My basment trap was clogged with mud, so obviously it caved in over the years, the house was built in 1938. I had it snaked and the french drain around the foundation finally drained with great relief. I still don't understand why traps are used in plumbing. Is it to prevent odor from returning into the basement drain or filling the house? Why the design and what purpose do they serve.

Traps in plumbing are to prevent sewer gas from entering the home by means of a water seal. However, traps need to be properly vented to achieve this goal properly. Without traps, it would be kind of like living in an outhouse, no separation between us and the smell of our excrement

Hi Plunge:

A traps function is to keep sewer gasses and odors from entering your home.

In older days they only used a single HOUSE TRAP... a 4" cast iron trap that kept the sewer gasses from the septic or the public sewer from coming back into the home. Over time, however, they found that the sewer pipes inside the home also presented health concerns.

Nowadays they don't usually require a house trap but instead require that all fixtures get an individual trap. The result is a much healthier home for all!

Any questions... let me know.

Mark

TIE post MGD... :p

In Plumbing, it keeps the odor in check, and likely help to stop bugs from getting in.
The Trap for Air Handler stop bugs, and keep from pulling in hot air.
When your AC is turned on for the first time, it will pull hot air from the outside through the drain till enough condensation fills the trap.