I am smelling sewage gas from my ac vents... mostly the return vent which is at the ceiling in a small hallway... the smell concentrates in that hallway.. there is a half bath in that hall as well... the smell is not so much in the bathroom... i also will get the smell on the other side of the house near a bedroom .. also near a bathroom... i thought the p trap with the ac was dry...however i bugged by builder and he cut the pipe and it is full... the house is only 6 months old.. i dont know if this is related but off that same hallway with is my laudry room and when i run water into my washing machine,,,i get a horrible rotten egg smell that goes away after the water runs for a bit... we are on city water could it be related
i do not get that smell from any other water source but that one.

I had the same problem, seems that the exit from the washer was such that it forced all the water to suck out of the TRAP that normally holds some water to cap off the sewer gas.

Mine started when I thought I would improve the loose fitting rubber hooked looking hose and made a nice cap to the drain! BAD IDEA the water would then siphon all out and leave an open sewer in my room. Your trap may not be deep enough, as it implies it TRAPS the gases from escaping every drain has one look at your sinks, they all hold a bit of water to cap the gases, even the toilets don't drain that standing water is a built in trap. If you slowly pour some water in it and the smell goes away you have at least narrowed it down.

Does any of this make any cents $$
Frank