In message , NY
writes
Our bath/washbasin waste pipe emerges through the wall and then, via an
elbow, goes very slightly *uphill* for a run of a couple of feet,
before another elbow joint connects to a vertical pipe to ground level.
The highest point is probably an inch or so above the point where the
pipe emerges through the house wall, but as far as I can estimate, is
below the level of the bath plughole.

Is this likely to be responsible for water that is run out of the bath
ejecting the water from the washbasin U trap, or water from the
washbasin flowing into the bath and/or ejecting the bath's U trap wate,
with copious gurgling? And also causing very slow drainage of water
from the bath (several minutes to empty the bath).

Seems likely. The uphill bit has to be full allowing the flow to suck.


I can see that even if it's not the cause of the gurgling and back flow
into the bath, it's not a good idea to have water standing in the pipe
because it will (and does!) freeze in cold weather: several times I've
had to dismantle the pipework and pour boiling water down the section
that is normally horizontal/uphill, to encourage the plug of ice to
emerge.

--
Tim Lamb