Back by Popular Demand: AC/DC, PWM, Serial Communication…80’s Rock Bands or ECM Communication Methods? February 4, 2026, 3:06 am

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Back by Popular Demand: AC/DC, PWM, Serial Communication…80’s Rock Bands or ECM Communication Methods? February 4, 2026, 3:06 am

February 4, 2026

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HVAC School – For Techs, By Techs

Real training for HVAC ( Heating, Ventilation, Air Conditioning and Refrigeration) Technicians. Including recorded tech training, interviews, diagnostics and general conversations about the trade.

In this session from the 7th Annual HVACR Training Symposium in Florida, Eric “Elk” Kaiser delivers a comprehensive workshop on airflow testing and measurement. Eric opens by challenging technicians to think beyond simply pointing an instrument at a duct and reading a number. Before selecting any tool, he argues, professionals must understand exactly what they are measuring — whether that is velocity, pressure, volume (CFM), or the mass weight of air — and why each of those values matters for designing ductwork, sizing equipment, and delivering comfort to customers. The session sets the stage for a deeper technical conversation about the physics of air and how those physics affect measurement accuracy in the real world.

A significant portion of the presentation focuses on air density and how it affects the accuracy of common industry formulas. Eric walks through the origin of the widely-used 1.08 and 4.5 airflow constants, explaining that they are derived from a theoretical “standard air” condition of sea level pressure (14.7 PSIA) and 0% relative humidity — conditions that virtually no technician encounters in the field. He demonstrates how changes in altitude, temperature, and humidity all shift air density, causing those constants to become variables. For technicians working at elevations above 2,500 feet, the density difference can exceed 10%, enough to significantly skew BTU calculations and equipment performance assessments if left uncorrected. Eric also walks through a real-world scenario involving measurements taken across an operating evaporator coil, where a 3.4% density shift between return and supply could easily be misread as duct leakage.

The workshop then moves into a thorough survey of airflow measurement instruments and the specific conditions each one is best suited for. Eric covers vane anemometers (large and mini), hot wire anemometers, pitot tubes, flow hoods (passive and active/fan-powered), flow boxes, the temperature rise method, and the digital TrueFlow grid. For each tool, he discusses accuracy considerations, density correction requirements, velocity limitations, placement requirements, and common mistakes. He is candid about the limitations of manufacturer performance charts, sharing a behind-the-scenes look at how one manufacturer evaluated static pressure using a six-foot plenum and four averaging probes — conditions that bear no resemblance to a cramped residential closet with a coil slammed on top of the furnace. The takeaway is that no chart, regardless of source, should be trusted without understanding the conditions under which it was created.

Throughout the session, Eric emphasizes a core professional philosophy: understand your instruments, understand their limitations, and understand what level of accuracy is truly needed for the job at hand. He introduces the concept of stacked inaccuracies — where instrument error combines with density correction error to produce readings that can mislead technicians into diagnosing problems that do not exist, or missing ones that do. He concludes with a strong endorsement of the digital TrueFlow grid for residential applications, highlighting its app-based forecasting feature that allows technicians to predict whether a new piece of equipment will work on an existing duct system before the installation begins. The session closes with audience Q&A covering topics such as using density-correcting instruments to compare supply and return readings, and measuring airflow in systems with multiple filter grilles.

Topics Covered

What airflow measurement actually captures: velocity, pressure, volume (CFM), and mass weight of air — and why the distinction matters
The origin and limitations of the 1.08 and 4.5 airflow constants, and when technicians must correct for non-standard air conditions
How air density changes with altitude, temperature, and humidity — including a 22% density drop from sea level to 5,000 feet elevation
Real-world example: how a 3.4% density shift across an operating evaporator coil can be mistaken for duct leakage
Instrument selection overview: large vane anemometers, mini vane anemometers, hot wire anemometers, pitot tubes, and in-duct flow devices
Passive vs. active (fan-powered) flow hoods — accuracy differences and the importance of using residential hoods for residential applications
Proper probe placement for in-duct measurements: ASHRAE guidelines, straight-run requirements, and how turbulence affects readings
Duct traverses: log Chebyshev point averaging vs. timed traverse methods, and best practices for each
Manufacturer performance charts and external static pressure testing: how lab conditions differ from field conditions and why charts can mislead
Motor types (PSC, constant torque ECM, constant airflow ECM) and how motor behavior affects static pressure measurement and airflow setup
Manometer selection: resolution, accuracy, auto-zeroing features, and why a precise-looking display does not equal an accurate reading
Temperature rise method for estimating airflow: appropriate uses with electric heat, and limitations with gas furnaces
Digital TrueFlow grid: application for residential retrofit work, CFM forecasting, and evaluating existing duct systems before equipment replacement
Audience Q&A: density correction on supply vs. return readings, multi-grille TrueFlow workflows, and commercial system setup strategies

You can watch the flow hood comparison video by TruTech Tools HERE. You can also check out all of the great free downloads and other resources TruTech Tools has to offer at https://trutechtools.com/resources.

Have a question that you want us to answer on the podcast? Submit your questions at https://www.speakpipe.com/hvacschool.

Purchase your tickets or learn more about the 7th Annual HVACR Training Symposium at https://hvacrschool.com/symposium.

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