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Old 09-03-2011, 11:47 AM   #1
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What size main trunk (extended plenum)?


What size do you guys think I should have for my main trunk that will be 20 feet long? 70,000 btu forced hot air, 2ton ac. 8 inches by ? I keep coming up with 8x20, 8x22, and once in a while 8x18. What do you guys think? Thanks in advance.

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Old 09-03-2011, 06:27 PM   #2
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What size main trunk (extended plenum)?


About 8X28

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Old 09-03-2011, 11:06 PM   #3
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What size main trunk (extended plenum)?


Sizing you main trunk has many things to think about. Is it a straight run? How many heat runs come off of it? what size are they? etc. assuming you have a 70,000 btu furnace your looking at about 12oo CFM which would be 28x8 duct if straight and only feeding 1 floor.
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Old 09-04-2011, 05:31 PM   #4
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What size main trunk (extended plenum)?


I thought we would figure 400 cfm per ton of ac, 2 ton = 800cfm's that's why I came up with 8x20?
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Old 09-04-2011, 07:04 PM   #5
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What size main trunk (extended plenum)?


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Originally Posted by Heatless View Post
I thought we would figure 400 cfm per ton of ac, 2 ton = 800cfm's that's why I came up with 8x20?
The duct work isn't sized to the A/C because its A/C. Its sized to the one that needs the most air flow. Which in this case is the heat.

Your furnace needs to move more air for heating then it does for cooling. Or you'll end up tripping on the high limit, having very poor heating, and could even burn out the heat exchanger.
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