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HVAC School - For Techs, By Techs

Nov 21, 2018

This episode is very exciting to me because we get to have Dr. Allison Bailes on the show. Today, he shares his knowledge about friction rate and duct design. Allison got his start teaching college-level physics before getting into the building design industry.

If you have a forced-air system that blows heated or cooled air through a duct system, that blower creates a pressure difference. Some of the pressure is used up on the filter, registers, and dampers, so you will see pressure drops. Anything left over is the available static pressure, which pushes air through the ducts. When you do a duct design, you must account for pressure drops and your blower's static pressure rating.

When designing a duct system, you want to minimize friction as much as possible. Counterintuitively, you want a high friction rate. Friction rate refers to the availability of static pressure compared to friction provided by the effective length, not the total amount of friction. Fittings significantly impact your total effective length. By extension, fittings can have a major impact on friction. In flex duct designs, the turns add additional resistance.

Oversizing often happens due to poor load calculation. While you increase capacity with an oversized system, there are plenty of drawbacks. The capacity will rarely match the load, you may spend too much on the equipment, have ineffective dehumidification, and you will deal with short cycles, which lead to comfort problems.

Allison and Bryan also discuss:

  • Home energy ratings
  • Equivalent length and total effective length
  • Flex duct design
  • Seasonal runtime
  • Surface area challenges
  • Unconditioned spaces
  • Filtration

To find out more about everything Dr. Bailes has to say about building performance and duct design, visit his site at: 

Learn more about Refrigeration Technologies HERE.

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