should be cleaned

HVAC systems should be cleaned when a visual inspection indicates excessive particulate debris or microbiological growth on any interior surfaces. A fiber optic system or video inspection system is recommended to document the condition of the system both before and after any cleaning. A limited amount of adhered dust is expected on the inside surfaces of HVAC systems and may not indicate a problem. Obvious problems that require cleaning and restoration would include visible microbiological contamination, significant amounts of particulate debris coming out of supply ducts, or deteriorated fiberglass insulation that was contaminating the supply air. In all cases, the source or cause of particulate contamination or microbiological proliferation must be determined and corrected prior to system cleaning.  
Visual inspection for cleanliness as outlined in Chapter 3 should be incorporated as part of the HVAC preventive maintenance schedule and should include all components of the system. The required frequency of inspection will vary depending on building use, occupant load, geographical location, and surrounding environment. In general, most HVAC systems should be inspected annually or biannually for cleanlines