one think i do with one return filter grill, to get better filtering, is to use a filter coating. When you buy filters they are often dried out too much so the finer particles can go through them. Using the filter coat will stop that without restricting the air flow. I use it all the time. Our local wholesale house stocks it.http://www.smithfilter.com/misc.html later paulbm

What size are the (2) filter racks? You could get by with 1800-CFM that is 360-CFM per ton of cooling - good for high humidity conditions, how much CFM are you trying to deliver? Five tons with 2000-CFM is a lot of RA to run through the filters. On linked page; Top of page second row left, Click-on, Air Filter Rack Sizing - udarrell Here is a link that might be useful: Sizing of Residential Ductwork Systems

I found this note on page 11 of the installation manual under upflow units: NOTE: Where the maximum airflow is 1800 CFM or more, both sides or the bottom must be used for return air. This is saying the bottom return by itself will be enough. There was note under the upflow unit section, but I assume it also applies to a horizontal installation. You want the filter and grill cover to be at least the size of the opening to the furnace. Making the grille bigger is not going to help if return itself is smaller. I don't know how this is going to work with the filter size you quoted. I would think you would need two 14X24 filters. It would be nice if you get get a 4 inch filter instead of the 1 inch. Otherwise you will be changing the filter every month. The 6x6 stud obstruction should be removed and you want a tightly sealed return duct. The return duct should not have a right angle. It should be a curved elbow to make the 90 degree transition. Sharp angles create more air resistance. How big was the original house and AC, and much area are you adding? Most houses don't have the properly sized duct work to handle a 5 ton AC. Do you have confidence in this HVAC contractor?

Okay, here is the problem I have with this: 1. We are only talking about MERV ratings rather than the ability of the filter to pass air with little restrictions. CFM is a critical issue, particularly with A/C and heat pumps. 2. I'd love to see some impartial research on the overall effectiveness of advanced air filtration in residential systems. I'm not interested in the effectiveness of the filter itself. I want to know how effective the entire system is when you have distributed sources of dust and such along with an air circulation system that isn't on all the time. I suspect that people are spending a lot of money on filters and such that won't really do much good. 3. Are you talking about a powered electrostatic filter unit or the new filters that claim to be electrostatic? 4. It sounds as if you have an add-on filter box to address the airflow restrictions that the filters introduce. That's a good thing - a lot of people just substitute the old filter with the high restriction filter.