Because nobody is running water. Trap seals evaporate when a suite sits empty, and floor drains and restroom fixtures are the first to dry out. Once the water seal is gone, sewer gas rises straight in. Refill every trap, and install trap primers on drains that dry out repeatedly.
What is happening
Each fixture and floor drain has a P-trap, a bend that holds a water seal. That seal blocks gas from the sewer line. The 2024 International Plumbing Code, as adopted in Phoenix, Section 1002.4, sets the seal at not less than 2 inches and not more than 4 inches. Section 1002.4.1 adds that seals subject to evaporation, like a floor drain, must be protected, for example by a trap seal primer. In Phoenix, with hot dry air, a seal that is never topped off can disappear.
The fix, step by step
- 1Walk the whole suite and find every drain: restrooms, sinks, mop sinks, floor drains, condensate drains and any janitor closet.
- 2Pour water into each one until it stands full. About a quart works for most traps.
- 3Air the space out and check again the next day.
- 4If the smell returns within days, the seal is not holding. That points to a leak, a blocked vent or a trap that was altered, not just evaporation.
Prevent it while the suite is empty
- Add trap water to your vacancy checklist, on a schedule you can keep.
- Put trap primers on floor drains that repeatedly dry out.
- If you close the water to the suite, the traps still need a fill, since the shutoff stops the water that would top them off.
- Keep the fan or ventilation running if the space has been closed up.
When water does not solve it
If the traps are full and the smell stays, the cause is usually one of three things: a broken or blocked vent, a cracked drain line under the slab, or a fixture that was removed and left with an open pipe. Any of these need a plumber. See how to trace a sewer smell in an office restroom for the method.
A quick safety point
Hydrogen sulfide is what you smell. ATSDR notes people notice it at very low levels, and that smell fades at higher levels because you adapt. If the smell is strong, or someone reports headaches or dizziness, leave the affected area first and arrange appropriate help. HQ offers 24/7 emergency service, is licensed and insured, and gives free estimates.
Hotels and lodging have the same problem in guest rooms and back-of-house areas. See hotel sewer smell in rooms and floor drains.