pressure drop, which is unseating your toilet's supply valve. It's running a little bit, and you're hearing that. It may be causing water to run out the tank overflow and into the bowl.

It may be exacerbated by difference in the toilet valves on the two units.

The bathroom faucets may not be on the same pipe, so maybe no help there.

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