The Ductopus or the Centipede - Which HVAC Duct System Is Better?

I've been thinking lately that I might be in danger of losing all my friends in the HVAC business because I've been a bit heavy on showing examples of what they get wrong but not so good at showing what they do well. If you read my article Why Won't the HVAC Industry Do Things Right?, you may recall that in answering the question I didn't place all the blame on HVAC contractors. There's plenty to go around.

Anyway, in this post, I'm going to show you an example of something that I liked when I saw it in a home being built. The photo above shows part of a trunk-and-branch duct system. What you see is the end of a trunk line, which carries the conditioned air from the air handler out to the branches, which distribute it to the rooms in the house. Here's why I like this:

  • The trunk line is made of rigid sheet metal, not flex duct.
  • They've kept all the branches a good 6" from the end of the trunk, which helps get better air flow into the branches.
  • The branches aren't jammed up against each other where they come off the trunk.

The house that I saw this in had some problems (HVAC, insulation, drainage plane...), but I'd much rather see a trunk-and-branch system like this than a radial system the way it's usually installed. Last week I wrote an article asking if flex duct should be banned from green building programs and concluded that I think it's OK when used as shown here - for relatively short branches in a trunk-and-branch system.

Ideally, I'd like to see whatever system gets installed go through a full HVAC design and commissioning process. Every house should have it. ENERGY STAR Version 3 requires it. Building codes require it, too, even if it doesn't get enforced in most places.

But if we go by the 12-step program mantra of progress, not perfection, every time we see a system like the one in the photo above, we should remember that this is progress...because if it looks like that, it doesn't look the what you see below, which we affectionately term the ductopus. And I'm sorry, that system wasn't designed, and it's not going to perform well either.

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Tags: HVAC, ductopus, ducts, radial, trunk-and-branch

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Comment by Joseph Lamy on July 28, 2011 at 10:37am

As Ed Minch has made quite clear, the issue of where in the attic to run our ducts is a significant one, once we establish that the SEAL is critical and then the placement is second. I have found hundreds of miles of flex duct branching off the trunks and strapped to rafters between 2 and 12 inches from a 150 degree roof day in and day out.

 

I have also found miles of shredded exteriors of these flex ducts where the HEAT has disintegrated the silvery grey layer of plastic and the insulation layers have fallen open exposing acres of the interior liner of gobs of house-bound, expensively conditioned air to de-condition and move toward the ambient temperature in attics (14 -153) rapidly on its tortured path through the attic. This extra-long marathon is not needed. Nor is it really a wise thing to pass the duct so close to the roof that the plastic covers evaporate and disintegrate and end up  exposing the plasticized slinky that carries our house air to the slings and arrows of attic conditions. Yet I see miles of this condition regularly.

 

Burying the sealed duct in a mountain of insulation eliminates all this disastrous waste. That one change would close down 14 or 31 power plants by itself! IMHO

Comment by Ed Minch on June 13, 2011 at 8:27am

Allison:

Great start to a thread.  Let's look at the ideal and back off from there.  If we HAVE to have ducts in the attic, then the metal trunk should run the length of the attic and be mounted on top of the bottom chord of the trusses (or on top of the joists if there are no trusses).  Branch lines can then come off the sides or the bottom of the trunk and slowly drop til they are between the joists at drywall level.  Every joint is then sealed like your life depends on it.  The voids under the trunk and the branches are filled with the right amount of blown material, then additional blown material goes over the top of the branches, and as high up the sides of the trunk as the builder is willing to pay for - hopefully over the top of the trunk.  The unit should be horizontal and on the attic floor to keep both supply and return trunks as low as possible.  Central returns will keep the return ductwork as short as possible to reduce the number of joints for leakage and surface area for friction and conduction.  If you have to put ducts in the attic, this design has the best chance at efficiency and comfort.

Like you say, progress, not perfection, so let's start getting a few of these and other good ideas started

Ed Minch

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