What To Charge For Adding Ducts

 
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Old 02-08-2007, 04:40 AM   #1
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What To Charge For Adding Ducts


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.


Last edited by 72chevy4x4; 02-08-2007 at 04:43 AM. Reason: clarification
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Old 02-09-2007, 10:12 PM   #2
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Re: What To Charge For Adding Ducts


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.

Last edited by Diesel65; 02-10-2007 at 08:55 AM. Reason: adding more info
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Old 02-10-2007, 02:53 AM   #3
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Re: What To Charge For Adding Ducts


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.
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