Life-cycle-cost analysis is something we do every day as consumers. It has taken us from bias-ply tires to radial tires, which provide longer life and better performance, and led many of us to purchase hybrid vehicles, a decision based on the price of gasoline and calls for energy independence.

During the development of ANSI/ASHRAE Standard 180, Standard Practice for Inspection and Maintenance of Commercial Building HVAC Systems, there was extensive discussion of the decision to run HVAC-system components to failure. In many cases, building owners find running equipment to failure cheaper than maintaining equipment over time. Given prevailing attitudes toward sustainability and concerns about environmental impact, however, running to failure is falling out of favor.

Properly maintained, the main components of HVAC systems—coils, fans, casings, controls—should last 20 years or longer. One component that has a major impact on the cost of maintenance, the lifetime of other components, energy use, and the protection of building occupants and processes, however, is considered disposable: the air filter.

Key Factors
Contaminant capture. In the selection of air filters, the primary consideration should be maintained contaminant-capture efficiency. Once that is assured, the life-cycle cost of air filtration can be analyzed.

Most critical to the analysis of air-filter life-cycle cost is pressure drop, or resistance to airflow, over the life of a filter. Filter selection should be based on the fan curve made by the original system designer. In some cases, with constant-speed fans, operating outside of original design pressure-drop parameters—by selecting filters with too little or too much resistance—can result in unnecessarily high energy use. In such instances, a blower may have to be re-balanced to its fan curve for energy savings to be achieved.

Fortunately, most systems today employ variable-frequency drives (VFDs), which adjust to system demand, allowing a wider range of acceptable resistance. In a VFD system, the lower the resistance, the lower the demand for energy required to move air through filters and, thus, the greater the number of air changes per hour, which results in a higher contaminant-removal rate.

Design plays a major role in a filter’s resistance to airflow. Generally speaking, the greater the amount of media within a given area (e.g., 200 sq ft of media in a 24-in.-by-24-in.-by-12-in. box filter vs. 58 sq ft of media in a 24-in.-by-24-in.-by-12-in. box filter), the lower the resistance to airflow, as air has a greater number of paths. It is analogous to moving 100 cfm of air through a 6-in. round duct as opposed to an 8-in. round duct; resistance in the latter is only 20 percent of the former. However, simply cramming more media into a filter could result in a higher resistance to airflow and a shorter life. Media configuration is important in the design of high-media-area products, perhaps even more important than the amount of media itself.

Contaminant loading. Air filters with long loading curves use less energy over their lives and require fewer changeouts. In states with additional fees associated with medical-facility refuse removal, the cost of removing final filters can be exorbitant, often dwarfing the cost of the filters themselves.

As media area increases, contaminant-loading curves decrease exponentially. Figure 1 shows the loading curves of three identically configured and sized air filters, each with a different amount of total media area. Relative period of time is shown on the X-axis, as loading curves are relative to contaminant loading, which is specific to a geographic area or facility.

In Maine, an air filter may last the equivalent of four periods before requiring changeout, while in New York City, it may last only one period because of increased contaminant load. Filter-service intervals must be based on local air quality. Thus, if a company is applying life-cycle costing companywide and it has buildings in New York, Los Angeles, and Toronto, it must establish the proper change interval for each of those locations.

Environmental impact. Materials used in manufacturing, disposal methods, and carbon footprint are important considerations in air-filter life-cycle-cost analysis. Many localities offer tax advantages for "green" products.