A little further research makes me think a Studor Mini Vent, or similar contraption may fix the issue. It is an Air Admittance Valve. I believe our plumber used one of these on the guest bath, as it does not vent thru the roof, just into the attic with one of these.

"I believe the problem is that the vent is not tall enough to "draw" properly."

Unlike chimneys, plumbing vent to not "draw"

making a vent longer than allowed fr the pipe size produces the exact opposite problem, the vent fails to operate properly.

I would look for a dried out trap before anything else.

This is a very common cause of sewer gas getting into houses

Not sure if the trap could be dry or not. My wife uses the washer every 2-3 days. There is also a sink right next to the washer that we use every day, but I don't know if they are plumbed together that way.

The funny thing is we only notice it on cold days. Mornings are worse than afternoons. That's why the placement of the vent made me think it was cold air traveling down the two steep roofs this vent sits between.

At any rate, I'm intrigued by the idea of the AA valve, but not ready to go cutting the pipe yet.

Are there any floor drains?

Make sure you check under the washer (and even the dryer).

Nope, no floor drains. Its at it again this morning, worse when doing laundry. Had no trouble yesterday afternoon doing laundry. Was pretty warm then.

The fact that this occurs on cold mornings makes me think that heating your home might have something to do with it? Do you have forced air heat? If so is there a re-circulating vent in the laundry area? This might cause enough negative pressure to suck air back up thru a trap causing it to burp gas, or possibly thru your washing machine?

Have you made sure the vent isn't plugged? It needs to allow air into the drain system to work properly, and if plugged then when you run water down the drain, the suction formed can pull the water through the trap. Normally the weight of the water above the trap is what causes it to flow. The vent on the other side of the trap (through the roof) keeps a suction from forming.

Not to sound crazy, but are you sure there is a trap on the drain line? A clogged vent will cause drainage issues, not smell getting in.

The only way to get sewer smell into the room is from a drain line with no trap or through a dried out trap (aside from serious negative pressure that effectively sucks the water out of a trap). Have you ever moved the washer to see if here is a drain under it? They used to put them right into the tile on the floor, right under the machine.

as a footnote to my post above.-- another source of negative pressure would be your dryer blowing air out of the room. Do you do laundry in the mornings when the smell is evident?

Its been awhile, but I think I have the problem fixed. First, the floor drain issue was not involved. A carpenter friend of mine and I built the house. There is absolutely no drain in the area that could have been dry.

I went ahead and put in the Studor Mini-vent with some reservations, and the odor is gone. The vent was clear, not blocked in any manner. I can't say exactly what was happening, unless it was my initial theory. I can say it is not happening now.

For me, that's all that matters. Thanks for all the opinions, it sure helps in one's decisions.

Glad you got it fixed. I've seen problems similar to your's that were do to the vent gasses released from the vent stack being drawn back into a laundry room do to outside air infiltration. If this is the first or one of the first vent stacks upstream of the septic tank the odor can escape from there and flow down the roof and into the room. Your solution would keep it capped off so that only air is admitted into the vent and no odor escapes. Not sure it that was your case but wanted to post in case others had a similar issue.