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.


In coastal Florida if your doors and windows are weathered and leak excessive amounts of humid air, a heavenly ocean view can allow for conditions that result in a hellish AC mold problem.

The information in this blog give a text book example of how warm tropical ocean breezes infiltrating into an AC system can result in humidity, condensation, and mold in and around your AC ducts and registers.

This information is from a Broward county Ft Lauderdale mold inspection that took place on 11-8-07.

Client moved in as a renter 7 days prior to this inspection. Client reported that his fiancee moved into the property with him and enjoyed the property, but because of severe allergic reactions that occurred while in the property she moved out after just one day.

Client requested this investigation to help determine if mold or other similar bio-allergen may be at the property that may account for her complaints.

The inspector observed that mold was a problem in the AC unit and in the AC ducts at the property, this mold requires food and moisture to live.

As is almost always the case in moldy AC units the mold in the AC and ducts received it’s moisture from elevated humidity and from condensation in the AC and ducts. It recieves it’s food from dust that bypasses the filter.

Humidity entered the AC and ducts by first entering from outside.

The view outside the unit is excellent, it is this high ocean front vantage point combined with leaky sliding glass doors that provided the moist breezes that allowed this mold problem to occur.

In addition the master bathroom is not vented to the exterior, whenever someone takes a shower that shower steam can enter the AC unit and form condensation as well. Either the non vented bathroom, or the leaky sliding glass doors overlooking the ocean alone would have been enough to cause a problem.

The sliding glass doors did not fit air tight, in fact the inspector could feel a strong ocean breeze blowing through the sliding glass doors from 2 foot away even though the doors were closed as tight as possible. Humidity blowing indoors from outdoors often results in mold and humidity problems, this is sometimes called the paston effect.

Gaps in these old doors allow much moist ocean air in.

Mold requires not just moisture but also food. The mold in the AC system and ducts at this property received it’s food from dust as does most AC mold.

In the above photo you can see a hole where dusty air can bypass the AC filters and enter the AC unit.

Dust can also by pass the filters because they are very low quality and do not fit properly, dust goes around the filters.

AC mold and condinsation problems

During the mold inspection the inspector observed that condensation sometimes forms on the AC registers.

The inspector observed the water stains left behind by condensation dripping off the AC registers. Note that condensation forming on AC registers is not normal, this condition indicates that so much humidity was present in the property that it condensed out on cold metal & glass registers that typically reach around 60F or so when in operation. The natural progression of this condition is for mold and condensation and water damage to form on the drywall around the register if the humidity problem is not corrected.

Elevated humidity and condensation inside ducts and AC units causes mold to grow in ducts and AC units just as it developed on the visible sections of the outside of registers. It is common for homes with mold on registers to have much more mold inside the ducts and AC unit because the interior of the ducts and AC is much colder thus much wetter than the surfaces of the registers, also the interior is duster thus has more food for mold growth.

Because mold in the AC unit has better growth conditions than mold on the registers it exhibits dimorphism. In other words these mold look different in different environments. Most of the mold in the AC unit is the exact same type that formed on the registers but being in a more healthy environment with more food and water inside the AC unit it grows more thick and lush and sometimes takes on a fluffy grey velvety appearance. In each case the mold is cladosporium sphearisperium.

Mold inside AC units does not produce many spores but instead grows and reproduce more vegetatively. This is significant because mold in AC units do not produce many spores but will grow profusely feeding off condensation and dust. AC mold by products are not lots of spores but are Mvoc’s, these Mvoc’s are foul smelling moldy odors that are released by the molds and this is why the clients fiancee got sick.

It is a well accepted fact that Mvoc’s or mold odors produced by AC type molds and other molds can and very often do result in strong allergy like reactions and irritations.

In this inspectors experience AC units that grow mold in the amounts seen at this property very often produce health complaints in building occupants.

In conclusion the AC unit’s ducts, registers, blower fan, evaporator coil, condensation pan, wires, and most of all interior fiberglass insulation are infested with more than normal amounts of mold growth, and the grow is extremely consistent with conditions that this inspector has observed to result in both odors and health complaints.

Because the AC unit is extremely contaminated with mold and produces strong foul odors it is recommended that the unit be professionally cleaned in accordance with NADCA standards, or the unit should simply be replaced because cleaning the unit will likely result in damage to the very rusty pan and coils.

Because of the amount of contamination in the ducts it is recommended that the ducts be replaced. Cleaning fiberglass ducts is not recommended they are supposed to be replaced if contaminated. Note that the interior of all sections of the ducts will not be visibly moldy but expect mold contamination to have spread to many different duct sections.

The following photos are of mold on AC registers, in the AC ducts, and in the AC unit at this property. Note that the ducts should all be bright yellow and resemble a tennis ball in color and texture.

AC duct mold

Typical cladosporium mold colonies on an AC register. Nearly all registers at the property were visibly moldy.

black air conditioner mold spots

Close up of cladosporium mold colonies on the AC register.

HVAC mold

Close up of mold in the duct, this duct used to be yellow.

Mold in the AC unit. The white mold is likely pen asp and the grey mold is cladosporium sphearisperium, it is the same type of mold observed as black spots on the register, but it looks different inside the AC because of different environmental conditions inside the AC.

Mold on the AC unit panel insulation.

Fluffy grey cladosporium sphearisperium mold on the blower fan inside the AC, this is very common.

About the Author:
Daryl Watters is a certified indoor environmentalist CIE # 1952 who provides mold testing and IAQ consultation services in South East Florida.