Trap type and diameter for clothes washer...
Posted by Curtis on April 04, 2004 at 20:30:06:

My clothes washer is backing up occasionally and spilling from the top of the drain pipe. Since I know nothing about plumbing, I hope you can help... My questions are as follows:

- Should I replace the S trap with a P trap
- Can I replace the 1.5" trap and pipe with 2" or larger diameter (is there a maximum or recommended size).

The top of the 1.5" washer drain pipe is around 4' from the floor which connects down to a 1.5" diameter S trap which is around 18" from the floor. This connects directly to a 90� connector which then goes to a short horizontal run of 1.5" pipe (only 8-10" long) to a Tee connector in a 1.5" vent stack. I am thinking that the extra loop in the S trap makes it difficult for the rush of water to get through when the washer starts to drain. I think a P trap would need less force required to get the water over to the vent stack, as it wouldn't have to climb the extra 3-4" that the S trap has. Can I use a P trap on a washer connection? or will suction (back-pressure)cause a P trap to clear out when the pipe drains?

I am thinking of replacing the 1.5" T in the vent line with a 2" one and using a section of 2" pipe with reducers above and below a 2-3 foot length. That would then allow me to use a 2" diameter trap and pipe which would carry more water from the washer. I would be switching to a 2" P trap (from a 1.5" S trap) at the same time which I assume should make the water run much faster/easier. Also, since the vertical pipe below the T connection is then 2", it should be able to buffer up a gallon or so more water in case my 50 year old buried drains are slow...

Any ideas you might have are greatly appreciated and thanks in advance.

I even thought of replacing the 1.5" Tee with a 2" cleanout and use a 45� connector to go horizontal to the new trap so the falling water leaving the trap would already be pointing downwards when it goes into the vent stack, I am not sure if that is approved in the local code and/or if it makes much sense. I just guessed that venting air is easier than water and a 90� T would restrict water flow more than a downward pointing 45� connector would. Since the total weight of the water plus piping in this system is only around 5 pounds, the 2" Tee or 45� should have no problem.

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