Positive and Negative. Two words that are often used to indicate how someone feels about a certain issue or to describe the particular temperament of an individual as in “he is always so negative.” These two words are also used in the math world to indicate whether a particular number is positive or negative. But, for the purposes of this blog, we will use these two words to discuss the physical relationship of positive and negative pressures as they relate to heating/cooling systems and mechanical air filters .

When people think about air filters they normally associate them with the one they have in their heating/cooling ( HVAC) system. They need to be replaced about every 3 months or so as they get dirty and can create some air flow problems in the HVAC system. Most people are unaware of the fact that air flow through the HVAC system is affected by a number of variables.

In order to move air through an HVAC system requires a motor powered fan system. The size of the motor, in horsepower, is dependent upon the size of the system. For the most part, there are four sizes; 1/3,  1/2, 3/4 and 1 HP. The fan/motor assembly has to have enough power to move hot or cold air ( and cold air is heavier so it requires more power to move the same amount of air ) through a series of supply and return air ducts and grills as well as through the HVAC filter. Most motors are now multi-speed or ECM digitally controlled motors that automatically adjust speed to fit the air temperature and pre-determined comfort levels for the home.

The fan/motor assemblies have to both pull the air from the return air ductwork, pull the air through the air filter and push the air through the burner or condensing coil and supply side ductwork. The pressure to do this is measured in inches of water. For example, a 1200 CFM 3 ton HVAC system would use a 1/2 HP motor and the total system static, less the air filter, would be .5 ” to .7″ of water. This total pressure does not include the use of a standard, throw-away fiberglass furnace filter. The pressure drop or resistance across a minimum duty filter clean is about .2″. For some of the more dense media filters like the 3M Filtrete filters the clean pressure drop can be as high as .4″ to .5″. Even clean these filters already have almost as much negative pressure as the entire HVAC system.

Once these filters or any air filter gets dirty the pressure drop across some filters can reach 1.0″. This means the motor/fan assembly will not be able to pull and/or push enough air through the HVAC unit to adequately heat or cool the home. As the pressure drop increases, unless you add more horsepower, the motor/fan assembly will actually slow down. This is why it is so important to understand the relationship between pressure ( positive and negative ) as it relates to the overall effectiveness of the HVAC unit.

For more information on understanding the adverse affects of filters on air moving devices please go to the “Pure Air University” tab of the following web site. www.pureairsystems.com




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About the Author: Don Musilli

My name is Don Musilli. I started Pure Air Systems in 1985 and then sold it in 2006. I continue to consult for PAS and write their blog and control their Twitter account. I also, on occasion, make changes to the website. The company is now almost 26 years old and has been a major player in offering commercial grade, high performance HEPA and Carbon based filtration systems for the commercial, institutional, industrial and residential markets.

I currently reside in Englewood, Florida where I write blogs and do social media marketing for a number of clients.


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