Had a contractor I have been working with out today, I asked him about the 25x10 being too small for a 4 ton a/c. He took out a chart and said that it was adequate for 4.5 tons. Said running returns to both sides of the furnace was for 5 ton and up. I have heard that rule of thumb from other contrrators as well. Didn't mention what you wrote since I don't think it would have been well received. This is hard because I believe what you say. Did you get the 1600cfm by multiplying 4 tons by 400cfm per ton? Where did you get the info saying that I needed a 24x18 return? If I can get a published source I can show it to him without him getting too defensive.
As far as having the returns on both sides I would be willing to pay for it but I will have trouble getting someone to do it. They will say I don't need it. Do you think that most people with a 3.5 to 4 ton A/C and a spaceguard filter are running high return static?

[Edited by heetseeker on 06-08-2005 at 08:49 PM]