Proper filter application remains one of the biggest mysteries in the HVAC industry for good reason. There are so many filter choices with so little information; it is difficult to match filter performance with system requirements. Consumers yield to flashy marketing not understanding the mechanics of the selection process, the contracting community unknowingly gives up early sacrificing performance, and the overall industry suffers the consequences. Restrictive, undersized filter selection generally ranks high on the list causing underperforming HVAC systems. There have been effective electronic air cleaners around for many years, but for our conversation within this article, we will focus on media or fabric material filtering.

Each furnace or air handler has some sort of filter housing and filter correct? So what’s the problem? 

Manufacturers must provide some type stop gap filtering to protect the equipment on initial start-up where air flow instructions or requirements are forgotten or ignored. The integral filtering has little to do with real life HVAC performance. Manufacturers must depend on installing contractors to navigate road blocks the structure brings to the table. Each job must be analyzed and viable solutions employed to insure the equipment is protected and the system flows sufficient air. If we read the manufacturer’s instructions thoroughly, it becomes apparent that one inch filters on anything but the smallest air handler or furnace are inadequate. Why? 

When we look at the dimensions provided for air to find its way to the blower wheel through the appliance chassis, we find the required cfm arrives at velocities that exceed filter manufacturers specifications. A 2 ton (nominal) furnace or air handler with a chassis opening of 16” by 20” must flow approximately 720 to 800 cfm to fill the duct system with performing air.  When we calculate the required velocity, we find a value of 320 to 360 fpm. For a 3.5 ton, the numbers run at just under 500 fpm and 5 ton, just under 700 fpm. These numbers exceed practical flows by a factor of two to three times, taxing filter effectiveness and pressures (static) within the duct system. The result is ineffective filtering with excessive noise, energy use and the potential for short equipment life. 


First of all, widespread use of residential air conditioning is not that old.  The 1970’s brought central HVAC to production builders. This leaves the industry a little less than 50 years old.  Those first air handlers and furnaces used wire loops, baskets and cages to hold filter media in place. These type products made filter changes a difficult task for home owners. The results were filters did not get changed regularly and evaporators were cleaned periodically. Older evaporators were more accessible at that time and fin per inch count (density) was such that a light shined through the evaporator revealed whether or not the evaporator was dirty or clean.  In some instances chemicals were sprayed onto the surface of the evaporator. These chemicals would loosen the dirt on the aluminum fins and a foaming action would push debris out of the spaces between the fins. The first attempt to provide a practical solution came when a company realized filter access shortcomings and devised a filter frame to fit beneath an up-flow furnace or air handler. This frame, when installed with new or existing equipment allowed a homeowner to change the filter in 10 seconds and leave the furnace or air handler access undisturbed. The product was very popular and gained market share quickly. As energy efficiency gained greater importance, manufacturers found adding more mass (copper and aluminum) to evaporators gave betters performance numbers. So gradually the evaporators became denser, air flow more restrictive and filtering more important. Today’s evaporators have more fins per inch and the fins are pleated or waved for more surface area. This fact emphasizes the importance of providing protection for the evaporator with proper filtering because new evaporators can no longer be cleaned in place. 


The lack of available product information has been an issue for years. Filter manufacturers are reluctant to publish data on filter performance due to complexity of the issue and the confusion the data can provide. Just as there are many formulas to obtain a particular conclusion to a mathematical equation, there are ways to test filter designs and justify their use. Two filters may look identical from a consumer’s standpoint however the particulate size the filter stops and the air flow rate through the material can vary significantly.  So what is a practical velocity flow to benchmark filters by? Leading utilities in California (Pacific Gas & Electric, Southern California Edison, San Diego Gas and Electric, and Southern California Gas Company) commissioned a study* to serve as a guide for air flow velocities that support efficient, effective filter performance. The study indicates 300 fpm face velocity for filter flows as a maximum. This is a far cry from the generally accepted velocity of 500 fpm most filters are rated at. Pressure drops can double at the higher velocities costing consumers comfort, noise and money in operating costs and warranty issues.


If we use one inch filters or inferior media material we must slow down the air flow well below the 300 fpm marker. When we slow down the air we require more filter surface. More filters mean more service points, possibility of higher infiltration and excessive replacement cost. There are many other factors to deal with when selecting appropriate filtering. We set aside technical aspects of filter performance (dust spot, particulate matter and gaseous pollutants) and have focused on the physical restrictions of the air delivery system. As a HVAC contractor you should assess air flow requirements and filter performance to insure adequate comfort and equipment protection on each job. We must pay attention to expected conditions within the home with regard to particulate size to assure we are making the proper choices to insure great indoor air quality. The contractor should then drive the customer to the appropriate filter market with education and /or a program for maintenance. 


Instructions?  We don’t need instructions!  We seldom review equipment installation instructions. The enclosed information states airflow and resulting static pressures are provided minus a filter. (Figure 1) That’s right no filter! Some manufacturers use a washable low density filter to keep wrenches and rocks out of the blower wheel. Think about it. The manufacturer has no idea of limitations on installed equipment, so they defer to the contractor.  Contractors must be aware of filter performance for each brand and make selections for individual customers based on need. While most customers will be happy with the performance of a low maintenance deep pleated media filter product, there will be occasions where we must employ a top end combination electronic / media type filter. When these select products are used, please review the performance statistics. The best filter on the planet is a pane of glass; however it will allow no air to pass by through. This is the problem with some dense filtering products on the market. They may offer superior filtering however low airflow and high energy demands on the equipment are the result. It bears repeating; if you have air handlers or furnaces that are over the three ton nominal mark, you will have to use two filters for proper operation in most instances. (Figure 2) [There are exceptions as one popular company offers a single media filter with a static pressure of only .06 on 5 tons (2000 cfm) of nominal air flow. This product must be used with horizontal air flow systems]. Most installation directions will instruct us on setting up proper airflow as it relates to static pressure. These instructions will caution the contractor to pay attention to transitions as well.  Air will not flow beyond 22 degrees, so transitions must maintain a 5/12 ratio. In other words, we need one foot of depth for every 5 inches of width outside the equipment airstream. (Figure 3) Of course most evaporators today are designed to fit furnaces. These rules hold true on both the return and supply side.  


In summary we should use the fewest filters possible due to costs associated with replacement and lower energy consumption. Generally, two higher performance filters installed correctly will accommodate just about any single system equipment situation. 

Never double or stack any filters as the costs in available energy kills performance.  

Disposable filters should never be cleaned and reused.

Filter selection can be complicated and discouraging. However with a little common sense and diligence your labor will yield amazing results. By cataloging your selections from research, you should be able to build a library of filters to suit just about every need.This area of contracting is one of the most overlooked factors of underperforming systems you will find.

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