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Intake vs filter size

Posted: Sat Oct 17, 2015 1:38 pm
by Mark440
After 5 short years, my blower motor died on my Goodman unit (ADPF304216). While the repair guy was here he made a comment that every downflow unit has repeated blower motor failures. (The installation and subsequent repair were done by separate companies.)

If I use a higher grade and pleated filter (20 x 25) the suction of the intake draws the center of the filter down a couple inches. If I use one of those blue fiberglass cheapies, it seems the unit strains less to move air. And somehow.....that triggered an old foggy memory.

The original unit had an A frame filter housing - setting directly over the A frame coils. Two filters were used - one on each side of the A frame. The unit I now have does have the A frame coils - but they taped a single filter housing to the top of the cabinet. Now I am wondering if the motor burnouts are caused by the reduced intake area of a single filter configuration.

Any thoughts?

Re: Intake vs filter size

Posted: Sun Nov 01, 2015 6:15 pm
by Mark440
I decided to dig in to this to get my own answers. And, for the most part, what I found simply stirs my hackles.

To start - my unit is 22"w x 24"d. The filter housing is for a 20x25 filter - but the housing is 22'w x 25"d. I dug through the specs on my blower unit to find its CFM capacities. Of course, Goodman rates the capacity based on a filter being in place - but they don't say what size or what rating for the filter.

So - I dug into filter ratings. We are all led to believe that the higher MERV rated filters are 'better' because they filter out more junk. That's all good and fine - but the higher the MERV rating, the higher the resistance of the filter - which equals less air flow.

Since Goodman failed to provide measurements with NO air filter - I had to use their "CFM delivered against external static pressure" table. From there I took every rating - and then did the calculations. My assumption is that the higher CFM rating is somehow relevant to the cheapy fiberglass filters. Following that assumption, the higher MERV rating filters would deliver the lower CFM ratings.

In every single instance - the filter size installed on my unit is too small - with the range going from 38 sq inches too small (cheapy fiberglass) to 100 sq inches too small (MERV 8+). So, no matter what filter media is used - the filter is too danged small.

Part 2 of this was finding information that connects restricted air flow to burned out blower motor. Not one mention of it anywhere on the internet. BUT!!! Virtually every site I went to all said the same thing: blower motor burnout can be caused by clogged air filters (read restricted air flow).

It seems there is a huge reluctance to simply say that an undersized filter WILL cause motor burnout. Ironically, there is absolutely zero degradation of performance if the filter is over-sized!! Additionally, research done somewhat recently indicates that higher MERV rated filters end up costing more in energy costs.

If folks can reconfigure their filter units to accommodate usage of the higher MERV rated filters and NOT burn out the motor - gees - some repair guys are gonna get hungry! And while I mean them no ill - that $300 for the blower motor replacement bill looks and feels much better in my pocket. (I should note - that was just labor. The actual motor was covered under warranty.)

I have since taken down all the measurements necessary to construct an A-frame housing that will fit on my unit and accommodate two of the 20x25 filters. And while this may well not eliminate blower motor burnout forever - I will hope that it at least puts greater distance between replacements. Believe me when I tell you that Goodman (and all the rest) have a portion of their bottom line profits riding on "post warranty parts sales" - not to mention the local contractors whose livelihood depends on those failures.

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