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HVAC School - For Techs, By Techs

Nov 23, 2017

Jack Rise returns to the podcast to share some duct design facts with us and talk about his Manual D book on the ACCA website.

Before we can even start thinking about duct design, we need to think about the total effective length; even before that, we also need to think about finding the critical path. The critical path is the path with the greatest resistance to airflow (from the return to supply); the fittings in the critical path contribute to the duct's total effective length.

Flex duct is a controversial and somewhat complicated building material. It's common in Florida, but Jack doesn't use it in his duct designs; he can't depend on others to install it properly. Very few people tend to install flex ducts as tightly as they probably should.

Noise is a problem for ducts, and takeoffs on the plenum are a significant contributor to noise issues. Instead, Jack suggests having a takeoff from the collar that goes straight into the appropriately sized duct for the desired airflow. (It's also worth noting that noise is subjective and is difficult to measure.) It's also unwise to position two takeoffs directly across from each other, as noise travels across those.

The rise of indoor air quality (IAQ) products also requires us to look at duct design facts. Filtration improves IAQ but increases static pressure and can impede airflow. We need to be able to plan for IAQ products when we design ductwork.

Jack and Bryan also discuss:

  • Selecting the equipment location and position
  • Balancing damper placement and leakage
  • Radial systems and symmetry
  • Plenum sizing
  • Why panning is not great (and illegal)
  • Why bay jumping is a bad idea
  • Duct design vs. truss positioning
  • Airflow in the occupied zone


Check out Jack's book, Understanding Manual D, HERE.

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