For a furnace output of 100,000 BTUH with a return air temperature of 65°F and supply air temperature of 135°F, what is the CFM?

To determine the correct CFM (Cubic Feet per Minute) for the furnace with the given parameters, you can use the formula that relates BTUs to CFM based on the temperature differential.

The formula used for calculating the airflow (in CFM) for a heating application is:

[ CFM = \frac{BTUH}{1.08 \times \Delta T} ]

Where:

• BTUH is the output in BTUs (100,000 in this case),

• ( \Delta T ) is the temperature rise, calculated as the supply air temperature minus the return air temperature.

With the given temperatures:

• Return air temperature = 65°F

• Supply air temperature = 135°F

Calculating ( \Delta T ):

[ \Delta T = 135°F - 65°F = 70°F ]

Now, substitute the values into the CFM formula:

[ CFM = \frac{100,000 BTUH}{1.08 \times 70°F} ]

Calculating the denominator:

[ 1.08 \times 70 = 75.6 ]

Thus,

[ CFM = \frac{100,000}{75.6} \approx 1323