How to determine CFM of Furnace??

Old 04-11-11, 06:49 AM
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How to determine CFM of Furnace??

I need to determine the CFM of a 1988 Trane XE70 furnace. 100k btu's. We are putting it in a single floor house and need to design the ductwork. I have some formulas to work with, but I do not know the CFM of the unit. I looked but did not see anything on the tags.

Does anybody know how I can determine what the CFM is? Or a close guess? Thanks in advance.
Old 04-11-11, 08:57 AM
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If you can find information for your furnace specifically, fine.

Otherwise, in general it takes roughly the same amount of air to strip the heat away from any 100,000 BTU furnace given a certain static pressure.

You can use the chart on page 2 of this furnace as a guide:
Old 04-17-11, 02:17 PM
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as long as you know the btuh of the furnace and have some mechanical skill and tools you can find it. the formula is CFM= BTUH output/(1.08 x DELTA T).

the fan needs to be on high if you have a multispeed fan. run the heating system and let it stablilize. take a temp reading near the furnace filter (return air). take a temp reading about 3 ft away from the supply air plenum. now figure out your delta T by subtracting your supply air (120 for example) and your return air (70 for example) thats a 50 degree DELTA T. so say your BTUH output rating was 100,000.

that will be 100,000 divided by 1.08 * 50. 1.08*50 is 54. so 100,000 divided by 54 is 1852 CFM. hope this helps
Old 04-25-11, 12:42 PM
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Check the motor HP; on high...

1/3 = 1200 cfm
1/5 = 800 cfm
1/2+ = 1600 cfm+

@ 0.5" WC external static pressure

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