refurn air size and Flex duct work ??

i just got new furnace GMH950904CX , is 3 ton 92,000 BTU with heat pump. my house is 1600 square feet. so i went to the ducting supply company got all my ductwork, the duct supply is 20 x8 , and the guy told me i need the return duct 20 x 16 at least or 20 x 20 , i wonder is that right, because due to the small space that i have , is very difficult to have big duct ( i only have 42 in width) how to calculate the return air anyway??? ( also he sale me all the flex duct r4.2 is good ??

 

Just off the top of my head 92,000 BTU/hr. seems awfully large for a 1600 square foot house. Did you do a heat loss calculation? What is the design outside temperature for your location? My home is 1540 square feet with a 66,000 BTU/hr. furnace and it is easily 50% larger than I need.

Also, a 20 x 8 inch main supply trunk is woefully small for that much heat. That is a cross-sectional area of only 160 square inches, about half of what you need at a bare minimum. The return at 20x16 inches is 320 square inches and much closer to what the supply should be. The general rule is that the return air duct should be no less than 1-1/4 times the size of the supply and 1-1/2 times is better. The sum of the area of all your branches should add up to the cross-sectional area of the furnace outlet at a minimum and a bit larger is not going to be a problem. Smaller ducts will be noisy. The area of the branch ducts needs to be calculated on a room-by-room basis for the proper airflow.

 

supply is to small unless your coming of a plenum on both sides at 20x8 return is fine 20in should be fine or 20x16

 

that what the duct guy give to me, supply duct 20 x 8 , and the return i think is 16 x 25. is be oki ??????

 

16x25 return will handle 2400 cfm
20x8 supply will handle 900 cfm

3ton is 1200 cfm

 

so the return air bigger is better may be???

 

yes bigger return is ok 16x20 is fine for return

 

oh thank , because i want to add 3 return air in each room around 6 inch, to the main return air, is that good ???, so it wont hurt right?

 

If your adding to it that's fine. If there all 6in then no not enough air 6 in return would only be around 50 cfm