Thread: Horizontal Duct to Plenum Connection Question

Hello, I've read in a number of home inspection sites to evaluate or repair ducts that run horizontally out of a plenum or into a boot.

What is the reasoning behind this recommendation?

Thanks!

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Originally Posted by ed morales

Hello, I've read in a number of home inspection sites to evaluate or repair ducts that run horizontally out of a plenum or into a boot.

What is the reasoning behind this recommendation?

Thanks!

Horizontally or vertically doesn't make a whole bunch of difference. And actually transitioning from rectangular to round - even flex - is not bad IF the joint is truly a transition. When the rectanglular duct or plenum acts as a boom box there is excessive pressure drop at each transition and that is what should be avoided.

Originally Posted by Rod Butler

Horizontally or vertically doesn't make a whole bunch of difference. And actually transitioning from rectangular to round - even flex - is not bad IF the joint is truly a transition. When the rectanglular duct or plenum acts as a boom box there is excessive pressure drop at each transition and that is what should be avoided.

Hello Rod, thanks for the quick reply.

You mentioned that transitioning isn't bad if the joint is truly a transition. When determining if a joint is truly a transition, are you referring to the use of turning vanes in duct board transitions or the use of Pant Wyes or Tee Wyes in metal duct transitions?

Also, I'm not familiar with the term "boom box" is this the same thing as a duct board triangle box designed to split airflow?

Thanks,

Well, that certainly came in larger than I expected. A boom box is a plenum where the air from the supply is tapped off without a transition. A transition would provide a smooth flow of air and not the abrupt hit of a tap. I will see if I can upload a detail.

The larger flex from the lower left came from a rectangular boom plenum with no transition and hopefully you can see that is dumping into another boom box, again with no transition. Then from there it pops out into several smaller flex ducts, once again with no transition. Is this against code? No, I don't think so but the east half of the house wasn't getting enough cooling and it is a direct result of not enough air due to poor duct layout. That may be why it is a flag item.

I'm sorry this photo didn't turn out. But maybe you can see there is an excessive amount of flex duct which is also a bad situation resulting in excessive pressure drop and exposure to attic hot and cold.

Thanks Rod I understand now why it's an item to check. The duct layout in your pics looks eerily similar to what I'm seeing in my attic. Looks like I've got me a project this weekend!