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-   -   Formula for finding CFM (http://www.diychatroom.com/f17/formula-finding-cfm-168816/)

JJboy 01-10-2013 11:21 PM

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?

ddawg16 01-11-2013 12:11 AM

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.

HVAC1000 01-11-2013 12:33 AM

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?

beenthere 01-11-2013 04:41 AM

Quote:

Originally Posted by JJboy (Post 1090830)
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.

JJboy 01-11-2013 02:41 PM

Quote:

Originally Posted by ddawg16 (Post 1090840)
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.

JJboy 01-11-2013 02:42 PM

Quote:

Originally Posted by HVAC1000 (Post 1090842)
Ya or are we talking how much air the fan will move?

how much air the fan will move:thumbsup:

HVAC1000 01-11-2013 05:50 PM

Quote:

Originally Posted by JJboy

how much air the fan will move:thumbsup:

It should be on the rating plate


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