HVAC: Cleaning Filters

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Filters play two important roles: they help maintain indoor air quality and they protect downstream components of an air-handling system (the evaporator coil and fan) from accumulating dirt that cripples performance. Figure 1 shows a typical rooftop unit filter arrangement.

Filter-changing intervals can be based on the pressure drop across the filter or (more commonly) by calendar scheduling or visual inspection. Scheduled intervals should be between one and six months, depending on the pollutant loading from indoor and outdoor air. More-frequent changes may be required during the economizer season, because outdoor air is usually dirtier than indoor air.

Measuring pressure drop is the most reliable way to rate filter loading. However, this approach requires some effort—most rooftop units do not have built-in pressure taps. Taps can be made by drilling into the cabinet wall and installing quarter-inch tubing with removable caps. A technician then can use a hand-held pressure meter or manometer to check filter status. (To get accurate readings, cabinet access panels must be shut tightly, with all screws replaced.) In facilities with predictable and regular filter loading, pressure measurements can be used to establish the proper filter-change interval; thereafter, filter changes can simply be scheduled.

Pressure-measurement taps are a bargain, given that they cost less than a single change of a high-quality filter. Complete air-filter pressure kits that include a dial gauge cost between US$50 and US$100. Hardware for installing taps that can accommodate a portable gauge costs less than $10, and this task can be performed by a service technician in a matter of minutes. Automated pressure tracking is a further step in filter management. It makes the most sense for very large rooftop units (20 tons or greater) and for large, custom-built air handlers (for which a single filter change may cost $100 or more). There are two ways to track pressure automatically:

• Install transducers that feed pressure-drop data to a building automation system. This data then can be used by building operators to check filter status, or it can trigger an alarm when pressure drop exceeds the design limit. Unfortunately, this strategy has a long payback when applied to small rooftop units, because the cost of pressure transducers starts at about $260 (for a model with ± 3 percent accuracy).
• A less-expensive option is to use a differential pressure switch that flips from “off” to “on” at a certain pressure difference across the filter. However, these switches require time and care to calibrate, and one experienced service technician maintains that they cause more problems than they solve.

Copyright 2006 - Platts, a Division of The McGraw-Hill Companies, Inc.