An Ounce of Prevention . . .
Today’s post is going to be all about how to prevent HVAC system mold issues. I wanted to write this post, because some of the most prevalent and costly to remediate indoor mold problems (including the one my family and I suffered from) are caused from mold growth inside HVAC systems. Thus, it should also come as no surprise that, second to reader questions about cleaning their belongings that were exposed to mold, questions about HVAC mold problems are the ones I get most frequently. Often, readers write to me about frustrations with trying to effectively remediate their entire mechanical system and/or ducts (you should ALWAYS do both if this mold issue affects your home) or, because they want to know how to figure out if the musty smells in their homes are related to when their HVAC system cycles on and off—aka if the musty smells are coming out of their vents.
Factors That Can Contribute to HVAC Mold Growth:
To begin, I think it is important to discuss the many factors that can contribute to this major indoor air quality issue in the first place. I am no HVAC professional, building scientist, or microbiologist, but many elements of how we configure, use, and maintain our mechanical ventilation systems seem to me to be the root causes of mold HVAC problems. The reasons for this are many, but if I had to choose my Top 5, they would include the following basic principles:
1.) HVAC systems combine all of the ingredients needed for mold growth (dust, aka organic material, high humidity, and water) perfectly inside one mechanical system;
2.) Most HVAC mechanical systems are set up essentially out-of-sight in our homes and are housed in unconditioned areas, like our attics, basements, or crawlspaces. This makes regular maintenance, leaks, and/or visible microbial growth not something that is glaringly obvious or at the forefront of our minds on a daily basis; (This is in strong contrast to when flooding, or a major leak or water incident occurs in a highly visible or trafficked part of our homes.)
3.) Almost all homes have central HVAC systems these days, so all indoor air is, at some point, channeled through the mechanical HVAC system. Thus, if you have mold growth inside of it, it is going to be blown throughout your home and is going to directly affect almost every, if not all rooms in your home and all of the air that you breathe when indoors;
4.) Most people rely on HVAC professionals to maintain their systems (if they maintain them at all) and do not take the time to regularly inspect, clean, or check on their HVAC system to make sure that is functioning properly—this includes checking for leaks, having the coil and condensate pan cleaned, and changing and properly installing filters;
5.) Visual inspections of an HVAC system aren’t always enough to find the culprit, because most mold problems occur inside the plenum or on the coil and cannot be found without some digging and mechanical knowledge, so that you know where to look.
The good news here is that while HVAC mold can be a complex issue to solve once it permeates your system, it is actually a rather straightforward issue to prevent. Before I get into addressing prevention, though, I think it will be beneficial to talk in laymen’s-terms and to discuss basic air-handler mechanics and why the process by which closed HVAC systems work actually fosters microbial contamination. My hope is that, armed with even the most cursory understanding of the way the air flows through your system, you can become actively involved with the factors (humidity, filtration, and maintenance) that improve overall indoor air quality and preclude mold.
The HVAC Basics–Operational Process and Mechanics:
To start, most of us have split systems, which include an outdoor cabinet containing a condenser coil and compressor, and an indoor evaporator coil, usually installed inside the furnace or air handler. The compressor pumps a chemical called refrigerant or Freon through the system. Warm air from inside your home is drawn into the system and blows across the indoor evaporator coil. The heat energy from the warm air transfers to the Freon inside the coil. That transfer is what “cools” the air. The Freon with the warm air is pumped back outside to the compressor, where the cycle begins again. In other words, the heat absorbed by the Freon is moved outside your home and blown out of the compressor, while the cooled air is blown inside. It is during this process that moisture that contributes to indoor humidity is also condensed out of the air. Further, your cooling system is usually combined with your central heating system, because they share the same ductwork for distributing conditioned air throughout your home.
So, to break it down again, very fundamentally:
- Air conditioners remove heat from the air in your home and transfer it outside through the use of two coils.
- Evaporator coils are located inside your home and use rapidly evaporating Freon to absorb heat from the surrounding air. This newly chilled air is then circulated throughout your house.
- The now relatively hot Freon then travels outside of your house to the condenser coil.
