You weren't an engineer were you?:redflip
If you have (just for numbers sake) 50CFM flowing through the filter at 90kpa absolute and you restrict half of it, your going to have around the same pressure and CFM because furnace ducts are not restrictive. The amount of available air is still there, it just has to go to the part where the joist is not.
Will the furance require more power to turn the blower motor? Won't even be measurable and the amount of the airflow will not change.
I'm not an engineer but I am a Respiratory Therapist and well versed in gas physics.
You're correct about the pressure being the same at the duct
and that the furnace won't be affected because the same volume of air will reach the duct opening at a faster speed.
However, that's the problem. With increased flow through the filter
, the pressure placed on the filter media increases thus making it less effective and that's the important issue.
Unless the filters he's purchasing are rated for a higher pressure than the standard blower unit produces.
Ed, you need to have an HVAC guy come out and do an air velocity/balance test on it. It may be no issue but at least you'll know if you need to do anything at all.
Hell, you may end up just having to spin the current filter 180* and use the clean side :laughing