During my years in the construction and home inspection industries, I have found that most people do not understand the purpose of the P trap.

I have heard comments that that is a place to keep large stuff from going down the drains, the place where the pipes leak, poor design causing clogs in pipes, or a way to accelerate the flow of the waste water.

The purpose of the P trap is to create a barrier to prevent sewer gasses from rising out of the plumbing fixtures. With the thermal (infrared) image above, you can see the cold water that is sitting at the base of the P trap once the waste water from this sink has stopped flowing (sink turned off). The sewer gasses cannot pass thru this water barrier and travel up thru the sink drain.

Drains that are intended for emergency overflows, such as a floor drain, may have a P trap primer. The domestic water pipe will drip very slowly into the drain to keep water in the P trap below as the water may evaporate over time.

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