Head scratcher.

Here’s the setup. My house is on pilings, with living space starting 10’ above the street, garage at street level, no slop sink. On that living level I have a rarely used outdoor shower, plumbed into the city sewer. I have a powder room on that floor, along with family room, kitchen w/disposer, and dining room in open concept design. On the secondary floor of living space I have two full baths one attached to the primary bedroom which is used daily, the second is a guest bathroom used roughly three times a week and the rarely used shower cleaned biweekly.

The issue: when we run the washer on delicates cycle the powder room toilet bubbles and gurgles. A full load on normal cycle this doesn’t happen. Two years ago the town cleaned the sanitary sewer system and the toilet splashed water everywhere and I could smell the cleaning agent.

Do I need to keep the trap in the outside shower full to stop this madness? Please help my plumber heroes. TIA!