How do I stop the subterranean vault toilet from bubbling or spraying when the upstairs toilet flushes?
Answers: check your vent, it may be clogged.
You should suspect a partial blockage downstream of the basement toilet. Water coming down should slip away right by the basement fixtures, but is building up a pressure, which cause the bubbling. The problem is not in the underground room toilet discharge, but further towards the sewer. Drano or the equivalent ight clear the blockage, but I'll bet it will take a snake.
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