tonyjeffs wrote:
I'm going to fit a wc below the stairs on the ground floor of our
semi, and attach to the same soil stack as the existing FF wc.
Is there likely to be a problem with syphonage - such that when I
flush the upstairs toilet, the ground floor toilet will gurgle?

Provided that a) the distance to the stack from your new toilet doesn't
exceed 6m (or less if there are a number of bends), or b) there is a
vent on the existing stack (which there should be anyway for a toilet
above the ground floor), then you shouldn't have a problem. Even if you
do, you could stick a 'Durgo' valve on the new branch.
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