You Cannot See It But You Breathe It
By Robert Weitz, Principal, RTK Environmental
When the first cold night arrives, most homeowners simply turn on the heat. But before warm air starts moving through the house, it is worth asking a basic question:
What else could your HVAC system be circulating?
After weeks of heavy rain and saturated conditions across the tri-state area, moisture may have found its way into basements, attics, walls, crawl spaces, or areas near or even in HVAC equipment and ductwork.
Mold can grow in these hidden spaces. You may never see it, but it can still affect the air you breathe.
That is why fall is an important time to take a closer look at indoor air quality, especially before the heating system begins running every day.
Why a wet summer can create fall air quality problems
Mold needs moisture to grow. Heavy rain can enter through a roof leak, basement seepage, foundation crack, poorly sealed window, or overflowing gutter. Even after visible water
disappears, moisture can remain behind walls, beneath flooring, inside insulation, and in other areas homeowners cannot see.
Your air-conditioning system may have added another source of moisture. Condensation can collect on coils, in drip pans, around vents, and inside ductwork. When dust and moisture collect in ducts, they create conditions where mold can grow.
That problem may become easier to notice once the HVAC system begins running regularly. A musty or damp odor when the system turns on, allergy-like symptoms, or dark discoloration around vents, drip pans, or ductwork can signal a problem that deserves a closer look.
Check the HVAC system before turning on the heat
A seasonal HVAC check should go beyond confirming that the heat works. Look for moisture, musty odors, dirty filters, clogged drains, visible discoloration, and damp areas around vents and HVAC equipment. Pay particular attention to coils, drip pans, filters, and accessible ductwork.
Clean or replace filters regularly. I recommend a MERV 13 or higher filter where the system allows it because it can capture mold, bacteria, dust, pollen, smoke, and other contaminants. If the air handler is more than 10 years old, check with an HVAC professional first because a higher-rated filter may restrict airflow in an older system.
I strongly recommend professional duct cleaning approximately every four years, although the timing may vary based on usage, pets, and climate. Maintenance can reduce the conditions that allow mold to take hold, but it cannot tell you whether mold is already present in the system or the air.
Know the warning signs of mold and poor indoor air quality
Mold is not always visible. Pay attention to what you smell, see, and feel when the HVAC system starts running.
- A persistent musty or damp odor, especially near vents
- Visible spotting or discoloration around registers or HVAC components
- Condensation on windows, walls, or other surfaces
- Recent leaks, flooding, water stains, or recurring basement moisture
- Visible discoloration around vents, drip pans, or ductwork
- Sneezing, itchy eyes, headaches, fatigue, or other symptoms that ease when you leave
If you see or smell something suspicious, do not try to scrub it away. Disturbing mold without proper containment can spread spores to other parts of the home. Professional testing can determine whether mold is present, identify the types and concentrations found, and help define the extent of the problem.
Keep indoor humidity under control
Humidity remains important after summer ends. I recommend keeping indoor relative humidity between 30 and 50 percent to discourage mold growth while keeping the home comfortable.
A simple hygrometer can show where your indoor humidity stands. A dehumidifier may help reduce moisture in the air, but it cannot remove mold that is already growing or repair the leak, crack, or water intrusion that caused the problem.
Test first if you suspect mold
If there is a musty odor, visible growth, a history of water intrusion, or concern about contamination in the HVAC system, start with an independent mold inspection. If you see or smell mold coming from a vent or unit, I recommend turning off the system and calling a professional mold inspector.
An inspection can identify whether mold is present and provide a clear plan for remediation when needed. After cleanup, follow-up testing can help confirm that the work was successful before the system goes back into regular use.
Do not wait until the heat has been running for weeks. A fall HVAC and moisture check can catch a hidden problem before the system circulates air throughout the house all winter.
Frequently asked questions
Can mold grow inside an HVAC system?
Yes. Mold can grow in an HVAC system when moisture and a suitable food source, such as accumulated dust, are present. Cooling coils, drip pans, vents, filters, and ductwork are common areas of concern.
Can turning on the heat spread mold spores
Mold can grow in ductwork and around HVAC components when condensation and dust collect. If you see or smell mold coming from a vent or unit, turn off the system and call a professional mold inspector before continuing to run it.
What indoor humidity level helps discourage mold
Aim for an indoor relative humidity level between 30 and 50 percent. A hygrometer can help you track conditions throughout the home.
Should every home have its air ducts cleaned each fall
RTK recommends having ductwork professionally cleaned approximately every four years. The timing may vary based on how heavily the system is used, whether you have pets, and the local climate.
