Just Because I Cannot See It, Doesn't Mean I Can't Believe It!
Just as carpets, windows, contents and walls accumulate dirt, dust and mildew; so do HVAC and duct systems. HVAC systems are often overlooked when a cleaning routine is implemented in both residential and commercial properties. Besides common dirt and dust, many other factors can exacerbate the sanitary conditions of duct work including: pet dander, cigarette or cigar smoke, water contamination or damage to the properties' HVAC systems, renovation and remodeling projects, and other harmful contaminants. If the registers that circulate air into a building appear dirty then chances are that the duct systems are just as bad if not worse!
Dirty systems can worsen symptoms in individuals with chronic lung issues, health problems, asthma and allergies. The air quality inside of a home or place of work can have different effects on different individuals, especially the young and elderly. Similarly, the HVAC system that works as the lungs of your building; breathing in air and breathing it back out inside of a property cannot work as efficiently when neglected of the proper cleaning. Clean ducts are also more energy efficient without having to work as hard to cool and heat air filled with dust, mildew, and other contaminants.
Photo By: http://www.city-data.com/forum/house/1683598-air-quality-house-open-ducts-crawl.html