- The compressor condenses the Freon, releasing the stored heat to the outside of your house.
- The cycle then begins again.
When HVAC Mold Growth Occurs:
As you can see, the coils play an important role in airflow, temperature, and moisture control. Thus, it should make sense that when you cool the warm air passing through the mechanical system to the dew point, it is going to cause moisture to accumulate on the evaporator coil inside the supply portion of your system. This is often the genesis of mold issues, because, if dirt, particulate matter, and/or dust bypass your HVAC filter or ducts and get inside your supply, that organic matter is going to end up on your moist evaporator coil as well. If the coil stays wet and dirty, mold is definitely going to grow. 
Then, all of the conditioned air passing over the coil is going to also contain mold spores. Then, those spores are going to be blown through your ducts and into your home.
In the winter, when you switch to heating your home, the air inside your system and home will dry out more, because there is less heat transfer to the outdoor air, and the heated indoor air naturally contains less moisture as well. This may seem like a good thing, because any mold on your coil and in your system will dry out and go dormant. Unfortunately, once mold is present, even when dormant, it can cause health problems. This is especially true when you are still using the HVAC system and blowing the dried dust that contains the dormant mold in it all over your home. Those mold fragments are easily distributed and inhaled. Then, if mold is left inside the warm, moist environment of your sinus, it can colonize and make you very sick. Further, and this is definitely something to consider, certain geographical locations have quite the climate overlap in the spring and fall where AC and heat are both used. For example, it went from 60 degrees this week to the 30s overnight. This constant switching from cooling the indoor air to heating can make it harder for the dehumidification cycle to complete inside your system. Another reason for mold growth.
Does all of this make sense? I hope so. In my opinion, it also makes clear why sometimes a mold sufferer, once sick, stays sick from season to season, but has varying and cyclical symptoms. A good example of this would be someone who experiences more allergy and sinus symptoms in the spring and summer, but then suffers from more intense GI, cognitive and hormonal symptoms in the in the winter. Which, also makes clear why mold illness can have such an expensive and confusing progression with multiple symptoms and continual misdiagnoses.
Mold Growth Prevention and HVAC Maintenance:
With the mechanics of an HVAC system better explained, I think this part will make more sense and will be more helpful. So, let’s get to the Top 5 basic ways to maintain your HVAC system that will prevent and solve potential mold issues:
1.) Install high-quality, pleated filters in your furnace. 
This is the first and maybe most important preventative measure you can take to ensure proper filtration of your HVAC system. In the past, I used to recommend using filters of a MERV 8 rating or higher. Now that I have read more of the literature and understand more about particulate size, I recommend the newer MERV 13 filters. These filters can trap much smaller particles and soil, so that it doesn’t get inside of your furnace and onto the coil where it can come into contact with moisture and grow mold. 
Also, you need to make sure that the filter you are using is fitted and installed properly inside your system to eliminate air bypass. Even the best filter, if air is getting around it, won’t work. In other words, good filtration equals the right filter with no air bypass, which then equals a healthy system. Finally, make sure to check and change your filters every 3-4 months at a minimum.
2.) Keep your evaporator coil clean. 
Since the coil is really where microbial and mold growth congregate, having it cleaned and maintained is an overriding principle to having a healthy system. The IAQ professionals I have spoken with suggest having the evap coil cleaned every 6 months with your regular, seasonal HVAC maintenance. A professional should do this work, because the coils are delicate and putting the wrong pressure at the wrong place may cause damage. Too much abrasion or force, or even using the wrong type of product for cleaning can also potentially cause damage, or worse, cause an electric shock to someone with untrained hands.
