I have solid plumbing and building knowledge, but a problem I can’t grasp. Hoping for direction from the experts. Builder and plumber have been Zero Help. Pre-engineered home 2018 build Single story Flat roof Crawl space Single bathroom Laundry I began experiencing sewer gas smell from the bathroom sink. My nose directly to the sink confirmed it’s coming from there. No smell from shower , tub, toilet , kitchen sink or laundry. Investigating myself a found an “s” trap installed In the crawl space connected to the bathroom sink. I assumed siphoning was occurring causing a dry trap and the smell. Told the plumber what I found. He changed the s to the correct p trap. It was bone dry. Smell went away for a while. smell came back. I started to track the factors I think could be related. . Cold weather. Dryer running. After I shower Or do a load of washing laundry. running water into the sink stops the smell but only until I stop running the water. Sorry for the long post. I wanted as much detail included. I’m reading about super air tight homes and dryers drawing fresh air. Not sure if I’m on the right path. HELP !! thanks for your time and expertise.