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01-11-2013, 12:21 AM   #1
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## Formula for finding CFM

BTHU output
CFM = _________________ =

Temp rise x 1.08

Trane service guide says to get bthu output from furnace nameplate and multiply by the furnace efficiency (ex: 0.80) but I saw in another manual that doesn't need to multiply by the furnace efficiency. Wich one is correct?

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01-11-2013, 01:11 AM   #2
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Are we talking the BTU/CFM of gas?

If so...

1 CFM of gas is 1000 BTU (average)

Hence, a 60,000 BTU furnace will need 60 CFM of gas to keep the furnace running at full output.

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Last edited by ddawg16; 01-11-2013 at 01:14 AM.

01-11-2013, 01:33 AM   #3
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Quote:
 Originally Posted by ddawg16 Are we talking the BTU/CFM of gas? If so... 1 CFM of gas is 1000 BTU (average) Hence, a 60,000 BTU furnace will need 60 CFM of gas to keep the furnace running at full output.
Ya or are we talking how much air the fan will move?

01-11-2013, 05:41 AM   #4

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Quote:
 Originally Posted by JJboy BTHU output CFM = _________________ = Temp rise x 1.08 Trane service guide says to get bthu output from furnace nameplate and multiply by the furnace efficiency (ex: 0.80) but I saw in another manual that doesn't need to multiply by the furnace efficiency. Wich one is correct?
If the furnace has a BTU output listed, you don't multiply the output by efficiency. If it only has a BTU input listed, you multiply the input by efficiency.

EG: 64,000BTU output, don't multiply.
80,000 BTU input, multiply, 80,000 X .8 equals 64,000

On a nat gas furnace, you should clock the meter to make sure it firing at its rated input. On LP, you should check the manifold pressure to make sure its firing at rated capacity.

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01-11-2013, 03:41 PM   #5
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Quote:
 Originally Posted by ddawg16 Are we talking the BTU/CFM of gas? If so... 1 CFM of gas is 1000 BTU (average) Hence, a 60,000 BTU furnace will need 60 CFM of gas to keep the furnace running at full output.
No, I'm talking about the air moviments.
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01-11-2013, 03:42 PM   #6
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Quote:
 Originally Posted by HVAC1000 Ya or are we talking how much air the fan will move?
how much air the fan will move
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01-11-2013, 06:50 PM   #7
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Quote:
 Originally Posted by JJboy how much air the fan will move
It should be on the rating plate

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