For those who experienced discoloration (cannot say the “m” word here) atop your air handler, improper plenum construction and implementation is most likely the cause. One of the many hands-on tasks at HVAC Technical Academy is constructing supply plenum.
HVACTA: Supply Plenum

Plenum Time
A supply plenum is an enclosed chamber that connects directly to the outlet of an HVAC unit, such as a furnace, air conditioner, or heat pump. Its purpose is to act as a central distribution manifold, gathering conditioned air (heated or cooled) from the equipment and evenly distributing it to the various supply ducts that lead to different rooms throughout a building.
When it comes to airflow, the supply plenum operates at a positive pressure to push air out.
The group is working on their supply plenum project right now, if the weather allows, our next post will highlight the project and get air flow at its peak.
If you enjoy daily challenges, working with your hands, and never a dull moment, the HVAC industry may be a good fit for you and HVAC Technical Academy can help you get there quickly, we are talking six weeks.
Danny
407-878-0474
2nd Chance University
HVAC Technical Academy
Certifications and Safety
DHuffman@HVACTA.com
Sanford, Florida