Wow.....allot of questions being tossed out here.
To the OP, if your return sp is greater than supply the return is undersized or you have dirty filter. Systems that I do I always have the return sp is less. No rule of thumb, but I want to keep return sp low because it's easier to push the air than draw the air.
There is talk of pd across the coil, bear in mind the coil will have a pd for wet and dry conditions....always design for wet conditions and a dirty filter. The only real way to know if the coil is dirty is take a reading across the coil, I.E., before and after. Hopefully you will have mfr literature to tell you what the pd should be at the cfm your system is providing, and coil status (wet or dry)
The question was asked about literature to read...I will think about that and see if I an find something, but SMACNA manuals or ASHRAE fundamentals comes to mind. If you have a specific question, post and I will try and answer.
I disagree that dirty filters etc. result in higher operating costs the system is working more. It is actually working less. Take a blower and run it wild with no restrictions....amps go up. Stick board in front of the outlet and restrict flow and amps go down...it's moving less air and doing less work. Your losing efficiency, which wastes energy and is a loss of efficiency causing the system to run longer.
What is important is tesp. If the coil is cut in, add that to tesp when looking at blower curves. If the coil is integral to the air handler, disregard unless your trying to find out if its dirty.
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