Two Common Causes Of AC Mold

January 6, 2008

Some people believe that AC ducts are destined to become moldy. While all AC ducts will likely grow some mold and will become discolored with visible mold or visible dust to some degree, it is not normal or healthy for ducts to become excessively moldy, or to produce a strong mold odor.

Mold spores from AC ducts result in health problems and even mold odors in the absence of spores have been shown to cause health problems. Keeping ducts free of excessive mold is important.

The information in this blog is intended to provide readers with information on:

1) Some reasons why AC ducts become moldy.

2) The importance of proper sizing of AC units.

3) In addition it draws attention to a very common indoor air quality problem called entrainment. Entrainment is when AC units pull contaminated air from inside attics or walls or other undesirable areas.

The following information is taken from a Broward County mold inspection we conducted on 11-17-07 it shows an actual example of how ducts became moldy and gives recommendations at the end.

The property inspected is a one story single family Broward county house built in 1974.

Client reported mold growing on AC vent registers. The mold started a few weeks ago. The client reported that a new larger AC unit was installed about 3 months prior to the mold starting. This inspection is for mold conditions at the AC registers only.

mold humidity and AC instillation

Location: Garage:

In the above referenced area the inspector observed a new air conditioner. The serial number on the unit indicates that it is a 4 ton unit, thus is it rated to cool a 2400 sf house. This AC unit is just a little large for this house.

The inspector also observed a new condenser / compressor unit. The serial number indicates that it is a 3.5 ton unit thus designed to cool a 2100 sf house. This is also slightly large for the house.

You have a newer larger AC unit.

The size of this newer AC unit is a major reason for why you have humidity related cladosporium on your AC registers that were previously not moldy.

The previous AC unit in use about 3 months ago was smaller than this AC system thus would have been more efficient at drying the houses air before cooling the houses air and shutting down. If an AC is to large it shuts off after the air is cooled but before the air is properly dried / dehumidified. Air that is not dehumidified and has a humidity above 60%RH to 65%RH can and often does cause humidity problems in Florida homes.

The above photo is of cladosporium mold growing in your registers.

Another reason why you have mold problems now is because of the two holes in your AC return. These holes in the wall studs allow the AC to pull humidity into the AC each time the AC turns on, this phenomenon is called entrainment and is a common cause of mold and humidity problems.

Yes all metal studs have these holes, but it is not typical for these open holes in the studs to be exposed and in such close proximity to an AC return.

Where does the humidity in your walls come from? it comes room the attic, the attic in turn gets the humidity from outside via the soffit vents like the one in the above photo.

Why did this problem not occur till recently? the older smaller AC was more efficient at drying air as stated above and thus was able to handle the humidity load created by these openings.

AC duct mold air conditioner mold

More photos of mold on and around your AC registers. Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. This mold problem is caused by elevated humidity in your ducts.

This is not likely to cause a toxic reaction, but this mold may result in allergic reactions or act as an asthma trigger in sensitive persons. The real concern is if it is growing excessively deep in your ducts then you will have much more mold than you can see on the registers.

Mold growth in the ducts can and often causes health complaints.

Excessive mold was not observed in your AC unit but you are recommended to have your ducts cleaned in accordance with NADCA standards or replaced.

More photos of mold on and around your AC registers. Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold.

Based on this inspectors observations and conclusions your inspector recommends using good quality foil mastic tape or other methods to seal two openings in your AC return wall studs.

Have a AC company install a humidistat and set it to around 50% to 60%RH always maintain indoor humidity between 30% and 60%RH.

Most people live with a little mold in the ducts and have no problems. Mold spore levels in your air was very low. No mold odors were detected. Much of the mold appeared to be primarily in the ends of the ducts just behind the registers.

Have an AC company clean the mold from the registers and from the ends of the ducts just behind the registers.

Duct removal or good professional cleaning in accordance with NADCA standards is recommended if additional hidden mold is discovered inside the ducts during cleaning.

Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.

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2 Responses to “Two Common Causes Of AC Mold”

  1. ROESEMARIE MARONE said

    Have had problems with mold on coils. This unit is slightly over rate for the house (2000sqft.) Learned of the problem from a new company who wanted to service the unit. The first time they came they noted that the unit was only a year old, and the coils were rusted and covered with mold, earlier in the summer the ceiling fell in as condsation was forming in the drip pan. It was also noted that the installing company had not drawn a county permit. They were reported to the county. They cthan pulled a permit and supposely corrected the problem. The inspector came and signed off on the job. This year when the unit was inspected the job still was not correctly done. I now have been experiencing breathing problems. I would like to have this problem re evaluated and corrected. Please call for an appointment after Dec 5th. Thank you 239 466 5616

  2. James said

    Hello–
    I was suffering from sleep problems due to the air being very dry from the heater in the winter and too much a/c in the summer. To lessen the problem, I simply taped the vent in my room so little air passes through. However, I notice condensation that drips after forming on the tape and now see lots of mold spots around the vent. Any real concern other than cleaning off the area and replacing the tape? Is this black mold? Thanks, James

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