Comments on: How to Install Flex Duct Properly

By: Phillip Norman

Phillip Norman — Mon, 23 Sep 2013 17:07:35 +0000

May we please have discussion
May we please have discussion of preferred placement of in-attic furnace ducts resting upon and within the floor insulation, as presented here:
http://energyconservationhowto.blogspot.com/search/label/Attic%20HVAC 

Good systems have little flow resistance in straight runs. Register resistance always dominates distribution. Keep ducts big for low resistance by using illustrated duct take-offs. Please see the captioned web photo album accessed by link.
https://picasaweb.google.com/108533770292578040917/InAtticFurnaceDucting

By: Brian Stewart

Brian Stewart — Sun, 17 Mar 2013 18:32:10 +0000

I've found that static and
I've found that static and fan watt do not move much when putting in extra metal elbows and such to make the ducts to taut.

By: David Butler

David Butler — Thu, 21 Feb 2013 04:01:39 +0000

Rob wrote:

Rob wrote:
"yes, Appendix 17 is for informational purposes only - to show what happens when it's done wrong."

Several committee members wanted to include some typical friction loss examples in the normative section, but ultimately ADC won that debate.

"I also love Table A1-2 that shows how much doors need to be undercut as effective return air paths - it shows how ridiculous the concept can be."

That's Hank's sense of humor for you. Here's a link for those who don't own the book:

Manual D Table A1-2: Minimum Door Cut For Return Air

By: Rob George

Rob George — Thu, 21 Feb 2013 03:46:44 +0000

yes, Appendix 17 is for
yes, Appendix 17 is for informational purposes only – to show what happens when it’s done wrong. I also love Table A1-2 that shows how much doors need to be undercut as effective return air paths – it shows how ridiculous the concept can be.

By: Allison Bailes

Allison Bailes — Thu, 21 Feb 2013 00:50:29 +0000
Adam G.:
Adam G.: Yes, if you're using zip ties, you absolutely have to have the tensioning tool. Even with pliers, you just can't get it tight enough.

David R.: Yeah, it's amazing the kind of stuff that finds its way into ducts. See the photo I recently shared on our Facebook page (facebook.com/energyvanguard). Here's the photo.

Debbie: Good advice. I agree with you about wider support straps being better. As with most minimum standards, they're the least you should do. Going above and beyond usually works better.

Stan: Yeah, I think nylon zip ties do have a shorter life in hot attics. I don't know what their estimated life is, but the first thing to do is try to get ductwork into conditioned space.

Ryan S.: Yeah, I don't think I'd want to hold flex duct to a connector with those screws either, now that I think of it. Now that I think about it, I don't think I've seen it done in the field, either.

David B.: Thanks for the inside info. It's nice to know that the data made it into the manual, despite their lobbying against it.

By: David Butler

David Butler — Thu, 21 Feb 2013 00:07:50 +0000
When the latest (3rd) edition
When the latest (3rd) edition of Manual D was in development, there was a lot of discussion about friction rates of poorly installed flex. Texas A&M;, having studied this, provided data from its research for inclusion in the manual, the idea being to give designers an idea of real-world friction rates, and hopefully persuade diligent contractors to follow recommended installation practice.

I should point out that the ADC lobbied heavily to keep this data out of the book, but in the end, the data was included as Appendix 17.

By: Ryan Shanahan

Ryan Shanahan — Thu, 21 Feb 2013 00:02:39 +0000
I've been told to avoid using
I've been told to avoid using metal screws with flex duct because the flex can tear easily once you create a hole. The plastic zip ties zipped tight with the tensioner seems like the way to go.

By: Adam Gloss

Adam Gloss — Wed, 20 Feb 2013 22:44:45 +0000

Stan – We haven’t seen any
Stan – We haven’t seen any break, but use high quality, heavy duty ties. We are also in the Pacific NW and 1) most of our duct systems are in crawl spaces, not attics, and 2)our temperatures are fairly mild. If you don’t want to use nylon ties, you can use metal clamps. You can even use three sheet metal screws behind the first rib or two of the duct, as long as you use mastic under the duct at the sheet metal connection.

By: Stan

Stan — Wed, 20 Feb 2013 22:30:54 +0000
What is the estimated life of
What is the estimated life of zip ties? I've seen several in attics that have broken, due to the heat I assume, so I'm not a fan of them.

By: Debbie

Debbie — Wed, 20 Feb 2013 21:57:11 +0000
living in a hot humid climate
living in a hot humid climate, I see the issues caused by lying duct on insulation on attic floor.

hanging flex with mininum sag with 3" duct wrap is easier said than done. I've always watched closely the guys who do it well and learned from them.

always -2 people to run flex. just to open the bag of flex & streach the inner liner is impossible for 1 person to do alone.
always- mastic.paint on mastic or mastic tape.
always -3" duct strap..wider width trumps narrow width and when properly suspended
the 3" will not cause the duct to compress reducing air flow.
always- 90 degree ells at supply boxes.

always- test duct for air leakage. always.

having killed the kracken & replaced it with ductwork that both performs & moves proper cfm of air, takes time, experience & the want to do it right.

most hvac companies have low man on the job installing ductwork. no room in the bid for the time to do it right.

I stumbled into ductwork repair/change out,by testing before and after ducts were sealed by hvac companies. some cases duct leakage & air flow were worse than before the work was done.
so, I learned.
once the homeowner understands that the 30% duct leakage is air they pay to heat and cool that simply isn't making into the living area..they understand.

next areas to address are oversized supply box pentrations, supply boxes that lift off the attic floor when supply grill is installed, and return air leakage.

if testing of ducts/returns was code this would be a step in the right direction.
as difficult to achieve as getting hvac companies to actually train their workers how to install ducts.

but in that perfect world....duct & return systems would be sealed tightly and the sham of duct cleaning, uv lights,
and air purification systems would die out.

good article.thanks.

Debbie