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A Good Duct System With A Problem

In a perfect HVACR world, all comfort cooling duct systems would be designed for maximum efficiency and minimal heat gain and installed properly so that objective would be achieved. But, as we all know ours is not by any means a perfect world. A duct system in a tract home, for example, is often far from perfect. And it’s not just because of the design of the duct system itself, or because the installation wasn’t done right. The design of the building is a factor. To make my point, I’ll use the example of a typical extended plenum supply duct system in which the main trunk is manufactured either from fibrous duct board or sheet metal, and the branches are flex duct.

It’s common to find this sort of design and use of materials in a home that employs truss roof construction that creates an ‘attic’ crawl space. And, temperatures in that space will be significantly higher than the outdoor ambient temperature, and fantastically higher than the temperature of the air being transported through the duct system. Not that the specific numbers are the most important thing to consider at this point in our discussion, but it’s not uncommon to find an attic temperature far beyond 100-degrees F, while, in fact the temperature of the air in the supply duct will often be in the mid 50s….at least that’s what it would be after leaving the indoor coil and just starting its journey to the supply registers.

But, imagine this: You decide to do a fundamental inspection on the duct system and the building described above, and as you enter the attic on a 95-degree outdoor ambient day, you look up and see the bottom side of the roof sheathing above you. No foam insulation sprayed on the plywood, just the sheathing. And, now imagine that you recall that the roofing on this building is the typical dark charcoal color shingles used on tract homes. And, of course, when you look down at the main trunk and the flex duct, you’ll note that it is resting on, or perhaps suspended just above, the correct amount of ceiling insulation.

Yes, a typical installation, with the ceiling insulation required by code, the main trunk properly constructed with a reducing plenum to promote the proper static pressure in the second and third segments, and the flex duct properly installed, tightly connected with nylon zip-type duct straps to the main trunk transitions and the boots for the supply registers, pulled tight with no extra ducting snaking around the crawl space or hanging awkwardly from a truss. An installation done correctly, according to design….and not working nearly as efficiently as it could.

The reason, of course, is the extreme temperature difference between the supply duct air and the crawl space. While the air may exit the coil at the design temperature when the outdoor temperature is relatively mild, you can bet that that the system won’t be able to perform as it should once the temperature rises. Consider this: Ductwork insulation may be rated as low as R-6 or even R-4, which means that the amount of inevitable heat gain that will result in, say, one branch of the system, means that the discharge air temperature from that register will be significantly higher than it needs to be in order to achieve the desired comfort level in that room. And, that will result in extended run cycles that affect equipment performance, and increase operating cost.

The solution to this type of performance problem isn’t rocket science. Foam insulation on the underside of the sheathing will bring the crawl space temperature down significantly, or adding extra insulation around all ‘exposed’ ductwork to increase the R-factor and reduce the amount of heat gain in the air supply system will also allow the equipment to do the job it is designed to do.

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