Trap Primers – Are They Really Necessary?

Wikipedia defines Old Wives Tales as follows:“Old wives’ tale is an epithet used to indicate that a supposed truth is actually a superstition or something untrue, to be ridiculed. It can be said sometimes to be a type of urban legend, said to be passed down by older women to a younger generation. Such “tales” are considered superstition, folklore or unverified claims with exaggerated and/or inaccurate details.”

I have been told, so many times, that trap primers are not necessary for floor drains installed in public bathrooms. Supposedly, enough water from daily wet-mopping the floor will find its way into the floor drain to overcome evaporation and maintain the trap seal. If this were the case, why is it that I often visit a public restroom, only to be assaulted by sewer gas odor? I often see the building maintenance staff mopping the floors?? It becomes obvious to me that floor drain trap seals do evaporate readily, even when the floors are regularly mopped with a wet cleaning instrument.

I am quite accustomed to pouring a coffee pot urn of water into the restroom floor drain when I detect this unpleasant odor occurring. It works; the source of the odor is immediately eliminated. Most exhaust ventilation systems in public bathrooms are sized for 2 cfm/square foot. With a typical 8 foot ceiling, operation of an exhaust fan sized in this manner will result in the replacement of the smelly air in a restroom in about 4 minutes (15 air changes per hour). The smell may stick around for a few minutes, but once the source of smell is eliminated from the equation, the ventilated restroom returns to an acceptable environment.

Sanitary traps serve as the automatic barrier between the inlet of a plumbing fixture and the sewer pipe behind the wall, or below the floor. Usually that sewer line, which is part of a building drainage system, is a direct link to the city sewer. A functioning sanitary trap keeps those smells out of the building. It is a remarkably simple device. The “U” shaped trap fills with drainage water and acts as a boundary to the introduction of annoying sewer gas inside the building. Every plumbing fixture has one, either external to the fixture, or integral to it. When the fixture has sat dormant for a period of time and no drainage has occurred into it, then the trap tends to dry out. These phenomena can be prevented or mitigated with the use of a mechanical trap primer or more recently, with a trap maintenance device.

The 2012 International Plumbing Code includes the following language:

1002.4 Trap seals. Each fixture trap shall have a liquid seal of not less than 2 inches (51 mm) and not more than 4 inches (102 mm), or deeper for special designs relating to accessible fixtures. Where a trap seal is subject to loss by evaporation, a trap seal primer valve shall be installed (emphasis added).Trap seal primer valves shall connect to the trap at a point above the level of the trap seal. A trap seal primer valve shall conform to ASSE 1018 or ASSE 1044.

Every trap seal is subject to evaporation. I believe that only those floor drains or floor sinks that have a regular flow of drainage into them, such as a floor sink beneath a commercial ice maker, have a legitimate argument that an automatic trap primer is not necessary. The code requires these relatively inexpensive devices, and they can prevent the introduction of unwanted sewer gas. A sanitary trap does not need to completely dry out in order to release sewer gas into a confined space. I have witnessed rooms that get totally inundated with sewer gas just due to a screw being inadvertently inserted into a PVC sanitary vent line. It does not take a lot of free leakage area to mess up an otherwise clean smelling room.

I have also been told by experienced building engineers that trap primers are a nuisance, because they fail eventually. Once they fail, they have to be replaced. This mandates that the installed trap primers are accessible. They can be located above a ceiling or behind a wall, easily accessed by a simple hinged access panel. As long as the device is accessible, it can be maintained – just like everything else. The mechanical trap primer must be located properly. Most of these devices require a pressure differential in the water line that they are connected to. When the toilet or urinal is flushed in a public restroom, the sudden flow of cold water creates this pressure differential, and a relatively small spit of clean domestic water flows through the device. The trap primer is connected to the floor drain bowl via a piece of ½” diameter copper water tube. If the trap primer is installed in a location that will not have the necessary pressure differential on a regular basis, it will not function properly and consequently will not maintain the trap seal.

A few years ago, a company named ProSet developed a very clever little device called a TrapGuard. This contraption is made of plastic and silicon and it is designed to easily slip into the tail-piece immediately below a floor drain or floor sink drain outlet. It is basically a one-way device that allows drainage flow to pass from the drain inlet to the sewer pipe but does not allow sewer gas to flow out of the drain. I often describe these devices as looking like the inflatable birthday party noise-makers common in the days of my youth. When fluid enters the drain, gravity will allow flow through the unraveling silicon seal. When no drainage is occurring, the seal folds up, and no sewer gas can back-flow through the seal. Works great. There are multiple manufacturers that make similar products that are proven and tested to work as advertised. A standard has now been developed for testing these devices (ASSE 1072), allowing more competition and resulting in a degree of uniformity and satisfactory performance from devices manufactured by various parties.

I recently attended a public hearing regarding a school in a near-by town that was experiencing mysterious sewer gas. It came and went periodically within the school, but created an ongoing nuisance and alarmed many parents and school employees who thought that the sewer gas was potentially dangerous. High priced consultants and educators, as well as representatives from the State health department came and addressed the public after studying the school and its history of indoor air impurities. During the presentation, it was mentioned that this problem occurred after the completion of a major addition and remodel of the school. The school principal admitted that occasionally upon detection of the sewer gas, water was poured in all of the bathroom traps. This seemed to solve the problem – for a time. Then the sewer gas came back. A permanent solution was demanded.

Sadly, the IPC was the governing code when this school was built. For unknown reasons, the Authority Having Jurisdiction (building department) did not require the use of trap primers – despite a requirement within the governing plumbing code that these be installed. It makes me wonder – did someone convince the building official that the floor drain traps would stay primed by daily wet-mopping?

My conclusion: DO NOT BELIEVE THE OLD WIVE’S TALE; the drains will not stay primed from daily mopping.