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HVAC School - For Techs, By Techs

Mar 2, 2023

Genry Garcia returns to the podcast to talk about pressures in the building envelope, namely the infiltration skeletons behind closed doors.

When doing load calculations (Manual J), we need to know how much of the heat load, especially the latent heat load, comes from leakage in the building envelope and the ducts. Opening/closing doors and windows can also worsen the issues that stem from infiltration due to upsetting the balance of pressures in the home. Smoke pencils and other similar tools can give you an idea of the pressure in a home and how it could change when doors open or close.

Since there is a lot of room for inaccuracy in extreme climates (especially those with high latent loads), many HVAC systems are oversized and underperform. Some building design features also exacerbate problems presented by oversized HVAC systems. To get the data we need to design systems that mitigate those issues, we need to do a blower door test.

Downsizing the tonnage in retrofits or replacements usually has advantages, but it must be done right, and customers may not always want to do that. It's the contractor's responsibility to give them a choice and educate them about the options and what the thorough diagnostic process looks like, including balancing the home and checking the pressure in relation to the outdoors.

Genry and Bryan also discuss:

  • Positive and negative pressure in certain rooms
  • Exhaust ventilation and pressurization
  • Using See Stack to see differences in loads
  • Leaving the fan in the "on" position
  • Useful tools
  • Variables in lab-based testing and field testing
  • Getting hung up on the 3 Pascals rule of thumb
  • Leaky rooms vs. whole-home leakage


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