Excellent information by P.student for all to follow.Originally posted by perpetual_student
All the people who recommend Manual D already know this but... filter area is only one part of the need. If you have marginal area for the filter, it will load up quicker and the dirty filter will raise ESP (External Static Pressure), perhaps enough to reduce airflow and cause other malfunction. I believe the disapproval we often hear from pros about 3M filters being too restrictive, applies especially in this case. On the other hand if you have lots of filter area in your system, I believe you can prove that ESP is not adversely affected (I watch ESP and sometimes use Arm & Hammer 8 MERV filters). Unfortunately the number of houses with too little filter area seems to outnumber those with enough.
The short form of that message is you are not guaranteed to be in safe territory by checking filter area. You also need to have some concern about the pipe area behind that filter. One rule within Manual D is that you want to measure the air speed at your normal CFM and keep it to a low level, perhaps 500 FPM (feet/minute). It is nearly impossible to have too much return area, so you won't need to worry about that.
With such a number of returns in your system, I would like to think the calculations of filter area and pipe area, will indicate you are OK. Running thru a Manual D analysis will be the most professional way to check this. I believe measuring your system ESP will also signal whether it is OK. And calculating FPM in your ducts is still simple enough, perhaps you can do this even though not many people can do a Manual D. Hope this helps -- P.Student
If you go to the newer deeper (4" or more deep) pleated filters it will provide more total filtration area within the dimensions of the return filter boxes.
The problem there is that some of those filters are so thick they will only fit in extra deep return air boxes.
They also put more pleats per linear foot of face area, (strange standard). Pleats column is the number of pleats per linear foot of pleat width.
Every filter should have the new "initial static pressure drop listed in large print."
If a designer were to follow "Best Practices," it could be said to be wise to have at least two return air Filter Grille Boxes that would accommodate 4" thick pleated filters.
It has been a while since I checked however then Hart & Cooley standard filter boxes were only 1" deep!
Do you know any suppliers' of 4" deep filter boxes?
If there were "two of these oversized sq. in. area," 4" deep low static pressure filters installed in A/C systems with user load sensor warnings, there would be far less waste of energy and cost to users'.