27 Oct 2005
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United Kingdom
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 approximately a metre from the bath?
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It is not likely to syphon if you have a bath overflow, because this acts as a vent.
And it won`t syphon at all, or allow back flow of smells.if you fit a Hep2-o waste valve to the basin ;)
Nice one NF! I haven't ever tried one of those - are they good, and do you find many cases where they're justified?
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They're expensive and you have to find somewhere to put them though. Why not just use the normal solution, a trap on the basin with a built in AIR ADMITTANCE VALVE - see definitions.
Granted, they`re expensive but the same size as a straight through trap. I`ve used the mini a.a.v`s too, Both things I have fitted as a prophylactic ;) measure....that and swept bends are my favourite specs. for waste pipes. I personaly think Hep2o`s valves are a brilliant to fit in place of basin trap requires 2x135 degree bends to alter pipe centre line.........OK you can`t use one if the pipe goes thru the wall horiz. from trap. But you can use them as a running trap.

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