I have recently completed a rebuild of three 1940's duplexes - total of six 2 bedroom, 1 bath units. Four of the six unit's bathroom sinks are emitting a foul sewer gas smell. All the plumbing in the home is completely new and was inspected by the city before the walls were closed up. Vent pipes were of course installed. Immediately after the tenants moved in, two complained of the smell. After a couple months, two other tenants complained of the smell but they all said the problem was there since the day they moved in. The remaining two tenants either don't have the problem or are not sensitive enough to notice the smell.

The vent pipes are clear and the smell can't be coming from rotting grease, hair or whatever else could be in the p-trap because the smell was reported before anyone had even used the brand new plumbing. The pipes are ABS and are brand new all the way up to the main sewer line (all original plumbing), which starts off about 6" above ground that runs 12" - 36" under the ground to tie into the city lateral main.

Vents can't be clogged because these are also brand new.

The plumber that did the work has a very good track record and runs a class-A outfit. He didn't have an answer to the problem. I thought that perhaps the p-trap doesn't have a long enough "U" portion? So that when the water turns on, it allows air to pass back up into the sink and into the bathroom? The smell only occurs when the water is turned on and when it runs. There is no smell once the water is turned off.

I am also using a unique modern sink - square in design and the base is quite flat. So it does not drain out quickly. Instead of flowing quickly towards the drain like most water does in traditional round bowls, the water settles slowly towards the drain because the base of the square sink does not slope steeply toward the drain. But I don't know why this would cause the smell to back up.

Also, what's interesting is that sometimes the water fills up the sink when there is no obstruction in the drain at all. The sink won't fill up all the way - but to about 50%, then all of a sudden the water rushes down the drain (nearly draining the sink in 5 seconds). Once 90% of the sink has been drained, it slowly begins to "back-up" again, until it reaches 50% capacity... just to rush out again. This cycle will repeat itself. This "cycle" does not happen all the time but it does seem to happen on all the sinks at least once in a while.

I appreciate any insight anyone might have. I'm totally stumped.