Duct Size Calculations
I am having a new HVAC put in, and doing some homework to spotcheck my contractor. I was looking to calculate the size of a return, but this looks a bit more complicated then anticipated. I am trying to figure out a discrepancy between different contractors installing a residential HVAC system. 2 state I need a new return, one does not.
The one that does not has very good references, but I am concerned that he is in the minority on the thoughts of the size of the return. He is also the one that seem all around to know what he is doing (except this one point)
I have a old furnace/AC unit, going to a Trane2.5 ton AC unit (15 SEER) and 95% furnace (85K BTU) - itÂs a 1900 sq ft house in PA.
The current return is 8X18 giving me either 725CFM (0.08) or 820 CFM at .1SP. The 2.5 ton requires 1000 CFM - meaning we should increase to 8"X22" @0.8. My contractor says I only need to be at 80% of the 1000CFM for the system to work correctly (where I am with the current return) and is fighting me on increasing the return size, stating its not necessary.
I would think if the 2.5 ton needs 1000CFM, it needs 1000CFM. Right now with my 8"X18 I am at around 800CFM (80%). Will this work? Is my contractor just not wanting to do the work or am I reading into things?
I don't know what to do, or how to convince him either way. He wrote in the contract "all ductworkd requried, etc", but when I realized that he wasn't planning on increasing the return for the reasons he said above, I am now questioning.
This isn't my profession, so I will have a difficult time to present a case and reason for my inquiry