Comments on: The Best Velocity for Moving Air Through Ducts

By: Mark Modera

Mark Modera — Mon, 24 Aug 2020 21:45:36 +0000

I like your article, as it
I like your article, as it gets across an important point about thermal versus fan energy issues. I would however not say: "If you put ducts in conditioned space, you can move the air as slowly as you'd like." If you slow things down too much, the thermal losses defeat the purpose of the the ducts, which is to get the heating or cooling distributed to particular rooms. I am currently analyzing ducts in conditioned spaces more carefully, but have done detailed experiments on the interactions between ducts and variable capacity heat pumps. I would be happy to share those if you would like.

By: Robert Jordan

Robert Jordan — Fri, 01 May 2020 16:46:16 +0000

In reply to Thomas Dugan. I just watched Joe Lstiburek
I just watched Joe Lstiburek's presentation on YouTube "To vent or not to Vent". I am reconsidering unvented attics.

By: Marvin McConoughey

Marvin McConoughey — Tue, 31 Dec 2019 14:32:26 +0000

In reply to Defiant Lion. Blow cigarette smoke? That's
Blow cigarette smoke? That's horrible. Your human air ducts are infinitely more important than the ducts in your HVAC system. Please, stop smoking and try a better approach to duct and airflow analysis.

By: David Butler

David Butler — Sun, 22 Sep 2019 19:15:38 +0000

I hate to be the bearer of
I hate to be the bearer of bad news but all seams (including boots) should have been sealed with duct mastic, not tape! Even the best tapes will eventually lose adhesion (faster if located in attic or crawl).

Without a written contract that calls out this detail, your only recourse may be the building code. So check with your building inspector before doing anything else. For example, some jurisdictions require mastic instead of UL-181A-P tape on metal seams. Likewise with your requested pre-insulation inspection. Unless you put it in writing….

As for evaluating the ducts, most jurisdictions now require a duct leakage test (unless ducts are fully within conditioned space). Either way, I recommend that you have the ducts leakage tested by an energy auditor or HERS rater.

If the ducts are all (or mostly) metal, removing & replacing all that insulation may be cost-prohibitive, assuming you have no recourse with the installer. If ducts are inaccessible, there’s a technique called AeroSeal that seals ducts from the inside, but it’s expensive. Perhaps even more than removing and replacing the insulation.

By: Andrea

Andrea — Sat, 21 Sep 2019 20:51:39 +0000

How can I tell if a duct is
How can I tell if a duct is poorly made? I have a subcontractor installing ductwork that he sealed with non UL listed tape and now I am examining everything very closely. Just stuck my head in a plenum and it seems very sloppy to me. He has installed insulation on the outside so I cannot tell if the seams are properly sealed. I asked him no to install the insulation before I inspected the seams and he packed up and left the job. Ugh

By: Defiant Lion

Defiant Lion — Wed, 04 Sep 2019 18:28:29 +0000

Take my house, built in the
Take my house, built in the 1920’s, frame made of Redwood, I’m located in L.A./OC area of California. 1 story with a attic and 3 gable openings & crawlspace. Heating furnace with duct branches in a uninsulated attic. Now when the heat hits dry heat is bearable, house cools a couple hours after Sun goes down. When it’s humid the house is like a oven after 3pm. The house does not cool till after 12am & all the hot air in the attic goes nowhere but down the side of the walls & out of every open surface available. At times it gets so humid I feel the heat is coming from the ground, I understand where on a living organism that’s needs to breathe too, especially when where suffocating it with concrete & asphalt all over this God given land. At times the red brick is cooler than the concrete & the freeway concrete gives off an extremely hotter amount of heat, being surrounded by it in the big city. There’s a lot of factors that play in my part. My point is that hot air in the attic goes nowhere but down & all around inside the house, even if there’s a cool breeze & clouds that came out of nowhere on which was a 95 degree day at around 4pm late afternoon in which the day was humid. The humidity left for a couple hours, but later after 9pm returned for the rest of the week. No I don’t have any venting in the attic nor soffits or wind turbines. I know, but getting your grandparents to understand what needs to be done is like teaching that old dog new tricks, some dogs don’t even listen to you!

By: Defiant Lion

Defiant Lion — Wed, 04 Sep 2019 14:57:27 +0000

In reply to abailes. What about blowback from lets
What about blowback from lets say a bathroom exhaust/vent. Especially with the little port & small ducts their connected to with a turn or two in the line/path, exiting the building through the roof. Like to know how I came to this very interesting point on which no one makes no mention of! Go inside your bathroom, close doors leave them open it doesn't matter. Light a cigarette & blow into the vent fan. You see your smoke being thrown back into the bathroom. Need to see it better, turn off lights light a candle, now watch your smoke as you blow it into the vent, some will leave into the vent but not all of it, as I pointed out before blowback will be seen more clearly with lights out, candle on & smoke source of your choice! If you have bends in the ductwork, the more air will be bouncing all over the place inside that duct. Air is not like water where water takes it's course of path. The best way that I know to test for leaks but haven't tried to put it to use in the ductwork system cause of the size, length is like checking if a water pipe has leaks, inhale while your thumb covers the bowl. Instant leak finder. Just hard to put it to practical use being that duct system are bigger & you'll need a partner with a big enough thumb!

By: John R McTaggart

John R McTaggart — Thu, 04 Jul 2019 16:04:33 +0000

In reply to David Butler. The issue with satisfying a 3
The issue with satisfying a 3 level home is usually a. a design issue and b. an installation issue. Ducts can and will carry the right amounts of air to the required space if the design is accurate and based on design conditions. Secondly, if we call for a 4" round and you put in a 6" that will change the air flow of the design. And frankly a 4" piece of round won't fit into a 2x4 pocket, so that needs to be addressed as well; carpenter issues or switch to oval as examples. But we as an industry can't change the design without consequences. Thanks for the great article and helping my compatriots to understand air flow.

By: Deniz Erkan

Deniz Erkan — Fri, 28 Jun 2019 07:46:05 +0000

In reply to Thomas Dugan. If I seal off my attic, and
If I seal off my attic, and my gas furnaces are also in the attic, where does the combustion air come from?

By: RoyC

RoyC — Thu, 27 Jun 2019 18:41:32 +0000

I agree with many of the
I agree with many of the overall conclusions, but I do have to nitpick about some of the specifics. Let’s start with Q=UxAxDT. If you decrease the air velocity, U also decreases. Why? Because U includes the convective resistance of the indoor air stream, and it goes up as the air velocity goes down. Thus, the overall R-value of the duct (wall insulation plus surface convective resistances on each side) goes up with decreased air velocity, but this effect is small compared to the wall resistance if you have insulated ductwork. Thus, I disagree with Allison’s statement that “The slower the air moves in a duct, the more BTUs each cubic foot gains or loses.”, if you are talking about per square foot of duct surface area. The dominant term in the above equation is area (A) which is the duct surface area. To reduce air velocity for a given fixed airflow rate (cfm), you do have to increase the duct surface area significantly. That is why duct heat gain or loss in unconditioned spaces goes up, even if U goes down with higher duct air velocities.

As for air “residence time”, that has no direct influence on duct heat gain or loss. I don’t know where that term started, but it is meaningless to me.