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Old 01-15-2011, 11:54 AM   #1
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Very small gas forced air application


My application is going to be quite small. Probably not 50 feet of pipe in the whole project. 6 or 7 registers. The unit I am purchasing has an output of 1200CFM. Are all these concerns about friction and the like, and calculations that important in something this basic??

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Old 01-15-2011, 12:09 PM   #2
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Very small gas forced air application


Not enough info. You should do a heat load and loss calculation to get the correct sized unit. 1200 cfm is good for 3 tons of AC. If that is the size you need in a small house then fine. If the ductwork is too small then the velocity will increase and you could get whistling and aggravating air noise from the vents. AC could freeze from poor airflow and furnace could overheat. More to it than meets the eye. Need an experienced sheet metal guy to look over the project is what I would recommend.Load Calculator

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Old 01-15-2011, 03:09 PM   #3
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Very small gas forced air application


No AC involved!
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Old 01-15-2011, 03:44 PM   #4
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Very small gas forced air application


Heating a garage? How many BTU is the furnace?
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Old 01-15-2011, 05:26 PM   #5
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Very small gas forced air application


7 supplies for 1200 CFM. better be gig duct runs, you ain't gonna get 1200 CFM through 7-6" supplies without sounding like a wind tunnel.

So if you don't mind lots of air noise from your registers, then no, friction rate isn't important in your home.
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Old 01-15-2011, 06:21 PM   #6
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Very small gas forced air application


There is a reason for everything, and it doesn't matter of the size, big or small, it always pays to do all the prep work prior to buying and installing a heating and air conditioning system.

Too much air flow and too large a burner, results in noise and short cycling. Not enough heat and air flow, results in uncomfortable atmophere and eary equipment failure. Both result in higher operating cost.

Do yourself a favor, spend some time and learn how to, or hire someone to do if for you, but you need to do either.

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