X2 on Old Biscuit's advice.
Just to clarify, the required dimensions are square inches, not square feet.
From Atwood tech manual:
#3 - Minimum Ducting Requirements
The various BTU sizes of 85, 89 and 2-stage series furnaces require a minimum number of square inches of heat ducting.
The most common size of soft ducting is 4˝ diameter. Therefore, since a 4˝ duct is equal to 12 square inches, we can call
out the minimum number of ducts needed for the models of furnaces noted below.
Furnace Model Minimum Minimum
8516, 8520 24 sq. in. 2
8525. 8531 36 sq. in. 3
8535, 8935, 8940, 1522, 2334, 2540 48 sq. in. 4
CAUTION - Ducting Installation
4˝ Flexible Hose
• each 90° bend adds the equivalent of 10 feet of ducting
• ducting should be securely attached to furnace
• each run should be as straight and short as possible
2˝ Flexible Hose
• 2 - 2˝ duct runs do not equal one 4˝ duct. A 2˝ duct is only 3 sq/inches
• 2˝ duct adapters are available
• 2˝ ducts are ideally suited for bathroom and holding tank compartments
• a closeable outlet does not contribute to the minimum of total outlets recommended for a furnace.
• furnace must be completely sealed to floor and plenum with a bottom discharge gasket, with no air gaps.
• if furnace is installed in middle of run, the main duct run must be a minimum of 24 sq/inches.
• if furnace is installed at the end of the run, the main duct must be 48 sq/inches.