The water seal in a drain trap slowly evaporates when a fixture goes unused, and that seal is what blocks sewer gas. Once it dries, odor rises into the room. Running water in each drain restores it. If the smell stays after refilling, the cause may be a damaged trap, a blocked vent or a leak.
Why unused rooms smell
Every sink, shower, tub and floor drain has a trap, a curved section of pipe that holds water. The South Davis Sewer District in Utah explains that this water acts as a barrier "preventing sewer gases from traveling back into your home or business." Then it says what happens when a fixture sits idle: "When a drain is not used often, the water in the P-trap can slowly evaporate." Once the trap dries out, sewer gases can escape into indoor spaces. The district lists a building "vacant for an extended period" among the causes, and notes that hot or dry weather makes evaporation faster.
A hotel is a natural set-up for this. Rooms that stay empty leave a bathroom sink, shower and floor drain sitting dry. Empty offices have the same trouble with restroom fixtures, janitor sinks and mechanical room drains. For the basics, see what a P-trap is.
The quick fix
The sewer district recommends these steps for seldom-used drains:
- Run water in all sinks, showers and floor drains at least once a month.
- Pour a cup or two of water into seldom-used drains to refill the trap.
- Where a drain is rarely used, a layer of mineral oil on top of the water slows evaporation. The district prefers mineral oil over cooking oil because it does not go rancid.
For a hotel or an office, that means adding a monthly walk-through to your checklist for every room with a plumbing fixture, including the ones nobody uses.
What the code says
The International Plumbing Code (IPC) calls for a liquid seal of at least 2 inches and not more than 4 inches in each fixture trap, with allowance for deeper seals on certain accessible fixtures, according to ASPE's summary of the 2024 edition. For emergency floor drains and traps subject to evaporation, ASPE says the code requires one of several approved protection methods. They include:
- A potable water-supplied trap seal primer valve conforming to ASSE 1018.
- A wastewater-supplied trap primer device conforming to ASSE 1044.
- A barrier-type trap seal protection device conforming to ASSE 1072.
- A drain from a lavatory or hand sink in the same room that keeps the floor drain wet.
Manufacturers describe the same idea. Jay R. Smith says its automatic primer "refills drain traps that open due to evaporation of the water seal from infrequent use of the drain or fixture." Precision Plumbing Products says its barrier device is "designed to open and allow gray water flow to a floor drain and then remain closed in the absence of flow," and recommends it be used with a mechanical trap seal priming device. Your local jurisdiction decides which code edition and amendments apply. For more on primers, see why commercial floor drains need a trap primer.
When the smell doesn't go away
If odors persist after refilling drains with water, the district says there may be another issue, such as a damaged trap, a blocked vent or a plumbing leak, and that a licensed plumber can help identify and resolve the problem. Read why a commercial building smells like sewer gas for other causes, and what a plumbing vent does. Strong odors, headaches or dizziness call for leaving the area first; see the first signs of sewer gas poisoning.
HQ finds the source of a persistent odor and can add primers to drains that keep drying out. We are licensed and insured, and you get a free estimate and upfront pricing.