My hall bathroom toilet flushes when we flush the one in the master bedroom. I've replaced the flapper because the water was leaking into the boil (dye test). But that hasn't stopped the sympathetic flush. What else could this be?

partial stoppage somewhere that may also be blocking a vent?

How long has this been happening?

Best Wishes

Had the same thing happen in a rental years ago...

Blocked Vent was the cause...

It is a blocked vent, poor piping installation, or some other condition we would have to be there to diagnose.

Thanks, all. We had the entire house repiped a few years ago. Toilet is an Elger, which seems hyper sensitive. We usually replace the flapper every couple of years. This has been happening now for about a month. Prior to that, it happened a couple years ago and relacing the flapper seemed to fix the issue. If I get on the roof and flush water down the vent with a hose, will that help dislodge if there's an obstruction?



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You mention vent in the singular with two toilets depending on how they are installed you may need two vents. If I recall code correctly for a single vent they must be just about be back to back installs....otherwise two vents & therefore your problem.