Environmental Comfort Team Clears the Air on Air Filters and Cleaners

FOR IMMEDIATE RELEASE
(Free-Press-Release.com) June 16, 2011 -- Many customers are not aware of all of the facts to be considered when selecting an air filter for the furnace or air handler. Generally filters that do a better job of filtering have a higher pressure drop across the filter.

I have to apologize for the technical approach to filters and air cleaners. The recommended velocity through air filters and cleaners is approximately 300 feet per minute. This allows for the air to go through the filter slow enough that it does a good job of removing particles. Obviously with smaller furnaces this is generally not possible.

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Environmental Comfort Team Clears the Air on Air Filters and Cleaners

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This means that if you have a three-ton air conditioner (36,000 BTU per hour) that the filter should be no less than 4 square feet. 300 feet per minute X 4 = 1200 cubic feet per minute or CFM. This means that the filter should be 24 inches by 24 inches or larger to meet the requirements. If the unit is a five-ton unit the filter should be no less than 6.6 square feet. This is equivalent to filter that is 30 inches by 32 inches. Two filters, one on each side of the furnace that are 20 X 25 inches would work for a five-ton air conditioner. Geothermal units typically have large filters, which also meet the requirements for air velocity.

There are different kinds of filters. There is the basic fiberglass filter, one inch pleated filters, two-inch pleated filters, four-inch pleated filters, electrostatic filters, electronic air cleaners, and air cleaners that also incorporate activated carbon or charcoal to remove organic chemicals and odors.

Most furnaces are designed to deliver rated airflow at one half inch of water column static pressure. This amounts to .018 psi of pressure, which is not very much. This includes the ductwork, air filter, and evaporator coil. What this means is that if the pressure drop across the filter is not very low then the furnace will not deliver the rated air flow. Every ton (12,000 BTUH) of air conditioning requires 400 CFM of airflow. A filter with a high-pressure drop can be a problem, because the furnace or air handler will not move enough air through the evaporator or “A” coil. If there is not enough airflow through the evaporator coil the coil will freeze. If there is not enough airflow through the furnace, when the heat is on, the temperature difference through the furnace will be too high and the furnace will cycle on and off of the limit switch. This will cause the limit switch to fail and it will cause failure of the furnace heat exchanger. In other words a filter with a high pressure drop makes it harder for the furnace blower to circulate air through the ducts.

Electronic air cleaners typically have the lowest pressure drop. Electrostatic filters have a higher-pressure drop. One inch pleated filters have a higher-pressure drop across them than four-inch pleated filters because the one-inch pleated filters have less surface area. The pressure drop across the furnace filter should be less than 0.1 inch of water column. If the airflow (CFM) is known the manufacturer of the filter or air cleaner should be able to determine the pressure drop across the filter.

More information can be found online at http://www.heatyourhome.com

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