Question about Goodman GDS80904BNA Open Flame Heater

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Need the cubic inches for the return air duct

I am setting up return air for a new goodman furnace. There will be two return air ducts that travel through the house. I don't know the cubic inches these two ducts need to be. The two return air ducts will come together at a 10 inch by 20 inch main return air duct at the furnace. The duct work has to travel through the house and I am not looking for the length, but the cubic inches of the end of the two ducts combined. the furnace I have is a Goodman model number GMH80904BN. The product number below has the letter "A" at the end of it and mine does not.

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For proper minimum opening you need to do duct calculation. This is not a typical math equation for the typical homeowner. And unfortunately alot of contractors do not take the time to calculate this.
If you insist on doing this yourself...... I would recommend the largest opening that a common filter will fit. probably 20" by 25". Going to big on a return air will never hurt. But going to small will.
20" x 25" will work safely 95% of the time. But... If you are running airconditioning that is 5 tons in size, you will need the design a return air system that will feed from BOTH sides of the furnace.

If you are just sizing the trunk line you should use a ductulator to calculate the minimum size. A good rule of thumb would be to size the "return air" trunk 1 - 1 1/2 times the size of the supply trunk line.

Posted on Mar 08, 2009


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