Air Duct Mold

What is Air Duct Mold?

Air Duct Mold (and mildew) are fungi. Fungi are neither plant nor animal but, since 1969, their very own kingdom. The fungi kingdom includes such wonderful microorganisms because the scrumptious edible mushrooms, the manufacturers from the “miracle drug” penicillin and also the yeast which makes our bread rise and our fine wines ferment. Biologically, all fungi have defined cell walls, lack chlorophyll and reproduce by way of spores. Roughly 100,000 types of fungi happen to be described which is believed there are a minimum of that lots of waiting to become discovered. Most fungi feast upon dead or decaying organic matter – they are among the principle agents accountable for natural recycling of dead plant and animal existence.

The most typical fungi are ubiquitous inside our atmosphere and we’re constantly uncovered for them. Typically, however, illnesses brought on by these common fungi are relatively uncommon and therefore are rarely present in people with normally functioning natural defenses.(1)Nevertheless, mold has lately experienced much talked about press coverage. There are a number of inflammatory press reports concerning lawsuits over quality of air in homes, courthouses along with other structures parental concerns regarding school classroom environments home insurers refusing to pay for mold damage and broadly distributed news reports on so-known as “toxic mold” Try not to panic. Mold could be managed effectively generally which guide can help you do this.

Mold Growth in AC Ducts

You will find 4 critical needs for mold growth – available mold spores, available mold food, appropriate temperatures and considerable moisture. Removing these products will stop mold growth. Let us examine each requirement, one-by-one.


Picture of Magnified sporangiophore of the Pilobolus.
Magnified sporangiophore of the Pilobolus — “the shotgun fungus.” The small dark knobs at the top of each stalk contain the spore sacks that are “shot” up to 6 feet into the air toward the sunlight by this unique fungus.


Air Conditioner Mold Spores 

Varying in dimensions from three to 40 microns (real hair is 100-150 microns), mold spores are ubiquitous – they’re literally everywhere. There’s no reasonable, reliable and price-effective way of eliminating them from environments that humans inhabit. So, attempting to control mold growth by reducing mold spores isn’t achievable.

Mold Food In AC System

If the 3 other needs are met, just about any substance which contains carbon atoms (organic substance) will give you sufficient nutrients to aid mold growth. The oil out of your skin that’s left whenever you touch a normally unacceptable surface, like stainless, or even the soap residue left from the good cleaning will give you sufficient nutrients to aid the development of some molds. And some of the most common materials present in homes like wood, paper and organic fibers are some of the most preferred of mold nutrients. Thus, eliminating mold food out of your atmosphere is really a virtually impossible task.

Appropriate Temperatures For Mold Growth inside AC Ducts

Regrettably, most molds grow perfectly in the same temperatures that humans prefer. Additionally, anybody that has washed their refrigerator rapidly knows that temperatures near to freezing aren’t cold enough to avoid mold growth and temperatures which are much warmer than humans prefer, like individuals from the tropics, will grow abundant amount of mold. Therefore, it’s not achievable to manage mold development in the house atmosphere with the charge of temperature.

Mold Considerable Moisture

Most molds requires the existence of considerable moisture for growth. Clearly, the term “considerable” is essential here. The mycologists (fungi scientists) make reference to “water activity” when describing the needed conditions for mold growth. The different types of mold have different water activity needs. A material’s “water activity” is the same as the relative humidity from the air that might be in equilibrium using the material at this material moisture content. Most mold species require “water activity” levels which are equal to material equilibrium moisture contents akin to relative humidities with a minimum of 70%. Actually, almost all of significant, large mold outbreaks inside structures occur where porous, cellulose-type materials have literally been stored wet by liquid water or sustained condensation.

