Common HVAC Formulas : 2.2 CFM for an Electric Unit

 Key Differences:

• Electric Furnace: Input energy equals output energy (no need to calculate efficiency).

• Gas Furnace: Requires efficiency calculation due to heat loss through flue pipes.

Ohm’s Law:

• Voltage (V) = Current (I) × Resistance (R).

• Important for calculating wattage in electric furnaces.

Steps to Calculate CFM for Electric Furnace:

• Step 1: Measure voltage and amperage from the unit.

• Step 2: Multiply voltage by amperage to get wattage (e.g., 240V × 40A = 9600 watts).

• Step 3: Convert watts to BTUs (1 watt = 3.413 BTUs). Multiply wattage by 3.413 to get BTUs/hour (e.g., 9600 watts × 3.413 = 32,764.8 BTUs/hour).

• Step 4: Measure the temperature difference between supply and return air (e.g., 120°F supply, 70°F return, difference = 50°F).

• Step 5: Multiply the constant for standard air (1.08) by the temperature difference (1.08 × 50°F = 54).

• Step 6: Divide BTUs/hour by the result from step 5 to get CFM (32,764.8 ÷ 54 = 607 CFM).

Example:

• Voltage = 240V, Amperage = 40A, Temperature difference = 50°F.

• CFM = 32,764.8 ÷ 54 = 607 CFM.