This paper presents a method for calculating the optimum filter change interval for central air handling units. The calculations consider the change in fan power as filter dirt loading (and therefore pressure drop across the filter) increases, the cost of filters (including disposal), and the cost of labor.

The calculations were performed using a bin method analysis for a variable-air-volume system in Boston, but the method can be applied to any variable- or constant-volume system in any location. The bin method calculations were performed using a microcomputer spreadsheet program.

The paper describes each element that goes into the calculations: estimating the annual average mix of indoor and outdoor air for the system under consideration; estimating the filter dirt loading characteristics for that mix of indoor and outdoor air; estimating the particulate concentration in indoor and outdoor air; estimating fan energy consumption and resulting electricity cost as filter dirt loading increases; estimating the annual cost for filters and fan energy so total operating cost can be minimized.

The paper explores two examples, but its significance is the method presented and its applicability to various system types rather than the specific results of the particular cases examined.

Units: I-P