People prefer humidities which are underneath the critical relative humidity for mold growth inside ac system. Thus, from the four fundamental needs for mold growth, moisture availability is definitely the simplest mold growth requirement to manage in environments that humans prefer to inhabit. Because you will see from the rest of this informative guide, and from most the literature on mold control, the consensus regarding effective mold control strategies includes the mixture of lowering the accessibility to moisture and killing and removing active mold growth colonies


Determining If You Have Mold

Image result for florida moldImage result for florida moldImage result for florida moldImage result for mold in ac ductsImage result for mold in ac ducts
Image result for florida moldImage result for mold in ac ductsImage result for mold in ac ducts

Image result for mold in ac ductsImage result for mold in ac units

Image result for mold in ac unitsImage result for mold in ac units



Image result for florida moldImage result for mold picturesImage result for mold pictures

Image result for mold picturesImage result for mold pictures

Image result for mold picturesImage result for mold pictures

Although most active mold colonies appear green to black (usual for mold growing on bathroom tile grout) colored, the options of mold colonies growing behind vinyl wall covering in structures assumes completely different characteristics. These mold outbreaks typically lead to pinkish to yellow-colored staining from the wall covering. They’re quite important simply because they indicate serious, harmful moisture accumulations inside the gypsum wallboard behind the wall covering that cannot be removed from your ac or dehumidification systems. Where these complaints appear, they often require the help of an expert outfitted with pressure measurement along with other diagnostic equipment to look for the source(s) from the moisture resulting in the

 Minimize Mold Growth In your AC System

The next practices can help minimize the development of molds inside homes situated in hot, damp climates like Florida’s. Mold growth around the outdoors of homes isn’t covered here and visitors cautioned these practices might not be relevant in other climates.

Ac Operation :

Always set the fan mode switch of the ac thermostat within the AUTO position, never within the ON position. Why? When set towards the ON squeeze blower fan runs continuously and also the moisture that has condensed in your air conditioner’s evaporator coil during cooling is re-evaporated and blown back to your house before it may drain from the coil and from home. This will cause the relative humidity in your house to become considerably more than when the ac thermostat fan mode switch is placed towards the AUTO position. Even just in the “auto” position, some air conditioning units run the blower for 1-3 minutes following the compressor turns off. To maximise dehumidification, it is advisable to disable this selection. A professional mechanical contractor will be able to disable this selection so the blower and compressor switch off concurrently.

Ac Selection:

If you’re creating a new house and may choose, then choose an aura conditioning system having a variable speed air handler as well as an operating selection way of “enhanced moisture removal.” This is an excellent choice for multiple reasons: the units are SEER 14 , they’re quiet plus they perform a better job removing moisture, particularly under part load conditions. They make this happen by beginning the environment handler fan in a lower speed during each cycle, which improves moisture removal. The variable speed fan motors are intrinsically more energy-efficient– they will use less than 270 W/1000 cfm of ventilation compared to the normal 450 W/1000 cfm. Each one of the major manufacturers ask them to. When used correctly, perfect for Florida’s climate.

Ac Oversizing:

Oversizing of air conditioning units is typical. The greater an ac is oversized, the poorer its humidity removal performance, especially at greater thermostat settings. It is because, during each ac on cycle, the moisture removal doesn’t achieve full convenience of concerning the first three minutes of operation. The greater the machine is oversized, the shorter the on-cycle where moisture is taken away. Thus, if your house is correctly sized having a 2-ton ac along with a 4-ton product is installed, the two-ton machine would perform a far better job removing moisture although the 4-ton machine had two times the nameplate humidity removal capacity (Btu/hr). Remember, the shorter the ac on-cycle, the less opportunity for effective moisture removal. This fact could be clearly observed in the figure below, that is obtained from FSEC test data.(2)



Graph: Impact of Air Conditioner Cycle Length on Humidity Removal.

The following table of results from experiments that were conducted by the Florida Solar Energy Center illustrate the relative humidity impacts of both thermostat set point temperature and the position of the fan mode switch.

Impact of Indoor Set Temperature and Fan Operation
Mode on Interior Relative Humidity*
Indoor T
Avg RH %
(Fan= Auto)
Run Time Fraction
Avg RH%
  *  AC operated alternatively for at least one week in each mode.

Interior Air Conditioning depressurization

Interior doorways ought to be stored open when ac unless of course your cooling and heating system includes a fully ducted return air system from each room of the house or unless of course specific and sufficient return air transfer pathways have been installed to make sure that closed interior doorways don’t lead to space depressurization problems in your home.

