I recently replaced my 44 year old furnace. Over the years we have had several construction projects inside the home. For many years breathing the heated air from the duct work caused excessive sneezing, sinus issues, and lung irritation. . The heat smelled like dust. To maximize air quality I added a humidifier, MERV-11 filter, and UV light onto the new furnace. To further improve air quality, I had the ducts cleaned and sanitized. I took before and after pictures of the inside of each return and output duct. They were filthy before, and sparkling clean and fresh after the procedure. Now when running the system, clean sanitized air flows through my whole house. A huge difference. It will make sleeping and being indoors during the winter months far more tolerable. I use window air conditioners, and run the system in the summer to keep the house air clean and fresh on hot summer days too. It works! Great decision!
You may be familiar with air ducts that are constructed of sheet metal. However, many modern residential air duct systems are constructed of fiber glass duct board or sheet metal ducts that are lined on the inside with fiber glass duct liner. Since the early 1970's, a significant increase in the use of flexible duct, which generally is internally lined with plastic or some other type of material, has occurred.
All modern air conditioning systems, even small window package units, are equipped with internal air filters. These are generally of a lightweight gauze-like material, and must be replaced or washed as conditions warrant. For example, a building in a high dust environment, or a home with furry pets, will need to have the filters changed more often than buildings without these dirt loads. Failure to replace these filters as needed will contribute to a lower heat exchange rate, resulting in wasted energy, shortened equipment life, and higher energy bills; low air flow can result in iced-over evaporator coils, which can completely stop air flow. Additionally, very dirty or plugged filters can cause overheating during a heating cycle, and can result in damage to the system or even fire.
Your HVAC unit and air ducts deliver heated or cooled air throughout your home year-in and year-out. The air your HVAC unit produces travels through your ductwork and heats or cools your home via the air vents in each room. The dust and other particles in your home’s air are known as particulate matter, which the EPA defines as "a complex mixture of extremely small particles and liquid droplets." The EPA also notes that "particle pollution is made up of a number of components, including acids (such as nitrates and sulfates), organic chemicals, metals, and soil or dust particles." Having clean air ducts means fewer of these particles moving through your home. If you remove your furnace filter and hold it up to the light, you can probably see several hundred of these minuscule particles floating in the air. 
The key to the continued and efficient operation of your heating and cooling system is periodic maintenance. Cleaning the air duct vent system, and servicing you heating and cooling equipment will keep the system operating at its peak performance. Air ducts are cleaned for two reasons. One: to prevent debris from entering furnace and air conditioning components. Two: to prevent debris from cycling through the home. The air duct cleaning contractor needs to employ cleaning methods which successfully removes debris, and ensures that debris does not enter the occupied space. Most contractors can verify the results of the cleaning through video or pictures.
Central, "all-air" air-conditioning systems (or package systems) with a combined outdoor condenser/evaporator unit are often installed in North American residences, offices, and public buildings, but are difficult to retrofit (install in a building that was not designed to receive it) because of the bulky air ducts required. (Minisplit ductless systems are used in these situations.) Outside of North America, packaged systems are only used in limited applications involving large indoor space such as stadiums, theatres or exhibition halls.
You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
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