Air Flow and Filters


We all know how important proper air flow is to both heating and cooling systems.  Without proper air flow, furnaces can cycle on  limits or the consumer feels it is always cold air coming out of the register.

Likewise, with Air Conditioning, air flow is even more critical.  Too little and the coil freezes up or we slug liquid back to the compressor, eventually damaging the compressor.  Too much and we lower our sub-cooling, raise our discharge pressure, and have a high saturated suction temperature.  In fact — what is actually happening is the increase load on the coil transfers too much heat to the refrigerant.  More refrigerant is vaporized, elevating the temperature & pressure.  The hotter refrigerant entering the condenser requires more of the condenser surface to reject the heat.  More condenser is needed, less room for sub-cooling. This equates to a lower sub-cooled refrigerant..  this just continues and becomes a vicious cycle until the compressor eventually stops or a safety opens.

So, why do I bring this up again? I have a whole post on proper air flow. Well, as we all know, certain aftermarket air filters are a key source of reduced air flow in furnaces and air conditioning systems.  Some of these filters can add as much as 1 inch of external static to the duct system.  These companies have done a great job marketing these to homeowners. Of course, the old saying is “the dirtier a filter gets the more efficient it becomes”.  While this is true, it also reduces the amount of air through the filter.  So now we paraphrase the saying, the more efficient the filter is “suppose” to be, the more air flow problems we are going to see.  The homeowner can add these high static filters at any time and reduce the air flow through the unit by as much as 40%.

Some of these aftermarket brands do not even use the MERV rating system so we have no way of knowing the static drop or true efficiency. None-the-less, homeowners see the “brand”, see they tout it as high-efficiency saying only needs to be changed once every 3 months, and they buy them and put them in. (It’s amazing what a “brand name” and marketing can do :>)).

The whole purpose of this post is to remind you, now that air conditioning season is starting, of how important air flow is and how easy it is to overlook something like a filter causing problems. When doing those clean and checks on furnaces or air conditioners, take the time to check the filter.  In heating, do a temperature rise.  In cooling a temperature drop across the coil.  Both are helpful, but the most important check is to measure the ESP (external static pressure) of the system. By doing this, you will solve a lot of problems both in heating and cooling.

Once you know the static, you can see what the air flow is.  If there is no “book” on sight, just keep in mind that almost all manufactures rate their blower performance at one half (0.5″) inch static.  If your static is over that, chances are you have air flow problems. Now you can go back to the manufacturer and actually see what the air flow is from their blower performance charts. Also, another good indicator is if the RETURN static itself is over 0.2 IWC, you definitely  have a return side air flow problem.   CHECK THAT FILTER!

total static measurement

Take the time to educate your customer and let them know that changing the filter once a month with a MERV 7 or 8 is much better that putting in one of those other unrated filters that cost 3 to 4 times more.  And, because the air flow is better, their operating costs will be reduced.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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4 Responses to Air Flow and Filters

  1. Albert Schevey says:

    how do you determine the correct capacitor for a single phase compressor reciprocating and scroll without any nameplate info

    • all you can do is try going through the unit manufacturer with their model and serial number or the compressor manufacturer with their model and serial number. usually one of the 2 are available

  2. Robert says:

    What are your thoughts regarding plenum returns verses duct returns. One of the obvious is cost but how about system performance.

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