Preventing Mold Growth in Your Air Conditioner and Home
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Preventing Mold Growth in Your Air Conditioner and Home

Strategies for reducing mold growth inside your air conditioner and ways to reduce moisture levels in your home.

With hot humid weather forecast around the country, you may think that if your home is cool, everything is fine. Unfortunately, air conditioners are excellent mold breading grounds as deposited dust can accumulate on the cooling fins and then becomes damp from condensation being removed from the air. The mold will not grow well when the air conditioner is running and cold, but between cycles or during longer periods when the air conditioner is not needed, the mold multiply rapidly and pose a serious health hazard.

This mold growth can occur is central air conditioning systems, portable air conditioners, and even window AC units. Cleaning your air conditioner regularly improves your health by removing dust that feeds mold and improves the efficiency of the air conditioner.

Mold requires mold spores, carbon based food, sufficient temperature, and adequate moisture levels to thrive. Unfortunately, it is almost impossible to eliminate mold spores and food, in the form or dust and dander, from your HVAC system for any length of time.

Your nose will tell you if there is a problem before you notice and mold colonies. Many mold remediators will admit that the human nose is very sensitive to the slightest amount of mold growth by detecting the musty odor that is common with mold growth.

Typical Air Conditioning Schematic


Most molds grow very well at the same temperatures that humans prefer. Temperatures close to freezing are not cold enough to prevent mold growth and temperatures that are much warmer than humans prefer, like those of the tropics, will grow abundant quantities of mold. Therefore, it is not feasible to control mold growth in our home environment through the control of temperature.


The vast majority of mold species require humidity levels that are equivalent a relative humidity of at least 70%. In fact, the great majority of serious, large mold outbreaks inside buildings occur where porous, cellulose-type materials have literally been kept wet by liquid water or sustained condensation.

Human beings prefer humidity levels that are below the critical relative humidity for mold growth. For this reason, moisture availability is by far the easiest mold growth requirement to control in the home. Limiting moisture and destroying mold colonies is the most effective way to reduce the threat from mold in your HVAC system.

Strategies to Minimize Mold Growth

Unit Operation

Always set the fan mode switch of your air conditioner thermostat in the “AUTO” position, never in the “ON” position. If your fan runs continuously the moisture that has condensed on your air conditioner's evaporator coil (also known as the “A coil”) during cooling is re-evaporated and blown back into your home before it can drain into the condensate pan and out of the unit. This causes the relative humidity in your home to be significantly greater than if the air conditioner thermostat fan mode switch is set to the AUTO position. Some air handlers will run the blower for a few minutes after the compressor shuts off. To increase dehumidification, it is best to disable this feature. A qualified mechanical contractor should be able to disable this feature so that the blower and compressor turn off simultaneously.

It is important to recognize that the thermostat in your home measures sensible heat, or the air temperature, while humidity is due to latent (hidden) heat that is caused by the moisture in the air. Manufacturers size their evaporator coils to maximize the sensible and latent capacity, but in some parts of the country a typical coil may not be adequate.

Oversized Units

Oversizing of air conditioners is all too common. The more an air conditioner is oversized, the poorer its humidity removal performance, especially when the thermostat is set at a higher temperature. During each air conditioning on cycle, the moisture removal does not reach full capacity for about the first three minutes of operation. The more the system is oversized, the faster the temperature in the home lowered and the shorter period for moisture removal. Thus, if a home is properly sized with a 2-ton air conditioner and a 4-ton system is installed, the 2-ton machine would do a better job removing moisture even though the 4-ton machine had twice the nameplate humidity removal capability (Btu/hr). Remember, the shorter the air conditioner on-cycle, the less chance for effective moisture removal.


