Air Flow and Mold

Most of our calls are homeowners and renters that are extremely worried about indoor air quality (IAQ) these days. Most of our customers do not have these worries. They know! We have diagnosed and solved their IAQ concerns, or worked with their own HVAC/R contractor to solve what we have diagnosed.

This is from a recent visit.

The homeowners spotted some “mold” around their supply registers in their living and dining rooms. They got extremely worried that they may have to move out of their house. A call to their insurance company got a mold investigator our to take some samples and send to the lab. The diagnosis is that it was mold, not black mold, but still was mold.

Possible mold near ceiling register

The mold investigator is not an HVAC/R expert so we received the call to diagnose their Air Conditioning system and find the cause of the mold. Initial checks to the system found nothing wrong. The installation was nice and tight, the contractor did a good job. System relatively new, well maintained, and clean.

However, using additional readings and tools we found that there was not enough return air to the air handler, and thus the supply air was about 50% of what was supposed to be coming out of the supply registers. This was the problem.

Why? Because registers have a thing called “throw” and the design of the register creates turbulence that mixes the cold air with the warm moist air in the ceiling to cool the room. The throw and mixing ability are directly dependent upon the air pressure, speed, and quantity in the ductwork. If you do not have the proper air quantity, velocity (speed), and pressures the cold air will not mix properly with the room air. This can cause significant moisture problems in a warm humid environment, such as in Florida.

Think of this example. If you take a glass of ice water and place it on a kitchen counter in a warmer room and then walk away from it for an hour, what will you find? That’s right, a puddle of water next to and around the glass. This puddle is not because the glass was leaking, but it is due to condensation. The humidity in the air is attracted to the cold glass, and condenses onto the outside of the glass.

This is the same thing that happens with the metal supply register that has cold air trickling out of it and not mixing properly with the room air away from the register. The cold air, usually about 20 degrees colder than the surrounding room air cools the metal of the register, and the ceiling right around the register. Because there is not enough throw if the air velocity and quantity is too low there is not enough mixing. So, the cold grill causes condensation almost exactly like the cold glass of ice water. Condensation with sheet rock is a recipe for mold.

In this case we also found significant buildup of possible biological growth around the air handler, on the supply plenum. In Florida depending on the location of the air handler a little growth is expected, but wet and really significant growth should be looked into.

So, what is the solution? This depends upon your system, the installation, the ductwork, and the numbers we get from our test instruments. Sometimes using a different filter will work, sometimes adding an additional return will work, and sometimes it becomes a more significant ductwork modification.

If you are having similar problems, or your system is too noisy, or you just want to know if everything is working properly, give us a call at


and find out. Please leave a message if you receive our voicemail. In order to keep our costs low, and thus keep your rates low, we do use voicemail in order to help manage our calls.