“For God did not send his Son into the world to condemn the world, but in order that the world might be saved through him.” — John 3:17
klein cooling ac university header graphic link

CALL NOW
Skip to main content
A/C University
Table of Contents
< All Topics
Print

Ride Along: Mold on a Supply Plenum in TrailMark — Why We Rebuilt the Duct Connection

Mold Confirmed on the Supply Plenum

We were called to a home in TrailMark, in the World Golf Village area of St. Augustine, after biological growth was found on the air-conditioning supply plenum. A mold assessor had confirmed that the growth was mold. Our role was to address the affected HVAC ductwork—not to perform mold assessment or remediation of the home.

The Original Ductwork Design

The air handler had an unusually short supply plenum, approximately four inches tall, mounted directly on top of the unit. From that small box, a short section of flexible duct extended upward to another duct box at the ceiling, where the supply ductwork continued into the attic.

We weren’t fans of this arrangement. It added a short flex connection and extra transitions between the air handler and the ceiling duct box without an obvious benefit in this installation. That does not prove the configuration caused the mold, but when replacing the affected ductwork, we saw no reason to reproduce a design we considered unnecessarily complicated.

What We Did Differently

We removed the affected supply plenum and connecting ductwork. Rather than install another four-inch plenum with a short flex duct rising to the ceiling, we fabricated a new, properly fitted four-piece supply plenum extending from the top of the air handler directly to the duct box coming through the ceiling from the attic.

We sealed the new connections from the inside and outside and checked the completed assembly for air leakage. No air leaks were observed at completion. The result was a more direct connection with fewer unnecessary transitions.

A Related Concern We See in Zoned Systems

Many two-story homes use zoning to control upstairs and downstairs temperatures separately. When properly designed, installed, and commissioned, a zoned HVAC system can work very well.

Problems arise when the ductwork, dampers, blower operation, and controls aren’t designed to accommodate the airflow requirements of the equipment under every operating condition—particularly when only one zone is calling and the other zone’s dampers are closed.

If the remaining open zone cannot handle the required airflow, the system may experience high static pressure, reduced airflow, excessive noise, and—in cooling mode—an increased risk of evaporator coil freezing. Poorly insulated or leaking duct connections and cold surfaces exposed to humid air can also create condensation conditions that support biological growth. High static pressure alone does not prove why mold developed.

Does Every Zoned System Need a Bypass?

No. A bypass duct is one possible design strategy, but it is not automatically required or always desirable. Some systems are correctly designed without one, using appropriately sized ducts, compatible variable-speed equipment, zone controls, or other airflow-management strategies. A poorly designed bypass can create problems of its own.

The important question is whether the system maintains the manufacturer’s required airflow and acceptable static pressure when each zone operates independently—not simply whether a bypass duct is present.

We’ve encountered several zoned systems in newer homes where the installation deserves a closer look. That does not mean all zoned systems, or all homes in TrailMark or World Golf Village, have a problem.

What Homeowners Should Watch For

Persistent sweating around supply ducts or vents, musty odors near ductwork, unusually loud airflow when one floor is calling, repeated coil freezing, or large comfort differences between floors are all reasons to have the HVAC system evaluated. Those symptoms can have multiple causes, and an actual diagnosis matters more than assumptions about the home’s age or zoning layout.

The Result

We replaced the affected ductwork with a direct, fabricated supply plenum, sealed the assembly inside and out, and found no air leakage upon completion. This work corrected the affected duct connection; it should not be mistaken for a claim that every possible moisture or zoning concern in the home has been diagnosed or permanently resolved.

If you’re in TrailMark, World Golf Village, St. Augustine, or the surrounding area and have questions about sweating ducts, supply plenums, or zoned HVAC airflow, contact Klein Cooling.

Comfort. Integrity. Legacy.