Formula for finding CFM

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?

__________________

**Always kill the power ** Hot air rises, but heat will always move from higher to lower temperatures. ** Real man shoot in manual. ** If it ain't grounded, it ain't dead.

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.

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HVAC1000

Ya or are we talking how much air the fan will move?

beenthere

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

No, I'm talking about the air moviments.

__________________

**Always kill the power ** Hot air rises, but heat will always move from higher to lower temperatures. ** Real man shoot in manual. ** If it ain't grounded, it ain't dead.

JJboy

how much air the fan will move

__________________

**Always kill the power ** Hot air rises, but heat will always move from higher to lower temperatures. ** Real man shoot in manual. ** If it ain't grounded, it ain't dead.

HVAC1000

It should be on the rating plate