Vent pipe related odor
Your toilet and washing machine standpipe probably empty into the same pipe.

There should be a p-trap at the bottom of your washing machine standpipe that always holds water, sealing it so that sewer gases cannot enter the building through the standpipe.

There should also be a vent pipe running upward from the washing machine waste pipe, just downstream from the standpipe's p-trap. It has to be in a position where the slope and distance down the pipe ensure an air gap (consult the plumbing code).

If there isn't a vent, there are a few things that could potentially happen.


One is that when water is flushed through the washing machine drain, the pressure could suction the water out of the p-trap, leaving it unsealed. Sewer gases would then enter the residence through the open pipe.
Another is that the sewer gases have higher pressure than the air in the house. So if the vent isn't there to allow the gas an easy path out of the house, it will bubble up through the water in the p-trap.
Another possibility is that the pressure from flushing the toilet is suctioning the water out of the p-trap at the bottom of the washing machine standpipe because there's no air break (the break--free flowing air--defeats the hydraulic pressure that could otherwise "suck" all the water out of the p-trap).


If there is a functional vent in the right place, the free flowing air through the vent will cause a break in the suction that might otherwise pull water out of the trap, and it also provides a low-resistance path for the higher pressure sewer gases to take, so that they don't bubble up through the trap into your dewlling.

It's also possible, assuming the plumbing is correctly vented, that the vent stack is plugged somewhere. Bird nest on the roof in the end of the pipe?

If it's a remodel, I suppose it's also possible that there is no p-trap on the washing machine standpipe at all, in which case sewer gas would flow freely into the laundry room.

It's also possible that you have a broken pipe, either the vent pipe or something downstream from the p-trap in the laundry room, or that you have a badly-done joint in the vent stack that is leaking gas into the room.