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I'm Trapped!

Updated: Mar 17, 2023

Have you ever heard of a trap primer or a trap seal? Well, if you’ve been in a bathroom or kitchen and smelled the sweet smells of sewer...then chances are you have at least smelled a non-existent or non-functioning trap primer or trap seal! The p-trap in a drain keeps the sewer gasses out by “trapping” them with a rim of the water. In a floor drain, the water can evaporate from the p-trap if not adequately cared for, thus leading to the gas smell you may have experienced once or twice in your life.

When choosing between a trap primer or a trap seal, consider three major factors: water pressure, types of fixtures, the amount of use of each fixture, and the cost of installation.

Water pressure is the basis for how the trap primer works. There usually needs to be a line pressure of between 35-80 psi with a negative change in pressure of about three to ten psi for the fixture to function correctly. If the trap primer is too far from the main, or if the main is undersized, or if a booster pump isn’t working correctly, then the trap primer may fail to function as intended.

The type of fixture will also affect the operation of the trap primer. A water closet with a 0.5-gallon per flush flow will not produce enough flow to cause activation of the primer. On the other hand, a flush valve will create a substantial drop in pressure and thus would make excellent placement for the trap primer.

The next thing to consider is how much use the fixture will get. There is a minimal amount of water that is released when the trap primer is activated. The trap primer is activated when the adjacent fixture is used. Infrequently used fixtures will not activate the trap primer enough to keep the water in the trap.

Another factor is the expense of installation. The piping extends between the trap primer to the floor drain, which means running down the wall from the primer, under the floor, and then finally to the floor drain.

For new work, this is relatively simple. Remodeling is a different story, which brings us to the trap seal. This item hasn’t been around long, but the popularity is increasing.

The trap seal is a device with a one-way valve that allows water to flow down the drain but forms a seal that stops air from escaping up from the drain. A trap seal is much easier to install and maintain, and there is no worry about fixture use, fixture type, or water pressure.

With the problems when using and installing a trap primer, trap seals are the clear winner on all fronts. Check with your local jurisdiction before using trap seals, as each locality adopts different codes. If your authority has adopted IPC 2015 or later, then trap seals are permitted and legal.

If you have any questions about this topic or other MEP related concerns for your project, please feel free to reach out to us at If you’d like to get an idea of pricing for your drawing package, complete this form to get a quote in less than 10 seconds!

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