While attempting to research a "closed" laundry drain system (assuming such a system exists, or is even possible), I found this image of a "correct" laundry standpipe installation at Terry Love's site:

A member of Terry's forum, "cacher_chick," posted the following (which appears to have been quoted from yet another source):

"The waste from an automatic clothes washer shall discharge through an air break into a standpipe in accordance with Section 802.4 or into a laundry sink. The trap and fixture drain for an automatic clothes washer standpipe shall be a minimum of 2 inches (51 mm) in diameter. The automatic clothes washer fixture drain shall connect to a branch drain or drainage stack a minimum of 3 inches (76 mm) in diameter.

. . . (C) Clothes washers. 1. âÂÂResidential types.â ResidentialâÂÂtype clothes washers shall discharge into the sanitary drain system by means of an airâÂÂbreak. A.) A standpipe receptor may not extend more than 36 inches nor less than 18 inches above the centerline of the trap outlet. B.) A standpipe receptor shall terminate at least 26 inches but not more than 48 inches above the floor on which the clothes washer is located."

Notable details from the standpipe installation photo:

1. There is a sanitary 'T' (instead of a standard 'T') at the washer standpipe connection to the main vent. Apparently, some "professional" plumbers are unaware of this code-compliant connector.

2. There is a secondary vent attached to the washer standpipe drain, just prior to the sanitary 'T' at the main vent. Previous to seeing this photo, I was completely unaware of this requirement.

3. The laundry sink supplies are tapped to also supply the washer inlets using PEX (unfortunately, PEX is not accepted by our local codes).

This post was edited by studio460 on Sat, Jun 15, 13 at 21:32

Well, I actually posted this topic on Terry Love's forum (makes sense!).