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Tech Tips:

Troubleshooting and commissioning are entirely data-dependent processes. How can you know that a system is undercharged unless you measure the superheat and subcooling? How can you know how much static pressure a blower motor has to overcome? In both cases, we use tools and measurements to help us determine the next steps. However, using the […]

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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 short podcast episode, Bryan talks about psychrometrics and the magic line inside a messy-looking chart. The psychrometric chart is a key tool for understanding the relationship between air and water.

“Psychrometric” comes from the Greek roots for “cold” and “measurement.” As such, a tool called the psychrometer (with paired wet-bulb and dry-bulb thermometers) could measure two different temperatures, and we could use these to determine how much water vapor the air can hold (even though the air doesn’t “hold” water vapor; it’s all about vapor pressures).

The psychrometric chart was developed with the contributions of James Apjohn and William Ferrell in the 1800s, but Willis Carrier was the one who drafted the paper titled “Rational Psychrometric Formulae” in 1911. He created a chart that plotted temperature, humidity, and enthalpy on a single piece of paper.

The horizontal axis represents the dry-bulb temperatures, the curved lines represent relative humidity, the left axis represents humidity ratios, the diagonal lines represent wet-bulb temperatures, and the other angled lines represent enthalpy. Every possible air condition can be plotted on the chart. Moving horizontally changes sensible heat (temperature), and moving vertically changes latent heat (moisture content); both are critical to human comfort.

The “magic line” is the dew point. When this line is crossed, that’s when surfaces like ducts or HVAC coils begin to sweat, as air can’t hold any more moisture. You can use this information and the psychrometric chart to help you make sense of customer complaints (e.g., “clammy”) and predict how air and moisture will behave if you adjust airflow or add insulation.

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Psychrometrics and the Magic Line – Short #265

11/11/2025

Bryan Orr

Learning Hydronics, Boilers and Steam w/ Ray Wohlfarth

11/06/2025

Bryan Orr