Short #64 – Diagnostic Duct Design

Balancing complaints are common in the HVAC industry. In this short podcast, Bryan explains how diagnostic duct design solves those issues.

“Diagnostic duct design” refers to using the duct system to locate and solve a customer's comfort problems. If a couple of rooms have problems with humidity control, then the duct system could be a culprit. However, before we even touch the ducts, we should look at the space to determine if we have issues. For example, radiant gains from a window could be contributing to comfort problems, not the duct system. Airflow may also not be an issue if comfort at night is an issue. That's a matter of the equipment cycling less often at night, and we can solve that by reducing the setpoint at night.

When we look for duct issues, we want to assess the pressure. You can do very simple tests with a manometer (or a qualitative test with tissue paper under a door crack) to look for pressure imbalances, which can cause discomfort in rooms where the door is closed very often. Flow hoods are good for assessing airflow, but you can also get an airflow approximation by measuring air velocity. Make sure you're hitting your targets; then, you can check your static pressure.

Since distributed airflow is a major comfort factor, you can take the total CFM and divide it by the square footage (factoring in each room's square footage) to determine the airflow distribution. Remember: Perimeters require more airflow than the centers of rooms, and rooms with more windows will have greater radiant gains to account for.

When you can't redesign the entire duct system, use balancing dampers in oversized ducts to help balance the airflow. (Make sure the register isn't oversized, though! Try to keep the static pressure down, too.)

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