CFM

A few weeks ago, I spent several days redoing a duct system and replacing a furnace for a friend. I had static pressure readings from before the system was replaced, and with the new furnace that I had picked out, I had created a static pressure budget and felt confident I could significantly lower the […]

I once heard someone say, and have since repeated many times that BTU's ride on the back of Cubic Feet of Air or CFM. As I've learned more I've come to the conclusion that the weight or mass of the air passing over the evaporator coil is far more important to understand both in terms […]

Every now and then, a tech will call me and give me the same story of woe. They're working on a system and have the following readings: Low superheat Low suction pressure Low head pressure They'll reassure me that the system airflow is correct. So, what could possibly be wrong? I'll ask how they could […]

  This article was written by my good friend, Neil Comparetto. He is one of the all-around best dudes in the industry and a guy who practices what he preaches on duct design. Thanks, Neil! These are some fundamentals for designing and installing duct systems that I’ve learned over the years. Included are links to […]

Recommended Duct Velocities (FPM) Duct Type Residential Commercial / Institutional Industrial Main Ducts 700 – 900 1000 – 1300 1200 – 1800 Branch Ducts 500 – 700 600 – 900 800 – 1000   As service technicians, we are often expected to understand a bit about design to diagnose a problem fully. Duct velocity has many […]

Service calls about condensing fan motor failure are quite common. Even though fan motor replacement is a standard procedure for residential split systems, it requires careful attention to safety and detail.  This article will give you a step-by-step guide to replacing condensing fan motors. Along the way, we will also explain a few best practices […]

When tightening down a blower wheel or a fan blade on a motor shaft, ONLY tighten it on the flat of the shaft. If you have more than one screw but only one flat surface on the shaft, only tighten the one set screw. Also… Refrain from over-tightening set screws; they need to bite into […]

This article is written by one of the smartest guys I know online, Neil Comparetto. Thanks, Neil! Recently, I posted a question in the HVAC School Group on Facebook that went like this: “When designing a residential duct system, what friction rate do you use?” As of writing this, only one answer was correct according […]

Measuring airflow is easy. Measuring airflow accurately is quite a bit more difficult. In many cases, when we as technicians measure airflow, we are trying to get to the almighty CFM (cubic feet per minute) volume measurement. You can take CFM readings fairly easily with a hood, like the Testo 420, but even a hood […]