If the sewer smell only happens when you have not used the washer for 3 weeks, it could be the trap drying out in the dry weather.
After hearing the input on vents, and seeing that there does appears to be a vent, or at least a roof penetration that looks like one in the adjacent laundry room, I'm thinking that adding what I guess is called a re-vent would be doable. Updated drawing below.
Existing drain in BLACK. New WC branch in ORANGE. Shower line is GREEN - that's one part I'm not sure about, since I can't see it, but I'm pretty sure it goes as shown. The bit between the old WC and the existing vent is a guess but I'd say I'm 90% sure.
The re vent would come off the vent stack in the ceiling at a straight tee, then travel across the ceiling void and drop through a partition wall next to the old toilet. It could join the drain there, using the old WC riser (stepped down obviously), I guess, but I'm reading elsewhere that I don't want the new WC riser more than 4 feet from the vent.
- Is vent sizing
an issue? i was thinking 1.5" just to make it easy to get through the partition wall, which is 1x3.
- Is the re-vent a good solution here
? Is rrunning the vent loop parallel to and at roughly the same height as the drain for a short distance kosher?
EDITED to correct 'loop vent' to re-vent.