Installation and Trouble Shooting Air Filters

May-June 2000

J.G. Barry
Director of Sales and Marketing
Dwyer Instruments, Inc.

Introduction: The Application of Air Filters

Air filters used in ventilating systems are called upon to remove an extremely wide variety of contaminants from the air, ranging from soot and smoke and the common dusts to mold spores, bacteria and pollen. Particle sizes vary from less than a micron (.000039″) to insects and leaves. The concentration will vary hourly, daily and seasonally. These variables combined with filtering requirements that range from the simple needs of a household furnace to the absolute filtration required for a clean room make air cleaning a complex science. Filter selection should, therefore, be made by a qualified engineer or in cooperation with the filter manufacturer.

The same variables that affect filter selection also contribute to the problem of determining when a filter has reached the tolerable limit of its effective life span as a result of the accumulation of contaminants. Because of wide variation in contaminant load in the air being filtered, time is an uncertain measurement. Visual inspection of the condition of the filter tells little or nothing. The most widely used method of checking filter condition is to measure pressure drop across the filter by means of an air filter gage.

The Function of the Air Filter Gage

A clarification of terminology is essential to understanding the function of an air filter gage. The air filter gage measures pressure drop across the filter for the purpose of determining whether or not the filter is operating within its design range of effective utilization. It does not measure filter efficiency.

Filter efficiency as defined by the ASHRAE GUIDE, “is synonymous with the term arrestance and measures the ability of the air cleaner to remove particulate matter from an air stream.” Several methods of determining filter efficiency exist, because of variations in filters and application requirements.

One method is by weight, in which a carefully compounded mixture of standardized dust is fed into the air stream and the amount trapped by the filter related to the amount released in the test determines the filter efficiency in percentage. A second method, the dust spot method originated by the National Bureau of Standards, compares the opacity of the dust collected on filter paper from two air samples of equal flow rate, one of which samples filtered air. The efficiency of the filter is thus evaluated on the basis of the darkness of the dust spots. A third method measures arrestance of DOP (Dioctyl-phthalate) smoke, a homogenous aerosol of 0.3 micron size, to test a special class of high efficiency filters used to control particulate matter of high toxicity or where extreme cleanliness is required.

The “dust holding capacity” of an air filter is defined by ASHRAE GUIDE as, “the amount of a particular dust that an air filter can hold and determines the operating life of the air cleaner which is fixed mainly by some figure of tolerable resistance.” Thus, an air filter gage, sensing the differential in static pressure across an air filter, will permit the air filter to be used to its maximum dust holding capacity as determined by the filter manufacturer and specified in terms of maximum filter resistance. It should be noted that servicing a filter when it has reached its point of specified maximum resistance is the only method which will guarantee that the filters are being used to their full dust holding capacity and at the same time not being used beyond the point where their increased resistance could affect balance of the system, increase air leakage and, for some types of filters, begin to pass contaminants.

Type of Filters and Recommended Resistance for Servicing

All filter manufacturers supply technical data which will permit full utilization of their filters. This data will include initial resistance
in inches of water column for the filter at its rated air flow and a recommended resistance at which point the filter should be replaced or serviced. The following describes the broad classifications of air filters: Viscous Impingement Filters have the filter element treated with an oil or adhesive which holds dust particles coming in contact. The initial resistance of a typical filter will usually run from .08 to .15″ water, and servicing will be called for at .50″ water. In the automatic moving curtain type, resistance will usually be maintained between .40 and .50″ water by means of a pressure actuated switch or timer with overriding pressure switch operating an electric drive. Dry Type Filters are available in a multiplicity of materials, in varying thicknesses, in batts, woven or bonded materials, natural or synthetic, and in pleated form or in tubes or bags to obtain greater filter area. Efficiencies are usually at least equivalent to the viscous impingement type and may run to virtually 100%. Dust holding capacity is high and resistance values vary so widely no rule of thumb figures are possible. The manufacturer should always be consulted for proper initial and final pressure drop figures for these filters.

Electronic Air Cleaners using the electrostatic precipitation principle have no characteristic increase in pressure drop as they accumulate dust and must therefore be serviced on a pre-set schedule. The mechanical filter normally used with such equipment should, however, be serviced on the basis of increased pressure drop. Significance of High or Low Initial Resistance Readings Due to the many variables inherent in an air distribution system, the initial pressure drop in some systems may be found to be below that specified by the filter manufacturer. If the system has been carefully checked over and found to be in order, this need not be a source of concern, as it simply indicates that less than rated air volume is passing through the filter.

Generally, this will mean increased filter life before the filter requires servicing. Conversely, if initial pressure drop exceeds the filter manufacturer’s rating, it indicates that a greater volume of air is being handled than the filter is rated for and filter life will be shortened.

Excessive variation from the filter manufacturer’s published initial resistance figure will probably indicate one or more of the following problems:

Low Pressure Drop

  1. Less than rated air volume being handled due to over design of filter
    area, improper air balancing of system, open bypasses, etc.
  2. Incorrect filter installed.
  3. Velocity influence.
  4. Air leakage around filters, past frames, and possibly through
    damaged filter.

IMPORTANT. An initial pressure drop 30% to 40% or more below the rated value for the filter means that approach velocities may be sufficiently low to impair the efficiency of some types of filters.

High Pressure Drop

  1. Incorrect filter installed.
  2. System handling more than rated air volume. Suggest thorough check of
    system to be certain all controls are properly set and operating. If
    unable to reduce pressure drop to less than ten percent or fifteen
    percent above rated value additional filter area should be provided or
    air volume reduced.

IMPORTANT. An initial pressure drop 10% to 15% or more above the rated value for the filter, means that approach velocities may be sufficiently high to impair the efficiency of some types of filters.

Installation Check and Trouble Shooting 

Before putting your air filter gage into service or in the event of initial pressure drop readings that don’t agree with the filter manufacturer’s specified pressure drop, make the following checks:

  1. Check zero adjustment of the gage. Remove both tubing leads or open
    vent valves to atmosphere.
  2. Check all tubing connections for tightness from gage to the static
    tip or fitting connection.
  3. Check static pressure tips or fittings to be sure they are not
  4. Check installation of static tips or fittings. Angle type static
    pressure tips must point directly into the air stream. Flange type
    static pressure fittings should be mounted on a duct wall in such a
    location that the opening is at a right angle to the moving air
    stream. A velocity pressure error can be created if the air blows
    directly into the opening.

About the author: J.G. (Jerry) Barry is the Director of Sales and Marketing at Dwyer Instruments, Inc. He has been with Dwyer Instruments for 10 years in this position and can be reached at

May-June 2000