The HEPA filter, when used in a negative air machine, is rated for approximately 40,000 hours of operation, or 4 years, under normal conditions and with recommended replacement of the pre-filter (air scrubber use will reduce filter life). If the red indicator light is illuminated, and did not extinguish after replacing the pre-filter, then the next step is to replace the main HEPA filter. For residential use, call your dealer to do this procedure.
For healthcare facilities, follow these steps (Fig.5):
Source: Carrier (www.shareddocs.com)