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HVAC School - For Techs, By Techs


Aug 5, 2021

In today’s podcast, Steven Rogers joins Bryan to talk about measuring total system airflow.

 

Fluid measurement is NOT restricted to liquids; a fluid is anything that flows, including liquids and gases. In HVAC, we deal with both of them, but airflow is strictly limited to the flow of gas. The airflow we experience in HVAC is exclusively turbulent flow, NOT truly laminar flow.

 

Total system airflow is difficult to measure directly; we can use flow hoods, but they have their limitations, especially on systems that have multiple returns. So, we measure pressures that give us clues about the airflow tendencies.

 

One of the most recognizable measurements is static pressure. Many factors can contribute to poor static pressure, including dirty filters and poorly designed supply and return plenums. 

 

All measurements that you use to calculate static pressure will require an average. You take readings at multiple points of the duct, so you need to calculate an average value, whether you’re using static pressure tips, a hot wire anemometer, or a vane anemometer. Measurements are also particularly difficult to take in the supply registers, as there are almost no runs of straight duct.

 

Recently, the TrueFlow grid has come on the scene to make airflow measurement easier. The grid relies on torque and RPM data to determine the total system airflow. The TrueFlow grid slides in where the filter goes and measures the total system airflow. The grid works with an app that considers the system tonnage to let you know how good or bad the airflow is.

 

Bryan and Steven also discuss:

 

  • Bernoulli’s principle
  • “Moving” CFM targets
  • Flow hood limitations
  • Laminar vs. turbulent flow
  • Static pressure probes vs. pitot tubes
  • TESP and fan charts
  • Precision vs. typical manometer
  • TrueFlow grid vs. filter restriction
  • Water heater backdraft
  • Depressurization and combustion air zone



Check out The Energy Conservatory’s Website HERE. You can also check out the TrueFlow grid on that site or at TruTech Tools.


If you have an iPhone, subscribe to the podcast HERE, and if you have an Android phone, subscribe HERE.