If I Have My HVAC System Cleaned, Will It Stay Clean?

If you've ever had your HVAC system professionally cleaned, you've probably wondered how long it will stay that way. Removing accumulated dust, debris, and other contaminants is one thing. Keeping them from building up again is another.

A study published in the peer-reviewed journal Indoor Air explored this question in ductless mini-split air conditioners across 17 homes. When researchers conducted a follow-up assessment one year later, they observed that fungal contamination had returned to approximately pre-cleaning levels.

Finding contamination in systems after cleaning raises an important follow-up question: What conditions may allow it to return?

Although the study did not establish exactly why contamination returned, several factors can contribute to microbial growth in cooling equipment:

  • Cooling creates moisture. As air conditioners cool and dehumidify indoor air, moisture naturally condenses on internal surfaces like coils and drain pans.
  • Indoor air carries particles. Everyday living introduces dust, fibers, and microscopic particles into the air.
  • Moisture and organic matter create opportunity. When airborne dust settles on damp internal components, conditions can become favorable for fungal growth to redevelop.

Ductless mini-splits operate differently from central ducted systems, and local climate conditions can influence moisture accumulation. While findings from mini-splits in humid environments should not be generalized to all HVAC equipment, the study illustrates a useful principle: cleaning addresses existing accumulation, while ongoing maintenance and moisture management help address conditions that may contribute to future buildup.

Practical Ways to Protect Your System

Keeping your system in good condition doesn't require an engineering manual. It comes down to everyday household care paired with professional maintenance:

  • Keep up with filter changes: Check accessible air filters monthly and clean or replace them according to the manufacturer's instructions. Make sure each filter fits properly in its frame to help prevent unfiltered air from bypassing it and depositing dust on internal components.
  • Manage indoor humidity: In warm or humid weather, keeping indoor relative humidity generally between 30% and 50% helps reduce excess moisture. While this range does not guarantee that mold cannot grow, it can help reduce conditions favorable to moisture-related problems.
  • Have a professional inspect condensate drainage: Standing water can create favorable conditions for microbial growth. During regular maintenance, have an HVAC technician verify that internal drain pans empty properly and condensate drain lines are clear.
  • Inspect before deciding to clean: Heating and cooling systems do not need cleaning on an arbitrary calendar schedule. Inspections can help determine whether accumulated debris, suspected microbial growth, or other system conditions warrant cleaning or further evaluation.

When an inspection indicates that cleaning is warranted, the work should follow appropriate source-removal methods designed to capture and extract contaminants. If microbial growth is suspected, identifying and addressing contributing moisture conditions is just as important as cleaning affected equipment.

If you'd like to better understand your HVAC system's condition or need help addressing accumulated contamination, use the Find-A-Pro directory at BreathingClean.com to connect with a NADCA member company.