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Tech Tips:

I remember it like it was yesterday. It was my first day of work as a trainee at my first technician job, just a wet-behind-the-ears kid fresh out of trade school. It was a Monday morning, and the technicians and I were standing in the dusty warehouse surrounded by stacks and stacks of brand new […]

How to Set a Charge by Superheat

So many techs are OBSESSED with how to set the charge rather than understanding all of the readings, specs, and system conditions that go into optimum system performance. Before “setting the charge,” I suggest (nay, I REQUIRE) that you do a full visual inspection of the system, have an understanding of the initial factory charge […]

Superheat Talk w/ JD Kelly

In this episode of the HVAC School podcast, host Bryan discusses superheat with guest JD, who recently co-authored an article on the topic with Ty Branaman. The conversation explores the often misunderstood concept of superheat, its importance in HVAC diagnostics, and common misconceptions technicians have when measuring and interpreting it. Bryan begins by sharing […]

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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 episode, Bryan and Roman talk about the psychrometrics of a hidden moisture failure… behind the drywall. Roman had an indoor air quality and mold assessment in his own home, and he learned that there were active mold spores in his home; he shares what he learned from the experience.

Roman had a bulk water issue from plumbing (though bulk water issues can also occur outside the building or from condensate drainage problems). Walls are NOT solid; they have plumbing and insulation inside, and the air spaces between the walls support a microclimate when moisture is in there, regardless of whether it’s water vapor or bulk water. In Roman’s case, the bulk water failure came from the plumbing leak; it saturated the slab for half the house. The water was absorbed by the concrete, and any surface that touched the concrete transferred that water over time.

The mold assessor used two different devices. The pin-type meter punctured the surface of wood or drywall and measured the resistance between the probes (water transfers electrons between them); the measurement revealed the water quantity. A pinless meter determined the source of the failure via high-frequency radio waves, and it measured the capacitance to determine whether moisture was present within or behind the surfaces. The results showed that water was behind tile, baseboards, and drywall.

Thermal imaging has also proven to be a useful tool to see the cooling effect that occurs when water evaporates from liquid to vapor inside a wall, but the cold spots go away when the space thermally equalizes. While HVAC professionals cannot assess mold, we can use tools like thermal imaging to help determine when it’s time for a customer to call someone who can help; we are still responsible for our customers’ health and comfort in their homes.

Learn more about building science concepts, especially moisture and its effects on buildings and comfort, from the Building Science Corporation’s many great resources at https://www.buildingscience.com/.

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Behind the Drywall – The Psychrometrics of a Hidden Moisture Failure – Short #298

08/11/2026

Bryan Orr

HVAC Education. What NOT to Do w/ Jim F., Roman B. and Craig M.

08/06/2026

Bryan Orr