What size return air duct should be installed on a new propane fired Warm Air Furnace rated at 100 MBTUH with a blower moving 1400 CFM?

To determine the appropriate size for the return air duct in a residential heating system, you can use the airflow requirement of the system, which in this case is 1400 CFM (Cubic Feet per Minute). The size of the duct must be sufficient to handle the airflow without creating excessive resistance or noise.

When calculating duct sizes, the standard air velocity range for return ducts is typically between 600 to 1000 feet per minute (FPM). Using this velocity, we can derive the required duct area for the desired airflow.

Calculating the duct area required for 1400 CFM:

1. For a target velocity of 800 FPM (which falls comfortably within the 600-1000 FPM range), the area (A) can be calculated using the formula: Area (A) = CFM / Air Velocity (FPM)

   Thus, A = 1400 CFM / 800 FPM = 1.75 square feet.

2. To convert square feet to square inches (since duct sizes are typically listed in inches), you multiply by 144 (the number of square inches in a square foot): 1.75 square feet x 144 = 252 square inches.