I once had a 12 year old ask me why kitchen sinks have P traps. This young man, had worked with his father under the sink to help fix a slow leak but never got an explanation from his dad. I asked him why he thought sinks needed P traps and he responded, “I’m not sure why, people aren’t supposed to pee in the kitchen sink.”
I laughed and laughed when I realized he thought P traps were installed to collect urine. He was right though in one respect. It IS a trap and it serves two important functions in drains. But the reason for its name comes from its shape, a sideways P.
For any drain that leads to a sewage line there is a major problem – STENCH. Stank. Odor, musk, reek, whiff, whatever you want to call it, nobody wants to smell what neighbor Bobby ate for lunch at Taco Bell on Friday night.
We all owe a great debt to the underappreciated man by the name of Alexander Cummings. In 1775, this heroic inventor came up with a revolutionary concept (pun intended) to cast away those smelly demons from the bathroom, kitchen and laundry rooms forever. His invention was the s trap.
The s trap, or s-bend, is a pipe fitting shaped like a side-ways S. You can think of it as two U’s connected at one end only. Now imagine water being poured down the kitchen sink. (A much better visual than a scenario involving the toilet.) I comes into one end of the U and then has to travel back UP the other U before gravity takes it the rest of the way. When water is flushed down, a portion will stay inside that first U shape and close off airflow from the sewer line to the drain line leading back to your sink and eventually into your kitchen.
This great pioneer sparked the fire to an array of letter-shaped traps. Next came the U-trap invented by (I am not making this up) Thomas Crapper in 1880 which was a smaller variation of the s trap and are sometimes, though incorrectly, used interchangeably.
The problem with the s or u traps is they both allow draining water to create a suction or siphon in the drain system. The weight of the water as it passed the last curve literally sucks everything else down with it and you end up with air that is able to pass through the system unopposed. As you can guess, this allowed for stinky kitchens until you poured a smaller amount in to refill the trap.
Enter the P trap. It’s currently the design used today and has replaced the other designs. The P trap looks just like an unconnected “P” that has fallen over. Instead of the drain line running directly vertically, it has a horizontal run which prevents any siphon from forming in the system. Almost any drain in your home will have either an s or P trap, including toilets. Although, toilets have a P trap built into them where the bowl acts as an enlarged entrance for the trap.
Finally, we can return and discuss the secondary function of a P trap and that is to collect heavy objects from entering the sewage line. If you ever drop a ring or small eating utensil down the drain and it makes its way through your garbage disposal, there’s a good chance you can find it within your trap which is why they are screw-on and screw-off. Other have different mechanisms for removal.
In addition to the objects you want to keep, it also traps bad objects that could create blockages in your main sewer line to your utility company or drain field or septic tank. Objects you need to clean out could be sand, dirt or other materials that shouldn’t have been poured down the sink.
Drain traps need to be properly maintained by the occasional cleaning. Even if nothing important has washed down your drain recently, you should clean out any drains that get a lot of use every year as part of a spring cleaning.