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itsnotrequired 05-07-2012 10:09 PM

ventilator as multi-bathroom exhaust fan, discharge vent low point question
i have a broan MP280 multi-port in-line ventilator that is being used as the bathroom vent fan for the two bathrooms and laundry room in my home. there is a spare port available for a future bathroom. bathrooms and laundry room are connected to the ventilator via 4" foil flex duct and the discharge is 6" rigid.

ventilator is located in the basement, attached to the floor joists in an unfinished utility area. as part of a basement finish project, looking to reroute the discharge line. shortest route to the exterior is perpendicular to the joists. duct would be supported by the joists but in the last joist cavity before the exterior, their is a toilet waste line parallel with the joists. it actually hangs below the cavity. clearance is such that a 6" rigid duct will not fit over the pipe and below the joist. to use this route and 6" rigid duct, i need to go under the waste line and then turn back up to get above the foundation sill plate to exit the rim joist.

so basically, i would form a giant 'U' duct under the pipe. i know low points (traps) should be avoided but would like to use this routing if possible. so here are a few different approaches i had in mind:
  1. do nothing and leave the U there. i have a low point in the existing vent routing (not as extreme as this U, however) and have not witnessed any condensate issues.
  2. install a drip leg or similar at the bottom of the U. i can't find much online about this or in the building codes. if i put in a drip leg, what is a good approach (materials, sizes, etc.)?
  3. run 6" flex instead of rigid and just 'pinch it' to get over the pipe. clearance is about 5".
  4. put in a reducer at the ventilator and take it down to 4", such that i can now route it over the pipe.
  5. install a round-rectangular transition piece at the pipe, such that the duct can squeeze over the pipe without sacrificing duct area.
  6. lower the waste pipe. this looks doable but would be ugly.
thoughts? anyone ever encounter this before? perhaps a good question to run by the local inspector? i'm leaning toward putting in the 6" rigid with the 'U' and no drip leg. i figure i can add one in the future if need be.

itsnotrequired 05-08-2012 09:03 AM

1 Attachment(s)
here is a photo to help explain it. i took another look this morning and it looks like i could run it on the left side of the picture, such that i am past the pipe. still a bunch of 90s though...

Attachment 50528

airventilator 03-13-2013 01:14 AM

thanks for this post..

old_squid 03-13-2013 11:25 AM

To make it look "nice" I'd opt for a 6" round to rectangular transition (register boot) go over the pipe in rectangular and then another transition on the other side and exit in 6" round. I'd use 6"rd x 4 x 10 straight register boots to reduce the restriction to air flow. Or, use whatever boots you can find that they also sell premade duct work sections for.

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