Why is there a "U" on the pipes under my sink?

Question

Under my sink (both in the kitchen and the bathroom) the pipes come up and then make a U turn before going to the drain. Why don't they just go straight up to the drain?

Answer 1

The "U" is a trap. Plumbing traps serve two purposes:

1. Under normal conditions it is always full of water, which keeps sewer line gases, insects, and other bad stuff from coming back through the drain into your house. (Note that if you don't use a drain for months the trap can dry out. You may then notice bad smells or small bugs coming out of your drain. The solution: Run a little water back down the drain to fill the trap!)

2. If something big is forced down the drain, it should get hung up in the trap, which makes it easier to unclog than if the obstruction occurred further down the pipe -- when it's behind walls or underground.

Answer 2

The "U" in the pipe is there to hold a small amount of water inside it to stop odor from coming back up the pipe into your sink.

Answer 3

This is mainly to prevent sewer gasses from backing up into the house. In the past, when they didn't have these 'traps' installed, sewer gasses would back into the house and people would be killed by the gas (it can contain Hydrogen Sulfide, which is poisonous) or the gas would collect and explode.

Answer 4

This is to prevent sewer gasses from coming back up. If you have ever had to change the wax ring under your commode you would know what kind of gas I'm talking about.

Answer 5

Stuff like rings etc falling in will stay in U bend instead of going straight to outside drain system where they will be harder to recover.

Answer 6

They are traps to keep sewer gas out of your home.