Cleaning the interior evaporator coil annually is the best way to prevent dirt from accumulating on the coil and lowering efficiency and promoting mold growth. See my article covering this topic at:  

Also see my article on hiring a duct cleaning contractor at:  

Keep Interior Doors Open

Interior doors should be kept open when air conditioning unless your heating and cooling system has a fully ducted return air system from each room of the home or unless specific and sufficient return air transfer pathways have been installed to ensure that closed interior doors do not result in space depressurization problems in the home. See my article which talks about jump and transfer ducts at:


It is important for homes in hot, humid climates to be pressurized slightly with respect to outdoors; meaning the pressure inside the home should be slightly higher compared to outside air pressure. If homes are under negative pressure with respect to the outdoors, hot, humid outdoor air will be pulled through the air pathways that exist in all building envelopes (walls, ceilings, floors, etc.). The air will most likely not take a direct route, but be pulled through the wall cavity at one point and then enter the living space at another location. This fills the interior wall cavity with warm, moist air. Take for example air entering a wall high on the exterior where an outdoor light fixture is mounted and exit the wall low on the indoors where an electrical outlet is located. If the home is air conditioned, the surface of the gypsum wallboard will be relatively cold and often colder than the dewpoint temperature of the humid outdoor air that must flow along that gypsum wallboard to that indoor electrical outlet. Moisture will condenser on the backside of the wallboard and begin to saturate the gypsum. This is the same process where water beads on a cold glass.

A few things that may cause space depressurization in homes are:

• Exhaust fans (bathroom, kitchen, attic, crawlspace, etc.)

• Cloths dryers

• Supply duct leaks

• Insufficient return air pathways due to interior door closure

• Blocked return grilles

Indoor Relative Humidity Levels

During the hot summer months, with the air conditioning on, the relative humidity (RH) should not exceed 55% during the day on a regular basis. If it does, you probably have problems either with leaks in your duct system or with your air conditioner unit itself. As stated earlier it could be too large, improperly charged or have insufficient air flow across the coil. Consult with a qualified air conditioning expert or mechanical engineer to determine the problem.

The highest relative humidity in your home will most likely occur during mild weather when your air conditioner is not needed very often during the day, overcast days, and at night.

At night, the humidity level outside can reach 80 to 90%; this is where morning dew comes from. If you open your home up on a cool evening, you will introduce all of this moisture into the building materials in your home. Regardless of temperature, you may have RH levels exceeding 70-75% for extended periods, and find it difficult to control mold growth on surfaces in your home. You may need to invest in some type of dehumidification system and should consult with a qualified mechanical system expert for advice.


Most bathrooms have higher humidity levels than the rest of the home due to hand washing, bathing, showering, and having a standing pool of water in the room known as a toilet. To control mold here, use a weak solution of water and common household bleach to regularly clean these areas and keep them free of mold. Low-noise bathroom fans are also recommended to remove excess moisture during periods when it is being generated by bathing or showering. Remember pressurization and make sure they are turned off when you leave or use a timer and let them run for a few minutes after you leave.

Other Recommendations

Avoid the use of whole-house fans when it is humid outdoors, especially if you have noticed mold growth in your home or you are having trouble controlling the relative humidity in your home. In addition, avoid opening windows for long periods when it is humid outside.

Change your filters regularly and use pleated filters. Once a year get your air-conditioners professionally serviced. At that time make sure coils are clean, the condensate drains properly and that the drain pan has no mold. You can purchase tablets that can be added to the condensate pan to kill any mold and bacteria growing there and also help keep your condensate drain line clear.

Single-pane, metal windows are still common in some homes and allow water to condense on the inside in winter. It is good practice to remove this condensation before it can run off and be absorbed by porous materials like wood casing or gypsum wallboard. Keep storm windows in place during the winter.

If you have a room on a concrete slab, you may notice condensation on the floor, especially in the spring when the ground is still cool and the home is warm and you have a tile floor. In some instances, condensation can occur under carpeting, so if you notice your humidity levels are high, check under the carpeting.

Make sure the clothes dryer vent goes all the way to the outside of the home, not to the crawlspace or to the inside of the attic or the house. The same goes for bathroom vent fans. It is also important for the kitchen range hood to vent to the exterior as well. Recirculating vents, like the ones on some under-cabinet microwaves, provide no removal of cooking moisture and limited control of cooking related pollutants compared with venting completely to the outdoors.

Additional Resources

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

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

ToolBase Services

Additional resources:

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Comments (4)

How true about mold in the A/C considering how it's recirculated air flow. Many time's people catch the summer cold's or illness's due to the bacteria, viruses carried in the recycled air. You are right on! Thanks, for that great insight.

Excellent information Daniel, very informative and in depth!

excellent excellent article, I buzzed it up

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