As far as coil-cleaning specifics go, (and this may be overkill, but I am including this, so that you can use it for reference with your technician, if needed) here is what I have gleaned from my interviews and research of some of the best and most-trusted professionals by the Indoor Air Quality Association and Healthy House Institute in this line of work: Conventional cleaning chemistry is based upon bleaches, oxidizers, acids and caustics, while the physics of cleaning involves wiping, vacuuming and sweeping. This traditional combination of physics and chemistry actually is NOT what works best for the biological issue of pathogens and biofilm on evap coils. This is because it introduces an equally dangerous and inflammatory chemical and/or chemical reaction into the indoor air. Thus, what works better and is healthier is using a high-pressure, high-heat steam-cleaning physical method, coupled with a non-corrosive, odor-free, non-chemical, but highly effective antifungal/antimicrobial agent to “treat” the coil, like EC3 Mold Solution, is best. Experts also suggest that coils be flushed from the “leaving” side of the coil to the “entering” side, working your way up until the coil is clean. Flushing at an angle is also best for particulate removal. This prevents all of the biological matter that is being flushed off of the coil from still remaining on the bottom part closest to the drain. (Note: Not all HVAC technicians, including those that claim to be IAQ professionals know what they are doing; call ahead and make sure the HVAC sends a tech that understands mold and the HVAC service required. Do not believe they all do! And if they don’t, call someone else.)
Interesting factoid: In 1986 well-respected researchers (Morey, P. R., Hodgson, M. J., Sorensen, W. G., Kullman, G. J., Rhodes, W. W. and Visvervara, G. S.) published a study, entitled “Environmental studies in mouldy office buildings,” that identified the presence of biofilm on HVAC coils and condensate pans that could not be seen by the naked eye and remained present even after routine cleaning. They concluded that the biofilm developed, because of the consistent water supply and presence of debris on both items. Further, they classified the material as biofilm, because, not only was pathogenic mold and bacteria present, but, much like the mold that colonizes the body in mold toxicity, it actually had created a protective barrier around itself on the coil and pan, so that it was extremely difficult to penetrate and kill. (Note: “Biofilm is a complex structure adhering to surfaces that are regularly in contact with water, consisting of colonies of bacteria and usually other microorganisms such as yeasts, fungi, and protozoa that secrete a mucilaginous protective coating in which they are encased. Biofilms can form on solid or liquid surfaces as well as on soft tissue in living organisms and are typically resistant to conventional methods of disinfection” www.dictionary.com). Thus, when biofilm is present on the coil, it is much more difficult to clean or to effectively remove without overhauling the entire HVAC system or replacing the coil itself—a costly endeavor.
3.) Make sure the condensate pan drains properly and stays clean and dry. 
You can use a mild dish detergent and EC3 Mold Solution Concentrate to maintain its cleanliness. The presence of substantial standing water and/or debris indicates a clog, leak or drainage problem requiring immediate attention. You do not want all of the moisture being removed from the air by your HVAC system to not be able to properly drain. This could cause pooling inside your system, or can cause the pan to overflow and leak into your home. You can also check any insulation near cooling coils for wet spots as in indication of an issue that needs to be resolved.
4.) Make sure ducts are properly sealed and insulated in all non-air-conditioned spaces (e.g., attics and crawl spaces). 
This will help to prevent condensation moisture from entering the HVAC system through your ducts. In other words, if you go into your attic or basement to check your system and notice condensation or moisture on your ducts, or on the outside of your mechanical system, you need to promptly address the issue. The use of self-draining dehumidifiers and proper ventilation in these spaces works best as a permanent solution.
5.) Make sure that your air conditioning unit is the proper size for your home and that all ducts are sealed at the joints. 
A unit that is oversized, will cycle on and off frequently, and cannot sufficiently dehumidify the air, because it doesn’t run for long enough intervals. This results in high indoor humidity, particularly in geographical areas with humid outdoor climates. Also, make sure that your system is designed to manage condensation effectively. This means that it is able to keep up with the volume of moisture that it is removing from the air as it is working. (Note: The US Department of Energy at www.energy.gov has a free webinar on their site that helps you to understand and calculate your home’s HVAC needs according to what part of the country you live in. Or, to read another helpful article on this topic that outlines why most homes are built without the proper-sized HVAC system, you can click HERE.)
There you have it! If you can investigate, maintain, and keep up with those 5 things, you are well on your way to preventing this costly and devastating mold issue and having a much healthier indoor environment. A side benefit, is that by maintaining and keeping your system clean, it will last longer and have increased energy efficiency. A happy money-saving bonus!
