For larger facilities (e.g., a multi-building campus), the on-site maintenance person or service technician may change the filters quarterly because the hours of operation are significantly greater than the run-time of a residential air-handling unit. This is primarily because residential units cycle on and off as needed to maintain the space thermostat set point.

In some buildings, the air-handling units operate constantly during the buildings’ occupied hours and then shut down for the evening. During unoccupied hours, this equipment may cycle on to maintain a night setback space temperature and then cycle off when the thermostat setting is satisfied.

For institutions where air filtration is critical for indoor air quality (e.g., hospitals) or to minimize air particle count (e.g., pharmaceutical cleanrooms), the filters may be changed more frequently to maintain the supply air quality being delivered to the building, its occupants, and/or the space. With this added emphasis on air filtration, as well as filter efficiency, the building operator may increase the frequency of filter replacement. Also, each filter assembly may come with a magnehelic gauge. This “mag” gauge senses the air pressure drop (resistance) upstream of the filter and directly after the filter to determine the differential pressure across the filter.

When a filter is installed, the initial resistance the supply air will encounter may be a pressure drop value of 0.3 inches static pressure loss. As the filter becomes dirty, capturing dust particles in the air, the filter resistance will increase to a higher value (e.g., 0.6 inch). The filter manufacturer documents, within their filter literature, the recommended initial static pressure drop (e.g., 0.3 inch), as well as the recommended final pressure drop (e.g., 0.75 inch). Once this “dirty filter” resistance is reached, it’s time to change the filter. Without a mag gauge, the service technician or maintenance person will not know when to change this specific dirty filter.

In the spring, when pollen is in the air, the air filter will be challenged to capture these particles, requiring the filters to be replaced sooner than the regularly scheduled servicing. Here again, the mag gauge will provide the needed data to prompt a possible early filter change. For the residential unit that has no direct outdoor air connection to the air-handling unit and recirculates 100% of the unit’s supply air, the filters will not be as challenged by the pollen from outdoors. This, and the fact that the air-handling unit cycles on and off rather than running continuously, minimizes the potential for the filter to “load up” with pollen in the spring. As a result, changing residential air filters may be done semi-annually, although other factors can come into play if the homeowner has pets that shed their hair during the year. That said, purchasing and changing one’s own filter more than twice a year may be considered money well spent to capture the dust at the residential unit and to keep the unit’s coils clean.

For building air-handling units that continuously introduce outdoor ventilation during occupied hours and/or operate 24 hours a day, introducing outdoor air mixed with return air, scheduled filter replacements may not be the optimum method to follow. The reason is that the introduction of pollen during the spring can result in an added burden to the air filter’s dust-holding capacity. At peak pollen time, a clean filter can be loaded up with pollen dust within a couple of days.

The scheduled filter replacement plan would not recognize this sudden surge in dust being introduced into the building from outside air. The filter change-out solution for a building operator is to install mag gauges across the various air-handling unit filter sections to record the dust particle build-up. A building maintenance person, on their scheduled tour of the equipment rooms, can check the gauge pressure differential to see if the readings are within the manufacturer’s recommended range. If a filter section has a sudden surge of pollen or dust, the mag gauge will identify an excessively high-pressure drop across the filter unit, indicating it is time to replace the dirty filter. For facility managers with a building computer automation system, and with the correct mag gauges, the pressure differential across a filter section can be linked to the computer, and a high-limit alarm may be programmed to notify the operator of the dirty filter condition and issue an unscheduled planned maintenance work order to change the filter(s).