Plumbing: Washing machine standpipe?

Question

Hello,

I'm replacing my old style washing machine drain hose (now hanging over the utility sink) with a Standpipe drain system. I understand that the building codes require me to use a 2" pipe and 2" solvent P-trap. The only problem is that my drain line is 1 1/2". So is it safe to go from 2" pipe to 2" P-trap to 1 1/2 drain line??

Thanks,
hrh

Answer 1

you may have to have the stand pipe rather tall to somewhat work . the codes actually mean for the whole line to be updated. sometimes a bummer
Are you having this inspected ? if so check with the inspector as to what he will allow.
You need the trap to keep the sewer gases from comeing into the building (often flamable let alone smelly) and into your freslhy washed clothes. plus a vent to keep things flowing without a "vapor lock".

Answer 2

Is the standpipe emptying into an open drain or are you proposing to physically connect it to the 1 1/2" pipe?

If it is draining into the open drain, there is no reason to have the P-trap - especially for a washing machine - there is a P-trap downstream of the open drain anyway. If you aren't having it inspected, don't bother with the P-trap. The 1 1/2" pipe should be OK.

***Just read Hanks answer. Sounds like he knows his stuff, so I'll tuck my tail in between my legs and retract my answer. Good job Hank!***

Answer 3

Drain pipe size is figured based on "fixture units". A vertical 1-1/2" drain can handle 2 units. (A unit = a drainage rate of 7.5 GPM) A sink constitutes 2 units, so adding your washer drain line to the existing plumbing will exceed the permitted flow.

A 2" drain is rated at 16 units. In order for you to met the UPC (Uniform Plumbing Code) with this arrangement, you would either need to run a 2" drain from the washer to the main sewer trunk, or replace the existing sink drain with 2" as far as the main sewer trunk and add your washer to it.

That sucks, I know. Like the first answer states, ask the inspector what he wants.

Answer 4

Honestly I'm pretty sure you would be fine to reduce down to 1 1/2", as long as you go 2" as long as you can. The tricky thing about building codes, especially when it comes to plumbing, is that if you're the homeowner, you can get away with pretty much anything.