It is always interesting to see the amount of new technology being developed every day. New computing devices, new 3-D TV’s, new ways to generate energy and yet we understand so little about how these systems actually work. We just assume that when it says to push the red button the system will turn on or off or clicking on this URL will get us to internet nirvana.

This lack of interest in actually knowing “how things work” can be to our detriment. If we don’t know the how or why of a product or system we will make a buying decision based solely on looks and price. For some items this is appropriate. But in many cases some basic research on the product or system is warranted.

Take for example the term “Delta P” or sometimes it is written out using the pyramid symbol with the P following it. This term means pressure differential, pressure drop, resistance in air flow as measured in inches of water and has a lot to do with some products you buy. Specifically air filtration systems and their relationship to their performance and performance of systems they are attached to.

Air filters or any type, style or media make up all create a certain amount of resistance when placed in an enclosed device like a furnace filter in a filter frame or a filter in a fan powered air filtering (air purifying) device.

When the filter is placed on the air entering side of the fan or air moving device it will create some resistance or increase the amount of pressure the fan has to see in order for the air to move through the filtering device. This pressure drop or Delta P is important as it affects both the air flow performance of the heating/cooling system or the filtering device itself.

When you have an air moving device like the fan in the heating/cooling system, the fan/motor assembly is designed to move a certain amount of air ( measured in CFM) at a specific pressure drop or resistance ( measured in inches of water). Let’s say the fan is designed to move 1200 CFM at .5″ of water. This is with a general duty, Walmart fiberglass furnace filter( this filter has a rated pressure drop clean of  .1″).

If you want to install a better, more effective filter in the system, say a MERV 9 or 10 filter, this media has an initial resistance or Delta P of .4″ . This means the furnace fan will have to work harder to pull the air through this filter. If the motor/blower system is only designed to move 1200 CFM at .5″ or .6″ then something has to give. What happens is the furnace fan actually backs down due to the increase in pressure drop and the air flow is reduced. Depending upon the type of fan and motor being used in the HVAC unit the air flow can be reduced by as much as 100 CFM per additional .1″. So if you add .3″ of static that the fan system is not designed for, you can reduce the air flow by 300 CFM.

This air flow reduction is significant in that it now requires the system to run 30% longer to either heat or cool the house due to the additional resistance across the filter. When using filter media such as HEPA filters the initial resistance across these filters can be as high as 1.0″. So it is important to understand the relationship between air flow and Delta P.

For more information on this subject matter and information on the complete line of HEPA filtration systems offered by Pure Air Systems please go to our website at:

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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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