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. 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. 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. Those first air handlers and furnaces used wire loops, baskets and cages to hold filter media in place.
The 1970’s brought central HVAC to production builders. This leaves the industry a little less than 50 years old. 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.įirst of all, widespread use of residential air conditioning is not that old.
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. 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?
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