AC Space Pressurization:

It is crucial that homes in hot, damp climates be pressurized slightly regarding outdoors. This is because fairly straightforward although not very apparent. If homes are depressurized with regards to the outdoors, then hot, damp outside air is going to be pulled with the really small air pathways which exist in most building envelopes (walls, ceilings, floors, etc.). To achieve with a home outdoors of the house towards the inside, this air frequently are required to follow circuitous pathways. For instance, the environment may go into the wall system high in exterior where an outside fitting is mounted and exit the wall system have less the inside where a wall outlet is situated. If the house is air-conditioned, the gypsum wallboard is going to be relatively cold – frequently cooler compared to dewpoint temperature from the damp outside air that has to flow along that gypsum wallboard to that particular indoor electrical socket. In Florida, it’s not uncommon for summertime outside air dewpoint temperatures to become more than 80 F! When this happens, the cooler gypsum wallboard can act much like that ice tea glass that “sweats” constantly whenever you go outdoors – it may condense the moisture from the air that’s flowing along its back surface coming towards the electrical socket that’s becoming its path in to the air-conditioned home. As highlighted within the figure below, this can lead to moisture accumulation inside the wallboard, which, consequently, can lead to significant mold growth.


Picture of Comparison of Wall System Moisture Contents.

  • The above mentioned wall diagrams from detailed computer simulations that model the combined impacts of warmth, moisture and air transport (3) illustrate the significance of this ventilation phenomena. The wall around the left bounds an area that’s pressurized with regards to the outdoors and also the one around the right bounds an area that’s depressurized. The Two Pa (Pascal) pressure gradient is extremely, really small – you will find 101,325 Pa in a single atmosphere. Clearly, it’s the direction from the pressure gradient instead of its magnitude that’s crucial here. You greatly would like your home slightly overpressurized in hot, damp climates to ensure that dry, awesome indoor air is pressed from the home with the walls (you’ll need left) instead of have hot, damp outside air drawn in to the home with the walls (you’ll need right). Fortunately, it’s relatively simple to pressurize a house – everything is essential is the fact that a little more air be introduced in to the home than is exhausted. This takes an optimistic mechanical ventilation system.Stuff that could cause space depressurization in homes:- Exhaust fans (bathroom, kitchen, attic room, crawlspace, etc.)- Cloths dryers- Supply duct leaks- Inadequate return air pathways because of interior door closureFans: Use fans within the summer time – they make you comfortable at greater ac thermostat temperatures. And they’ll save ac energy costs if you are using the best ones (see Gossamer Wind® Series – offered at Lowe’s stores) and switch them off when nobody is within the room.Appraise the RH in your house: Purchase a digital temperature and relative humidity (RH) sensor (about $20) and observe your indoor relative humidities. Two sources of these sensors are Radio Shack and Therma-Stor Products. Throughout the hot summer time several weeks, using the ac on, the RH shouldn’t exceed 55% throughout the day regularly. Whether it does, you most likely have issues either with leaks inside your duct system or together with your ac unit itself – it may be too big, incorrectly billed and have inadequate ventilation over the coil. Meet with a qualified ac expert or mechanical engineer to look for the problem.The greatest relative humidities in your house will probably occur during mild weather whenever your ac isn’t needed throughout the day. In Florida, the outside relative humidity reaches very close to 100% of all nights, no matter daytime temperatures. If your house is available to the outside over these periods, the types of materials in your house will adsorb moisture out of this very damp air, again whatever the temperature. If the moisture isn’t removed during the very next day, the “water activity” from the materials in your house can remain at or over 75% for longer periods and mold will probably grow on these surfaces. For those who have RH levels exceeding 70-75% for longer periods, and find it hard to control mold growth on surfaces in your house in spring, winter and fall, you may want to purchase some form of dehumidification system and really should meet with a qualified mechanical system expert for advice.


Picture of Pink Indicator for Mold.
Note the pink “splotch” at the bottom-center of this photo. It is the telltale warning sign that there is a likely mold “bloom” behind the vinyl wall covering.



