HVAC technicians are often the first to discover mold and asbestos in a home. Service calls for musty odors, routine maintenance on older equipment, and duct inspections put you in direct contact with these hazards regularly. Understanding how to identify mold and asbestos, and knowing what to tell your customer, protects everyone involved and ensures the homeowner gets the right help.
Advanced Health and Safety has been serving Dane County for over 25 years. We work with HVAC contractors and other trades, and we offer training for those who want to learn more about identifying mold and asbestos in the field.
Musty Smells and Odor Calls
A musty smell from the AC or vents is one of the most common reasons homeowners call an HVAC technician. That smell usually indicates mold growth somewhere in the system, whether in the unit itself, the ductwork, or both. When an air conditioner smells musty, the source is often mold growing on the evaporator coil, in the drain pan, or inside the air handler. Furnaces can develop musty odors as well, particularly if there is moisture intrusion or condensation issues. Mini splits and window AC units are especially prone to mold growth due to their compact design and the condensation they produce.
Dirty sock syndrome is a term HVAC techs know well. That distinctive smell comes from bacteria and mold growth on the evaporator coil, typically in heat pump systems during certain weather conditions. While coil cleaning can help, persistent odors often indicate a mold problem that extends beyond standard maintenance. When you get called out for musty smells from vents or mold odors from the AC, a visual inspection of the equipment and accessible ductwork can help identify the source. If mold growth is visible or the smell persists after cleaning, the homeowner may need professional mold remediation before the system can be fully restored.
Mold in AC Units and Equipment
Mold in AC units is common because the conditions inside HVAC equipment are ideal for mold growth. Dark, damp, and often warm environments allow mold to thrive on coils, drain pans, and interior surfaces.
Black mold in AC units frequently appears on evaporator coils where condensation collects. Mold on AC coils reduces efficiency and circulates spores throughout the home every time the system runs. Mold in air handlers can spread quickly because of the volume of air moving through the unit.
Window AC units and mini splits are particularly susceptible to mold because of their self-contained design and the difficulty of accessing interior components for thorough cleaning. Mold in these units often goes unnoticed until the smell becomes obvious or the homeowner experiences health symptoms.
Mold in furnaces is less common but can occur, especially in systems with humidifiers or in homes with high humidity levels. Any HVAC equipment that handles condensation or humidity can develop mold under the right conditions.
Standard coil cleaning and drain pan treatment can address minor mold issues, but visible mold growth on interior surfaces, insulation, or in areas that cannot be fully accessed typically requires professional remediation. Mold that has spread beyond the equipment into surrounding materials or ductwork needs specialized containment and removal procedures.
Mold in Ductwork and Vents
Mold in ductwork is a common problem that often goes undetected until odors become noticeable or visible growth appears on vent covers. Duct systems provide a perfect environment for mold when moisture is present, whether from condensation, humidity, or water intrusion.
Black mold in air ducts can develop on the interior surfaces of sheet metal ducts, but flex duct and duct board are even more susceptible because of their porous materials. White mold in air ducts is also common and should be treated with the same concern as black mold.
Mold on air vents and AC vents is often the first visible sign of a larger problem inside the duct system. Discoloration around vent covers, visible growth on the register, or a musty smell when the system runs all indicate mold may be present in the ducts.
Duct cleaning alone does not always solve mold problems. If the source of moisture is not addressed, mold will return. Ductwork with extensive mold growth, particularly flex duct or insulated duct, may need to be removed and replaced rather than cleaned.
When you encounter mold in ductwork during an inspection or service call, recommend the homeowner have a professional mold assessment. An HVAC mold inspection can determine the extent of the problem and whether remediation is needed before duct cleaning or system repairs make sense.
Asbestos in HVAC Systems
Older HVAC systems may contain asbestos in several components. Asbestos was valued for its heat resistance and insulating properties, which made it common in heating and cooling equipment.
Asbestos tape on ducts is one of the most frequent discoveries. This tape was used to seal joints and seams in ductwork and often appears as a white or gray fabric tape. Asbestos duct wrap and asbestos duct insulation were used to insulate heating ducts and plenums, particularly in systems connected to boilers or older furnaces.
Asbestos in boilers and furnaces can be found in door gaskets, cement seals, insulation blankets, and pipe connections. Asbestos furnace cement was used to seal combustion chambers and flue connections. Any gray or white putty-like material around an older furnace should be treated as suspect.
When you suspect asbestos is present in an HVAC system, do not cut, tear, sand, or disturb the material. Asbestos fibers become dangerous when they are released into the air. Intact materials should be left alone until they can be tested and properly removed by a licensed abatement professional.
What to Tell Your Customer
Discovering mold or asbestos during a service call requires clear communication. Homeowners rely on your expertise and appreciate straightforward information about what you found and what should happen next.
For mold, you might say something like: "I found mold growth inside the air handler and it looks like it has spread into some of the ductwork. This is causing that musty smell you have been noticing. I can clean the coils, but the mold in the ducts and on the interior surfaces really needs to be handled by a mold remediation company. They can contain the area and remove the mold properly so it does not keep coming back or spreading through the house."
For asbestos, you might say: "This tape on the ductwork looks like it could contain asbestos, which was commonly used in older HVAC systems. I am not going to disturb it because that can release fibers into the air. You will want to have it tested and removed by a licensed asbestos abatement company before any work is done on the ducts."
In both cases, you can let the homeowner know that Advanced Health and Safety handles mold remediation and asbestos abatement, and they can reach out directly for an assessment.
Training and Resources
Advanced Health and Safety provides training for contractors who want to learn more about identifying mold and asbestos in the field. Our team has over 25 years of experience in Dane County and works with HVAC technicians, plumbers, remodelers, and other trades on a regular basis.
If you are interested in learning more or want to schedule a training session for your team, contact us.
Contact Us Today
Have questions about something you found on a job site? Want to learn more about our contractor training?