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HVAC School - For Techs, By Techs

Jul 13, 2023

This podcast episode is one of Alex Meaney's HVACR Training Symposium presentations: HVAC Design Backwards, Forwards, and In Between.

Load calculation factors in all three means of heat transfer: conduction, convection, and radiation. It doesn't directly tell you the tonnage; it just tells you how many BTUs (sensible and latent) are entering or leaving a structure. When designing systems after doing load calculations, we need to be mindful of industry standards and their pitfalls, as well as the climate conditions and the difficulty of obtaining manufacturer data.

Equipment selection by tonnage is only part of the picture when it comes to HVAC design; we also need to factor in airflow and duct design, especially duct sizing. However, many rules of thumb and poorly explained terms are counterproductive to a thorough understanding of HVAC design. In some cases, the best way to design a system may seem "backward," especially when starting with blower selection instead of ductwork.

Duct design is particularly difficult, especially when software identifies several problems with designs that seemed to look good on paper. However, the software points out areas where you can adjust the duct size and manage restrictions to allow the fan to do its job without being derated by friction.

Alex also covers:

  • Insulation and efficiency ratings
  • CLTD Groups
  • Tricky radiant gains and losses
  • The relationship between BTUs and tons
  • AHRI ratings
  • Shortcomings of Ductulators in duct design education
  • Pressure vs. friction in ductwork
  • Static pressure vs. velocity pressure
  • Measuring friction with pressure
  • Regulations vs. reality
  • Furnace static pressure range
  • Differences between commercial and residential duct design
  • Oversized and under-ducted systems
  • Variable-capacity systems
  • ACCA manuals and tables


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