Hi, I need to divert a 32 mm waste pipe (from a basin) into a 40mm waste pipe (from a bath), but I am worried that when the bath is emptied that the water in the basin trap might be siphoned out and that I will get foul air traveling up the 32mm pipe.

Is this likely, I would be Tee(ing) the 32mm pipe into the 40mm pipe approximatley a metre from the bath?

In article ,
andymason79 writes:

Hi, I need to divert a 32 mm waste pipe (from a basin) into a 40mm waste
pipe (from a bath), but I am worried that when the bath is emptied that
the water in the basin trap might be siphoned out and that I will get
foul air traveling up the 32mm pipe.

Is this likely, I would be Tee(ing) the 32mm pipe into the 40mm pipe
approximatley a metre from the bath?

Bath traps are often shallower because of lack of space,
and it is thus more likely you will empty the bath trap
first. If that doesn't already happen, then adding a basin
waste isn't likely to make it do so. You can get basin
traps with built-in air admittance valves for only a
little bit extra money if you are worried, and that will
also stop the glug-glug noises. I used one in the case of
a condensing boiler condensate drain tee'd into the same
pipework as a basin so the condensing boiler's U-trap
couldn't get sucked dry after draining a basin full of
water.

--
Andrew Gabriel