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HVAC School - For Techs, By Techs

Dec 19, 2019

David Richardson with NCI, author of Duct Dynasty, joins us on the podcast to talk about mixed air temperature and more topics of interest.

When you bring outside air into the home, you introduce positive pressure into the home. That way, you can offset air lost via mechanical ventilation or through cracks, improving air quality. We often assume that the building will "breathe," but tighter constructions make it difficult for the home to bring in enough fresh air to offset harmful chemicals and VOCs.

We need to measure two different kinds of airflow: fan airflow and outside air. When we have these numbers, we must figure out how much air is coming through the outside air intake. The fan airflow represents 100% of the air content after mixing has taken place. You can perform a duct traverse to get the airflow measurement; when you plot the fan airflow, subtract the two to know how much return air you're getting BEFORE mixing with the outside air.

Once you have your airflow measurements, you must break those into percentages. You must determine the percentage that matches up with the temperature you want to use for the mixed air. Subtract the outside air from the fan airflow to get the CFM from your baseline. You could get 95% of your airflow from the return and 5% from the outside. Once you know the outside air temperature and the percentage of outside air, you will know how much the outdoor temperature will affect the return air and space temperatures.

David and Bryan also discuss:

  • How David got into writing
  • Blown-in cellulose insulation
  • Designed and undesigned leakage
  • Duct traverse tools and procedure
  • Dry-bulb vs. wet-bulb measurement applications
  • SEER vs. AFUE
  • Duct leakage

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