What Makes Summer the Peak Season for HVAC‑Related Mold Growth

We at Air Duct Cleaning Spokane have documented a sharp uptick in duct‑borne mold every June–August as humid outdoor air collides with chilled metal and dust inside local cooling systems. Spokane’s arid climate fools many homeowners into thinking mold is only a winter problem, yet summer conditions inside your air handler transform it into an incubator.

High Outdoor Heat Drives Dew‑Point Condensation on Coils

When 90 °F (32 °C) air at 44 % relative humidity meets an evaporator coil running at 50 °F (10 °C), the airstream’s dew point collapses, releasing a continuous film of moisture across fins and drain pans. ASHRAE Standard 62.1‑2022 flags a 60 °F (15 °C) dew‑point ceiling as the tipping point for persistent dampness; most residential systems hover below that threshold only when the compressor is off. Once surface wetting becomes “typical,” spores germinate quickly and biofilms start reducing heat‑transfer efficiency within days.

Relative Humidity Spikes Inside Equipment—even in “Dry” Spokane

Outdoor gauges show Spokane’s mean July humidity bottoming out near 44 %, but inside sealed ductwork the relative humidity at the coil exit routinely exceeds 95 %. The U.S. Environmental Protection Agency warns that any indoor RH above 60 % supplies enough water vapor for mold to take hold; spores need only 24–48 hours of dampness to root into fiberglass liner or dust.

Continuous Fan Cycles and Dust Feed Rapid Colonization

Summer thermostat settings favor long blower runtimes. Each cycle redistributes settled skin cells, insulation fibers, and cooking aerosols—an ideal carbohydrate buffet. Add steady condensate, and common HVAC isolates such as Cladosporium and Aspergillus can reach visible colony density in two weeks, releasing musty MVOCs that ride the supply air into occupied rooms.

Health Exposure Peaks When Homes Are Closed Up

Americans spend roughly 90 % of their time indoors, where pollutant concentrations run two to five times higher than outdoors. Locked windows and recirculating AC trap mold fragments and mycotoxins, elevating risks for asthma flare‑ups, allergic rhinitis, and hypersensitivity pneumonitis—especially in children and older adults who stay inside during heatwaves.

The Financial Hit of Summer Mold Outbreaks

Ignoring that faint summer odor leads to expensive remediation. Nationwide cost surveys peg average mold removal at about $2,365, with HVAC‑specific jobs climbing to $3 k – $10 k once duct liner or coil insulation must be replaced. Early intervention costs a fraction of a full teardown, and insurers often deny claims when maintenance is deemed “deferred.”

Data‑Driven Strategies to Break the Summer Mold Cycle

1. Target 45 % RH – Install Wi‑Fi hygrometers in the return plenum; program the thermostat’s over‑cool/reheat or integrated dehumidification feature to maintain sub‑50 % humidity even on milder days.

   
   

2. Dry the Coil – Run the blower or dedicated “coil‑dry” fan mode for 10–15 minutes after each cooling cycle to evaporate residual condensate.

   
   

3. Keep Dew Point < 60 °F – Variable‑speed compressors paired with smart thermostats prevent oversizing and short‑cycling that leave coils wet.

   
   

4. Seal and Insulate Supply Plenums – Vapor‑tight mastic and R‑8 insulation stop warm attic air from condensing on outer duct shells.

   
   

5. Install UV‑C or LED Germicidal Lamps – A 254 nm fixture positioned 6 in (150 mm) from the coil surface can cut biofilm mass by up to 99 % in three months.

   
   

6. Schedule Pro Cleaning Each May – A professional sweep removes organic dust before peak humidity arrives; Air Duct Cleaning Spokane performs coil sanitization, drain‑pan leveling, and airflow balancing in a single visit.

   
   

Conclusion

Summer’s heat loads create the perfect storm—elevated dew points, long fan cycles, and tightly closed houses—that turns HVAC systems into mold factories. Controlling humidity, keeping coil surfaces dry, and removing accumulated nutrients are the proven levers for stopping growth before it jeopardizes health or forces costly remediation.

Learn how Spokane’s Air Quality Index directly impacts indoor duct pollution in this detailed blog post.

FAQ

Q1. How fast can mold grow on AC coils during summer? 

With moisture present, colonies can germinate in as little as 24 – 48 hours, making prompt drying critical.

Q2. Does running the blower alone prevent mold? 

A post‑cool fan purge helps but only if combined with RH control below 60 %; otherwise, spores still find enough water vapor to thrive.

Q3. Will duct cleaning permanently eliminate mold? 

Cleaning removes existing colonies, but lasting results require fixing moisture sources and maintaining dew‑point and humidity targets year‑round.