How to Inspect AC Ducts

When a flood occurs in a building the cause and location of a mold problem is often quite obvious, and the inspector does not have to investigate very much to find answers. When inspecting in cases where the presence of mold and moisture problems are not obvious, AC duct inspections are without a doubt one of the most important and revealing type of mold investigation to be conducted.  Unfortunately many inspectors do not inspect ducts and AC systems in any detail. A proper duct inspection and AC inspection must be conducted whenever hidden mold or unusual odors are the clients concern.  The following is some information on duct inspection.

To remove or not to remove registers.
When inspecting AC ducts for mold it is important to know how to inspect AC ducts. Start with the most basic and most common since approach. Use a strong flashlight and simply look into the ends of the ducts through the registers. Only remove register covers if you are certain you will be able to reinstall them. When inspecting AC ducts if you remove the register covers you will find removal easy, but it can sometimes be very difficult to reinstall and screw the registers back to the end of the ducts properly.

Do not forget the bottom of ducts
When inspecting ducts it is very important to note that the top and side walls of the duct interiors may appear satisfactory, while the bottom or floor of the same exact duct interior may be severely contaminated with mold and dust. The reason is obvious, when the blower fan shuts down air stops flowing through the ducts, gravity takes over and dust settles out on the bottom of the duct interior leaving the top and side walls of the ducts interior much cleaner. When viewing a duct while standing on the floor the bottom of a dirty duct may not be visible. Standing on even a small ladder can often make contamination on the bottom of the duct accessible for viewing.

What about borescopes
I sometimes us a digital borescope with a flexible fiber optic cable to aid in the visual inspection of AC ducts. At times a borescope can aid in such investigations. Even the bottom of the duct becomes visible when using a borescope. Such devices can be a useful tool but they are not perfect, they have limitations. One limitation of borescopes is that the size and color of contamination in any location inspected with a borescope is distorted. With or without a borescope you can only see so far into a duct. If the duct makes a sudden 90 turn then you will not be able to see beyond that turn. Though borescopes can be an aid, when possible a visual inspection with the unaided eye is often better.

Opening Ducts
If there is an attic space with accessible ducts it can be beneficial to enter the attic and open fiberglass ducts sections for internal inspection. This way the tops, bottoms, and sides, of ducts will be in plain view.  90 degree turns will not hamper inspection. This provides a perfect opportunity to inspect and photograph the interior of ducts for your clients. When opening a duct sampling is easy, but even if you do not open a duct a swab can make sampling through registers easy. You must make sure to never open any ducts unless you are ready and equipped to properly and thoroughly seal each and every duct section after inspection.

Duct endpoints
Fortunately the last few foot of any duct just behind the register is not only the most accessible duct section for inspection, it is also one of the first parts of any duct to become contaminated. Why the ends of the ducts? Warm humid indoor air rises up into the ducts from kitchens, bathrooms, and other humid areas. This moisture condenses in the end of the cold duct before it travels further than a few foot. I have seen this many times while conducting mold inspections. If a duct has only some sections that are contaminate you are more likely going to find that contamination in the more accessible end section of the duct within about 18 inches of the register. Not always true but sometimes it happens this way and benefits inspectors checking ducts for mold.

Inspecting the Supply Plenum
The one duct section more prone to mold problems than the bottom or floor of the ducts, and the last 18 inches of a duct is the supply plenum. The supply plenum is at the furthest point from the registers and the ends of the ducts. The supply plenum is the massive typically square duct that attaches directly to the AC. The supply plenum brings cold air from the AC to the rest of the ducts.  The supply plenum is where mold is often heaviest, and it is often made of fiberglass duct board so it is very easy to cut open with a long sharp knife. Home Depot sells a knife made especially for cutting open fiberglass plenums. Bring foil tape to re seal the plenum after cutting it open. On rare occasions it may be necessary to clean the duct with soap and water for the tape to stick to the duct. Also bring mastic to seal the duct if it is one that will not accept sticky foil tapes adhesives. If you are not inspecting the interior of supply plenums then you are missing an enormous amount of mold and doing your clients a great disservice.

Inspecting The Return Plenum
The return plenum is another important area to inspect, often returns are below AC units thus water from the inevitable condensation pan leak ends up inside and around the return plenum. Often mold will grow unnoticed for months or years inside the return plenum after the condensation pan overflows. Even in the absence of any leaks this area is prone to becoming contaminated with heavy amounts of dust if the filter is missing or does not fit properly.

Trust your Nose
When checking ducts finally the inspector should check for mold odor coming out of the ducts. Make sure the AC is on and smell for mold. I have found to trust my nose more than expensive high tech test for microbial volatile organic compounds blowing out of duct. If you can smell mold odors with your nose then in my opinion it is a problem. Your nose can often pick up problems that our eyes cannot see. If you can smell mold odor from the ducts then it may not be necessary to inspect further in the unit or ducts because you know the AC and the ducts will have to be cleaned professionally.

Remember the Coils
I know this is supposed to be an article on inspecting moldy ducts, but as we all known the interior of the air handler is a good place to inspect as well. The bottom of the evaporator coils is an extremely important area to conduct inspections. The bottom of the evaporator coils will often be moldy when the rest of the AC and the rest of the home are fine. So remove that filter then check for condensation leaks, check for mold and dust in the return, and check for mold on the bottom of the evaporator coils.

What is that Grey Stuff?
That fuzzy grey stuff that looks like velvet on the bottom of many evaporator coils and inside supply plenums is Cladosporium mold. The black dots on registers are also often Cladosporium mold and sometimes Pen / Asp. The Clear slimy or clear jelly like substance on the bottom of coils and in the condensation pan is bacillus (rod shaped) bacteria. I know this to be true because I have samples materials inside AC units hundreds of times. I have analyzed these samples many times directly under the microscope, and I have sent such samples to various labs countless times and have almost always found the fuzzy grey material to be Cladosporium mold. Of course AC systems are bound to have various types of mold growth including Pen Asp and many others, however it is a fact that the majority of the mold you find inside AC systems is Cladosporium mold.


About the Author
Daryl Watters is a licensed mold inspector with 54 hours of air conditions I and 54 hours of air conditions II courses under his belt. In addition he has written extensively on the inspection of mold in AC units, ducts, and homes. He has preformed mold inspections since 2003.


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