Comments on: The Science of Sag – Flex Duct and Air Flow https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/ Building science knowledge, HVAC design, & fun Fri, 24 Dec 2021 16:43:59 +0000 hourly 1 https://wordpress.org/?v=6.6.2 By: David Butler https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9137 Sat, 12 Mar 2016 23:05:13 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9137 In reply to Max S.

@Max, I’m guessing that
@Max, I’m guessing that contractor made up such cockamamie story because his crews don’t know how to work with metal. The underlying duct material is irrelevant in terms of heat loss/gain as long as it’s equivalently insulated and sealed.

That said, I would rather see a crappy flex duct system fully inside the envelope than a beautiful metal duct system in a Phoenix attic. With a large enough blower, it’s usually possible get a poorly installed duct system to balance out. The additional blower energy required to do that would be a lot less than what’s lost by running ducts in an unconditioned attic.

If enough home buyers would ‘just say no’ to attic ducts, builders and home designers would come around.

]]> By: Max S https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9136 Fri, 11 Mar 2016 23:40:54 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9136 In reply to John Proctor.

Just a home-owner here with a
Just a home-owner here with a question. Regarding the practice of using metal for trunk runs and duct for straight-branch runs… does this recommendation hold true no matter where the ductwork is located (in conditioned or unconditioned/attic space)?

I ask because one of the local HVAC contractors I am talking with (after requesting a metal trunk run) mentioned it didn’t make sense (to use metal duct) here in Phoenix. He claims metal ductwork gets “too hot” when in the attic space. Is there any merit in his claim?

]]> By: John Proctor https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9135 Thu, 17 Sep 2015 21:19:39 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9135 My understanding is that the
My understanding is that the Air Diffusion Council once was a good source for information about terminals and terminal design. As far as I can tell it was taken over my the flex duct manufacturers who use it to promote their product in spite of all the failings and problems with the product.
As mentioned above, If you insist on using flex duct, use it only for straight runs– use metal for the rest.

]]> By: Jamie Kaye https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9134 Thu, 17 Sep 2015 15:42:09 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9134 All too common to see the
All too common to see the bends, twists and compression of flex ductwork. It is also ALL TO COMMON for a homeowner to ask “why” their energy bills went up when they just put in a higher SEER unit with a Variable speed air handler. It’s too hard to say “just look at the duct”. We use an analogy of airplanes and arrival times to make our point. Goes something like this:

You booked your flight to London 6 months in advance and know exact arrival times (barring delays at the gate). The problem the airline has is that they have NO idea of the winds and weather that day, but they have to get you there on time. With the wind at their back…they get you there at the same time with less effort (less gas $). With the wind in their face, they get you there at the same time with MORE effort (much more gas $). The variable speed air handlers are acting like a rigid time schedule and doing their best to get you there (to your point of comfort) and do it either with the wind…but more times AGAINST the wind! They maintain your comfort, but at a cost!

Thanks for posting Allison! As always…great stuff!

]]> By: Bob Ellenberg https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9133 Thu, 17 Sep 2015 05:46:38 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9133 I agree with all of this but
I agree with all of this but it is a shame as an excellent install can be achieved with all hard elbows and turns, tight straight runs of flex and only a little more labor than it takes to do a poor job. I have reworked the duct work in my house and a friends and in each one we spent less than $200 on hard fittings, straps and high end tape. Two of us spent about 12 hours (6 each) total on each job and removed more than 1/3 of the flex. The improvement in air flow and comfort in both houses was almost unbelievable. It would have taken only a tiny portion of those 12 hours we spent redoing the system to have done it right to begin with.

]]> By: Morgan Audetat https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9132 Thu, 17 Sep 2015 01:54:53 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9132 My father was a commercial
My father was a commercial tinner, so some of my earliest memories are of the two of us installing gravity for forced air conversions. The skill level required, at least when working with Dad, was considerable and all but obsolete at the residential level. Which is why we started installing Unico.

I can design a forced air system for high or low efficiency but training a tinner takes 4 years and training for a proper high velocity system takes three installations. The drawings and flexibility of design and installation, taking into account the central, round plenum, common return and ease of installation, I would not be investing in conventional tin furnaces for new construction.

The future is clearly in a hybrid.

]]> By: Cameron Taylor https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9131 Wed, 16 Sep 2015 21:42:35 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9131 John Poole touched on
John Poole touched on something that crossed my mind as I read Allison’s fine article during breakfast this morning: it’s been my understanding that the original intention for flex duct was to act as the final run of duct off a metal trunk to feed diffusers mounted in a suspended ceiling, as this method delivered both sound attenuation and flexibility for the spaces beneath the ceiling should they be reconfigured at some future date.

If this is true, it makes me wonder where and when it was decided to do whole house installations with this material. While the research data and field experience do state that if flex duct is installed properly (as a whole house delivery method) it will perform adequately for its intended design, the ease of which this material can be installed tempts the installer to become fuzzy in decisions regarding how it should be run and supported. It seems each time we make a form of fluid delivery easier to install (flex duct, CSST for black pipe, etc.) it reduces the skill level required for installation while raising the consequences for poor installation techniques. While metal duct systems are not immune to poor design and installation decisions, how often does one come across poorly designed and installed residential metal pipe duct systems vs. poor flex designs/installs?

I know flex duct use will not disappear. Therefore, information such as in Allison’s article is vital for HVAC personnel as well as end users to know about. The only way to raise expectations about our trade, that I know of, is to teach how it should be done properly to all affected by it. Thanks, Allison, for another great contribution to this effort.

]]> By: John Poole https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9130 Wed, 16 Sep 2015 18:38:03 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9130 Allison,
Allison,

One of the “big takeaways” I gleaned from your Mastering Building Science course (where you touched on this topic), was that very short runs of flex duct were fine, and actually desirable, for connecting branches of rigid duct to supply registers, in the interest of ease of making the connection, and also reducing the transmission of noise or vibration to the register. So, when I re-do a few of my existing Krakens, that’s the only application of flex duct I’m planning leverage.

~ John

]]> By: Kurt Schorsch https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9129 Wed, 16 Sep 2015 16:28:42 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9129 Thanks for the article.
Thanks for the article.

]]> By: David Butler https://www.energyvanguard.com/blog/the-science-of-sag-flex-duct-and-air-flow/#comment-9128 Wed, 16 Sep 2015 13:35:31 +0000 http://energyvanguard.flywheelsites.com/?blog_post=the-science-of-sag-flex-duct-and-air-flow#comment-9128 Well, if you won’t say it, I
Well, if you won’t say it, I will. The flex duct industry (represented by Ralph Koerber of ATCO and ADC Engineering Committee chair) fought tooth and nail to keep Culp’s research out of Manual D. The ADC position was (and presumably still is) that flex should be installed with 4% or less compression and that data regarding the impact of poor installation practice has no place in the standard.

However, in recognition of ACCA’s education mission (its design standards have always tended to be somewhat instructive), the consensus of the Manual D Review Committee was to:

“Instruct designers to follow flex duct manufacturers’ installation instructions (in the body of the standard) and to provide guidance (in an Informative Appendix) that allows designers and others to assess the impact of (and possible diagnoses after the fact) poor installation practices.”

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