Why do commercial floor drains need a trap primer?

The water seal is the only barrier between the room and the sewer. When it evaporates, gas from the drain line can come straight up through the floor drain. That gas is mostly a nuisance smell, but sewers can also contain hydrogen sulfide. OSHA describes it as a colorless gas with a "rotten egg" odor at low concentrations, with effects that range from headaches and eye irritation to unconsciousness and death at higher levels. It also warns that after a while, or quickly at high concentrations, you can no longer smell it. So a smell that comes and goes is not a safe way to judge the air.

A dry trap is a common cause of the sewer smell in a building with no other plumbing problem. Our page on sewer gas smell in a commercial building walks through the other causes.

Which section applies depends on the city and the code it adopted. Most Valley cities build on the International Plumbing Code (IPC): Phoenix, Mesa, Chandler, Glendale, Surprise and Tempe use the 2024 edition, Scottsdale and Queen Creek use 2021, and Gilbert and Maricopa County use 2018. Phoenix also adopted the 2024 Uniform Plumbing Code (UPC) as a second path. The trap seal rules sit in IPC Section 1002.4 and in UPC Section 1007.

The IPC-style text says trap seals of emergency floor drain traps and trap seals subject to evaporation must be protected by one of a short list of methods. The list, as it appears in the model code, is: Emergency floor drains often sit next to eyewash stations and safety showers; see what plumbing an emergency eyewash station needs.

• A potable water-supplied primer valve, which meets ASSE 1018 and discharges to the trap above the seal on the inlet side.
• A reclaimed or graywater-supplied primer valve, also ASSE 1018, using water that meets the valve maker's quality requirements.
• A wastewater-supplied primer device, which meets ASSE 1044.
• A barrier-type device in the drain, which meets ASSE 1072 and is installed following the manufacturer's instructions.

The UPC version is narrower. Hawaii's UPC-based code, shown on UpCodes, lists potable-supplied primer valves under ASSE 1018 and drainage and electronic primer devices under ASSE 1044. The ASSE 1072 barrier device appears in Hawaii's state amendment, which describes it as an added option. The UpCodes pages we cite show Hawaii and Texas copies of the model text, not the Valley cities' adopted books, so treat them as an illustration of the language and check the section your city adopted.

The Phoenix UPC and IPC amendment documents and Chandler's 2024 amendments, all on the sources list, do not rewrite the primer sections. Which drains in your building need protection is decided by the code your city has adopted and the plan reviewer, so check the drawings rather than assume. Wikipedia notes that codes usually require primers in the spots where drains are known to dry out, such as drains that only see water in a flood.

There are three common approaches:

1. 1A water-supplied primer valve. It taps a potable water line and opens for a moment when it senses a pressure drop, for example when a nearby toilet flushes. It needs a supply line to the drain, so it is best planned at rough-in.
2. 2A drainage type primer. It borrows a little water from a nearby sink or fixture drain and sends it to the floor drain. No extra water supply is needed, but the source fixture has to be used.
3. 3An electronic primer. A timer or sensor opens a valve on a schedule. It works where no fixture use can be counted on, and it can be set for a very seldom used drain.

A barrier type device is different. It is a valve or float inside the drain that closes when the water is gone, so gas cannot pass.

If a drain has no primer, the stopgap is to add water on a schedule. Pouring water down each seldom used floor drain now and then puts the seal back. It is a habit, though, not a system, and it fails when someone forgets. If a drain in your building keeps going dry, ask a plumber to add a primer or a barrier device, which may also need a permit. See when a commercial plumbing repair needs a permit in Phoenix.

Also check the floor drains that see water only in an emergency: mechanical rooms, boiler rooms, water heater closets and elevator pits. These are the first to dry out, and the smell may not be traced to them right away. Our guide to floor sinks, floor drains and mop sinks explains where each is used.

### How deep should the trap seal be?

The model code sets the seal depth. UpCodes' copy of the trap-seal section says each fixture trap must have a liquid seal of not less than 2 inches and not more than 4 inches. That water is all a primer has to keep in place. Because the seal is shallow and open to the air, it can dry out in a seldom-used drain, which is why a drain that "has always been fine" can start smelling later.

### Where does the primer feed the trap?

The model text puts the primer's discharge in a specific spot. In the copy we checked, the pipe from a water-supplied or wastewater-supplied primer connects to the trap above the trap seal, on the inlet side of the trap. That detail matters on a remodel. A primer that feeds the wrong side, or a loose line that no longer reaches the trap, leaves the seal to evaporate while the equipment looks installed. Water-supplied primer valves must meet ASSE 1018, and wastewater-supplied devices must meet ASSE 1044. The same section lets a reclaimed or graywater supply feed a primer valve, subject to the valve maker's water-quality requirements.

Wikipedia adds a practical point: most designs need an accessible spot where the primer can get periodic maintenance. A valve buried above a hard ceiling with no access panel is a repair waiting to happen.

### Is a barrier device an option instead of a primer?

It can be. A barrier-type device, made to ASSE 1072, protects a floor drain trap seal from evaporation, and the model text says it must be installed following the manufacturer's instructions. Hawaii's amendment shows why the choice is written as an option: it says it gives code users "the option to utilize an alternative method or design which the user considers to be beneficial to their system requirements." Because the choice is up to the design and the local reviewer, ask before you swap a primer for a barrier device on a drawing.

Hawaii's sustainability appendix also says drainage-type primer devices shall not be limited in the amount of water they discharge. That is one state's rule, but it reflects a real design question: a drainage primer that moves only a trickle can leave a large drain dry.

### What about the drains in your Phoenix-area building?

The Phoenix and Chandler amendment documents don't rewrite these primer sections, so the adopted model code language is what the reviewer works from. Which drains need protection is decided per project. If you're planning a build-out, mark every floor drain on the plans and ask the reviewer which need a primer or a barrier device. For an existing building with a recurring smell, a plumber can trace it to the drain, check the seal and tell you what would keep it wet.

### What should you check on a maintenance round?

• Look at every seldom-used floor drain and pour in water to see whether the trap holds it.
• Find any primer valve and confirm it is still connected and accessible.
• Note drains under equipment that gets serviced only a few times a year, such as boilers and cooling equipment.
• Write down which drains you refilled and when, so the next person knows the schedule.

HQ is licensed and insured and gives free estimates and upfront pricing for commercial floor drain and trap work.

Vacant hotel rooms and empty offices are a common place to find dry traps; see why they smell like sewer after sitting unused.