Impermeable interior surfaces like coverings can lead to severe mold problems in hot damp climates for example Florida’s. Moisture originating from outdoors build up that’s behind the wall covering. This normally occurs because of house depressurization where outside air has been drawn in to the home with the very minute air pathways which exist in most normal wall systems. Where this issue occurs, outbreaks of mold frequently occur underneath the wall covering on the top of wallboard. This mold growth is generally characterised by pinkish to yellow-colored “splotches” around the vinyl wall covering. The moisture accumulation is also severe enough to result in the gypsum wallboard to badly deteriorate and be “mushy.” For those who have observed these signs and symptoms, a structure science professional ought to be consulted. Positive pressurization of your house is a approach to minimizing the possibility occurrence of the problem.




AC Return Air Pathways:

It is crucial that there be adequate ventilation pathways for that supply air that’s sent to each room of the home to go back to the environment conditioner’s air handler unit (this area using the blower fan). Otherwise, negligence the house that contains the primary go back to the environment handler unit is going to be “starved” for air, leading to depressurization of the space with regards to the outdoors. If the occurs, outside air is going to be attracted with the small pathways which exist within the exterior building envelope. In hot, damp climates like Florida’s, these air flows can lead to the buildup of moisture inside the gypsum wallboard, particularly if it’s vinyl wall covering. This, consequently, can lead to the rapid and abundant development of molds – remember, the cellulose (paper) on gypsum wallboard bakes an excellent, preferred mold food.If room doorways are stored open, you will see sufficient return air pathways. However, if rooms doorways are closed, the rule-of-thumb is the fact that there must be about 50 square inches of “free” air transfer position for each 100 cfm (cubic ft each minute) of supply air towards the room. Within this situation, the word “free” means an easy, obvious hole within the wall between your room and the rest of the home. If, for appearance and privacy reasons, this hole will be included in grilles on every side from the wall, then your overall return air path area must be elevated by about 40% to take into account the ventilation resistance from the grilles, or about 70 square inches per 100 cfm of supply ventilation.

Mold in Bathrooms ac vents:

Most bathrooms, particularly tile around showers and tubs is frequently wet. Consequently, most bathrooms grow mold and wish regular cleaning. An inadequate solution water and customary household bleach may be used to regularly clean these areas and them free from mold. Low-noise bathroom fans will also be suggested to get rid of excess moisture in times when it’s being generated by bathing or showering. (See also exhaust fans.)

Whole-House Air Ventilation Fans

Opened up Home windows: Avoid using these fans when it’s damp outdoors, particularly if you have observed mold growth in your house or else you are getting trouble manipulating the relative humidity in your house. Additionally, avoid opening home windows for lengthy periods when it’s damp outdoors (e.g. during nights and nights) if you’re experiencing mold growth problems in your house.

Ac Maintenance:

Improve your filters regularly and employ pleated filters. Annually get the air-conditioners professionally serviced. In those days make certain coils are clean, the condensate drains correctly which the drain pan doesn’t have mold.

Exterior Water Management:

Redirect water from the home’s exterior – redirect sprinklers so they don’t spray around the walls. Don’t landscape with hillsides that direct waterflow and drainage for the home. Use gutters. Keep lower-spouts free from debris and direct output from the home.

Small Leaks: Even small water leaks may cause mold problems. Rainwater leaks from incorrectly exhibited home windows, wall and roof penetrations and plumbing leaks ought to be quickly repaired. Periodically inspect under sinks and vanities for indications of water leakage. Make use of your nose and smell for “musty” or “earthy” odors – they often indicate the existence of mold. Fix all water leaks quickly.

Water Damage And Mold:

Water damage and mold from flooding or any other major water invasion in homes ought to be dried within 24 hrs if possible. For severe flooding and severe water damage and mold in excess of 48 hrs, an experienced restoration professional ought to be consulted regarding cleanup procedures. Visitors also asked to consult the American Red Mix site at the end of the page for more information.

