What is a Plumbing Trap and what is it Used for?

  • For this article I will be using the ‘P’ trap, but ‘S’ and ‘U’ traps are used widely as well.

A plumbing trap is a simple but ingenius contraption used essentially to block the less-than-pleasant smell from drifting up various drain-holes located around your house. If you take the letter P and rotate it 90 degrees clockwise and remove the line segment between the U shape, you’ll get the general shape of the P trap. Now imagine piping extended from each end of the P, with the one pointing up going to a given sink drain, and the horizontal one leading through the wall and into the sewer.

Ugh, did I say the sewer? That’s right – the rat and germ-infested but absolutely necessary sewer! In most developed nations, all household waste – whether it’s laundry and/or kitchen waste or toilet waste – ends up going down the same pipes. It all Illustration of a P trapleads to sewer-treatment plants where it is then processed for eventual release back into the environment and/or fertilizer. Some countries even process it back into drinking water!

All that to say, unless you’re (illegally) draining your waste into the nearby river, your sinks, shower drains, and toilets are most-likely all attached to the same mother-pipe, also known as the city sewer. This means that if something isn’t blocking the smell from wafting up say, your face-washing sink, chances are you’d be at serious risk of passing out the next time you decide to lean over your sink and wash your face.

This is where what is called the P trap comes into the picture to save the day. As you run water down your sink, the water will first accumulate in the P trap before overflowing and continuing down the pipe. Now even when you stop the water, what has accumulated in the trap will stay “trapped” and just sit there obediently. This trapped water acts as the barrier between your face and the horrendous smells that would otherwise engulf you.

Complications Involving traps when Connected to Flush Toilets

You should know that if your trap is connected by way of piping to a toilet that flushes, the flush will suck the trapped water out of the P trap. The flushed water plummeting down the pipes creates a difference in pressure between the upper and Illustration of drain waste ventlower ends of the P trap – essentially a siphon – thereby causing the trapped water to be pulled down along with it.

This particular sink will now be defenseless until someone runs the sink water again, refilling the trap. The solution to this is to install what is called a drain-waste vent (DWV). These vents are simply piping that’s connected to the outside ensuring neutral air pressure inside the piping at all times. They should be installed adjacent but connected to your toilet piping, and shouldn’t have any traps located between it and the toilet.

Now when you flush, the DWV will introduce air as needed to maintain neutral air pressure, protecting the precious water in the traps. One last thing you should keep in mind is that the trap of any given fixture such as a sink will dry out given enough time. Generally speaking, a month of not being used will result in the evaporation of water in the trap to the point of trap failure. Simply run water periodically to ensure “healthy” traps.


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