72chevy4x4

I'm working on a commercial site-one floor has a 550 sqft room supplied by two 10" flexible ducts-no returns. The goofy part is one duct is from one split system, the other from a second split system. Whenever the want to use that one room (w/o using the rest of the rooms on that floor), both systems must be turned on.

I've proposed piping both 10" ducts to one system and installing a return. Being as there are no returns currently, would 2 10" returns be ideal, 1 10" or possibly 1 12"?

edit: the units use 3.5 ton AC units-not sure of the gas heating output. The floorplan includes approximately 11 rooms, most of them less than 350-400 sqft each-only the room in the original question is served by two heating systems.

Diesel65

3.5 tons is 1400 cfms, at .075 friction loss(which is the standard ductboard and flex friction loss #) you would need 2 12" or one 18" or one trunk line of 20"x12" to accomodate your return needs at a minimum.

Your two 10" supply should also be 12" or a duct 20"x12"

Unless the system design is .2 (which is not likely) with flex duct, but if it is your 2 10" flexes will work.

Best I can do for you not knowing the design parameters of the complete system.

AirPro

The first thing you need to do is get return air out of that room, into a common area where both units are serving. Rule of thumb here is to go 2 sizes bigger on return air vs supply air (with flex duct). You could use one thermostat to control both units if this is not the ONLY room requiring heat/cool at any given time. Other than that, the correct way to do this would be to reduct and zone both systems according to manual D calculations.
I use .1 static per 100' as a standard for flex duct if I don't have a manual D, that being the case....you will be taking too much off of one system either way, as a 10" flex duct @ .1 = approx. 415 cfm (about the same that is required for 1 ton of air). There really is no easy way to get around this problem without doing it properly. I see headaches in your future if you try to do it any other way. Good luck.