Moisture Condensation:

Single-pane, metal home windows, that are common in Florida, generally condense water inside in the winter months. It’s good practice to get rid of this condensation before it may elope and become absorbed by porous materials like wood casing or gypsum wallboard. Condensation also occurs on other surfaces in homes. If condensation is observed on interior surfaces in summer time, it might indicate numerous problems, including lack of ability to manage indoor humidity ac supply registers aimed directly at interior surfaces duct leakage problems and pressure imbalances or the suggestions above. If you see indoor surface condensation during summer time, you need to speak to a professional to assist identify the reason. However, during springtime once the ground continues to be awesome, it is extremely easy to experience some condensation on tile floors on slab-on-grade homes which are available to the outside. This should not be any regular occurrence, only something which occurs rarely.

Exhaust Fans:

Make certain the garments dryer vent goes completely towards the outdoors of the house, to not the crawlspace in order to within the attic room or even the house. You have to bathroom vent fans. It’s also essential for your kitchen range hood to vent towards the exterior too. Recirculating stove and kitchen vents provide no elimination of stove top moisture and inferior charge of cooking related pollutants in contrast to venting completely towards the outdoors. A significant deterrent to using kitchen oven hoods is noise. Choose an ultra-quiet, inline ventilation fan for the range hood. Kitchen and bath exhaust fans must only be utilized while cooking or while using bathroom to get rid of excess moisture generated by these activities.It’s best practice either to have bathroom vent fans interlocked using the light switch so they don’t get left on and have them switched with a manual timer which will shut them off after some time, or control them by humidistat.


Fungi such as the dark and closets are hardly ever provided with conditioned air like a standard a part of method. So it isn’t everything uncommon to possess mildew or mold exist in closets, especially on leather. Departing the closet doorways available to provide more conditioned air flow or departing the closet lights up with the doorway closed in order to enhance the temperature (which lowers the RH) can help to eliminate these complaints.

House Plants: Minimize live house plants, particularly if you have trouble manipulating the relative humidity in your house.

Recommendations to manage Humidity in Vacant Florida Homes

Fortunately, FSEC has been doing research backed by Florida Power and lightweight Company regarding how to treat vacant Florida homes to ensure that humidity levels are controlled:

http://world wide

Essentially, within that relate, two strategies are suggested to manage moisture:

1) Convey a dehumidifier on the timer in your home in order that it runs for several-4 hrs each day. Frequently it is advisable to drain the condensate in the dehumidifier to some sink. Leave the Air conditioning system trigger with this particular strategy, although fans on low speed may be set to flow the environment in rooms. Doorways to rooms ought to be left open.

2) Utilizing a programmable thermostat set the ac to 74 F from three-6 AM. This operates the air conditioning constantly noisy . morning hrs when indoor coil temperatures is going to be achieved leading to good moisture removal. Setup the thermostat to 85 F during all of those other day. This seems to be effective in most homes and under an array of climate conditions, although any significant duct leakage should be repaired.


Additional Resources

U.S. EPA – “A Brief Guide to Mold, Moisture and Your Home”

Building Science Corporation – online articles on mold and moisture control:

National Association of Home Builders Research Center – ToolBase Services site: “Mold in Residential Buildings”:

American Red Cross – “Repairing Your Flooded Home” provides excellent advice for cleanup of major water damage in homes:

The authors of this web page are Philip Fairey, Subrato Chandra and Neil Moyer of the Florida Solar Energy Center.


1. Readers are cautioned that there is not full consensus among the medical and health science community on the potential health consequences of exposure to molds. Individuals with compromised immune systems, allergies, asthma, other lung diseases or other medical problems should consult their physician before physically contacting or disturbing any mold growths.

2. M.K. Khattar, M.V. Swami and N. Ramanan, “Another Aspect of Duty Cycling: Effects on Indoor Humidity,” FSEC-PF-118-87, ASHRAE, 1987.

3. Swami, M.S., Lixing Gu and Philip Fairey, 2001. Appendix K, Moisture Analysis and Condensation Control in Building Envelopes, Heinz R. Trechsel, Ed., pp.183-184, ASTM Stock No. MNL40, ASTM International, West Conshohocken, PA.