Here is the corrected link:
www.ihateticks.com/smell.html

Here is a link that might be useful: link to pics

I smell a rat.

You know what I think is, that you will find a floor drain dried out. It is allowing sewer gas to fill the room. And It may actually have a small animal decaying in it. First, look for any such drain and pour water into it to refill the trap. Then, please reply.

Pinoke

I don't believe the problem is associated with your drain, waste & vent system.

As you pointed out in the photo's the top of the standpipe is 36"+/- above the trap. The IRC code requirement is that the top of the standpipe must be not less than 18" nor more than 42" above the trap. UPC code is not less than 18" nor more than 30" above the trap so it would appear that in either case the standpipe height is basically correct.

The trap is connected to a 1/4 bend before going into the wall so it obviously has the code required minimum horizontal distance of 2 x the pipe diameter from trap weir to vent.

This drain is back to back with the 1/2 bath and most likely shares a common drain line. There is no mention of odor or trap problems in the 1/2 bath so we must assume that the line is venting correctly.

You stated that you can see a vent on the roof which no doubt is the vent for these fixtures. You mistakenly assume it to be a vent for the toilet but in fact under the IRC a toilet is the only fixture in the house that technically does not require a vent.

I have measured! Top of trap to top of waste pipe is exactly 30" Should I shorten it??

I went into the attic.... There are 2 pipes coming out of the attic and through the roof. One is the radon-mitigation system, the other is a vent stack (see photo) from the approximate location of the (non-stinky) 1/2 bath toilet and sink.

In the basement, I have a picture of the washer waste pipe, connecting into the 1/2 bath plumbing stack. The pipe travels several feet horizontally before hooking into the waste stack. ("A" in this picture is a few feet below "A" in the other picture. )

And about the basement drain... the AC and the gas furnace and the humidifier all drain into it... It is not stinky!

Here is a link that might be useful: more pictures

Based upon the general layout of this line it appears that your jurisdiction is primarily under the International Residential Code.

The height of the standpipe is fine.

By necessity there is always a horizontal section of pipe between the vent and the trap. Under the IRC the minimum lenght of that horizontal section is 2x the pipe diameter and the maximum length is determined by dividing the pipe diameter by the required pitch. All pipes 3" or under are required to have a 1/2" per foot pitch so if this is a 2" line the maximum developed length of horizontal pipe from the vent to the trap may not exceed 2 / .25 = 8'.

The horizonatal section of pipe connected to the stack is running parallel to a concrete block wall. A standard concrete block is 16" long and the pipe appears to be equal to the lenght of 4 blocks so I am estimating the length at 4 x 16" = 64" plus another 12" for the offset to the riser. I am assuming the top of the riser is directly behind the quarter bend protruding from the wall at the standpipe location. Allowing an additional 4" for each 1/4 bend we get 64"+12+4"+4" = 84". 84" /12" per foot = 7' so the length of the run is within the allowable 8' however there are two serious problems with this installation.

1. The stack to which the standpipe line is connected is black pipe plastic pipe. We would need to see the writing on that pipe to be sure because they did at one time make black PVC pipe but normally black DWV(Drain, Waste & Vent) pipe is ABS plastic and code prohibits directly gluing dissimilar pipe. (ABS to PVC or vice versa). While that would be considered a code violation, in this case it would have no effect on the problem at hand so for the moment lets disregard that problem.

2. The second problem is that there is a toilet discharging into the stack at some point above where this connection is made. This means the vertical stack is a wet vent and code prohibits connecting a fixture drain to a wet vent downstream of a toilet.

There is an easy solution:

The IRC will permit an unlimited use of Air Admitance Valves (commonly called Studor Vents). On the horizontal pipe in the basement cut the pipe on the offset or near the offset end of the horizontal from the stack. Install a Wye & 1/8 bend or a COMBO with the side opening point upward. Stub a short riser vertical about 12" and put an 1-1/2" female thread adapter on the top, then screw an AAV into the thread adapter. This would then make the connection legal because the pipe from the trap to the AAV location would be considered the fixture drain but the line downstream of the AAV to the stack is now a vented branch line, which is legal to connect to the stack. Not only would this make the installation legal but it would also provide the additional venting required to resolve your problem.

Well, it was a DUH moment ... we had a very competent contractor come by to quote us on a Studor valve and also a major plumbing (shower)addition in the basement. He quoted us on the Studor valve, and then pointed out an open 2" pipe above a trap, going into the main drain stack for the house. I had not noticed this before, even when I took photos, because it was black pipe and very far behind the (very big) furnace. Also, I just was not expecting a trap there! The laundry room is just above the furnace room.

It looks like every time the washer pushed vent water into the waste stack, some stick air would come up from the dry trap.

Well, I squeezed behind the furnace, poured in a gallon of water (so the trap would be filled) and placed an end cap on the open pipe. I even had the end cap in stock, from a different project. I hung a bag of zeolite odor absorber in the furnace room, and now the problem is fixed!

Thank you all for your help...

If the trap is not going to be used for anything you can float a layer of mineral oil (about 1/8 inch) on the water to slow evaporation.
If it is realy inconvenient you can just fill the trap with mineral oil.

Klaire, We have the maytag washer and the smell is a problem with the washer. We had a problem with our washer and when they repaired that problem the repair person said that there is a part replacement witch lets all of the water drain out of the washer.

Doug