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the bath shouldnt just drain if the saniflo is switched off. the water should just stay in the bath...
Water wants to find its level. Assuming it is all connected to the same waste then the level of the water in the bath will be quite a bit higher than the level of the water in the Saniflo and the toilet.
But that would assume that all of the waste (including the bath) goes through the Saniflo. No sane setup would have that surely...
This sounds like bollox.
Saniflo or not, how does water escape from a bath, without the tap being left on and unattended?
If the Saniflo wasn't working, they'd pull the bath plug and the water wouldn't drain, which shouldn't be a huge drama unless they tried to fix it