Usually dry traps: guest bathroom floor drains and tubs in rooms that sit unbooked, plus back-of-house floor drains in laundry, kitchens and mechanical rooms. Other sources are blocked vents, failing toilet seals and a shared stack. Refill the traps first, then trace the rest.
Guest rooms
Each bathroom has a sink trap, a tub or shower trap, a toilet trap and sometimes a floor drain. In an unbooked room none of them is refilled. The 2024 International Plumbing Code, as adopted in Phoenix, Section 1002.4, requires a 2 to 4 inch seal, and Section 1002.4.1 requires protection for seals subject to evaporation. Housekeeping can help: run each fixture for a minute when a room is checked after a long vacancy.
Back-of-house
The higher-risk spots are the floor drains in the laundry, the kitchen, the pool equipment room, the boiler or mechanical room, and the loading area. Floor drains that catch condensate or overflow only get water occasionally. These are the drains that benefit most from a trap seal primer, which tops the seal off automatically. See floor drain vs floor sink vs mop sink if you are unsure which fixture you have.
Beyond traps
If the water seals are full, look at:
- Roof vents that are blocked or damaged, especially after monsoon weather.
- Toilet seals in a specific room.
- A shared drain stack that serves rooms in a vertical line, if several rooms on top of each other complain.
- The kitchen grease system. See why a restaurant smells like sewer.
The legal angle is different
Arizona's residential landlord-tenant act excludes transient occupancy in a hotel, motel or recreational lodging (A.R.S. 33-1308). For a transient guest, a complaint is a guest-service and operations issue, not one handled through that statute's repair remedies. Longer-term residents of a hotel or motel are not automatically outside the act, so check the arrangement. Your brand standards and insurer will care more than the statute does.
Safety and next steps
ATSDR notes hydrogen sulfide is noticed at very low levels, and that smell can fade as you adapt. Do not put a guest in a room with a strong odor. Move them, ventilate, and have a plumber trace it. HQ offers 24/7 emergency service, is licensed and insured, and provides free estimates and upfront pricing.
For a related odor investigation, see why does only one apartment unit smell like sewer.