New equipment: what's the right return size?
I decided to replace the downstairs unit.
We live in Dallas, Texas. Our 3670 sq.ft 10 years old home is a two story house with approx 2200 sq.ft downstairs ( with an open floor plan living room with 21 ft high ceiling, approx 700 sq.ft living room).
We have two 4 tons unit, the downstairs one needs to be replaced.
According to a load calculation we need 54600 BTU (downstairs).
Considering our desired winter temperature of 76 and summer indoor temperature of 78/79 the chosen contractor suggested a 4 ton Lennox unit (7 out of 9 contractors contacted suggested a 4 tons, one a 3.5 and 1 a 5 tons).
Indoor unit SL280-V 4 Ton 80 % (should be a 90k BTU)
Coil CH-33 50/60
Outdoor unit XC-21-048 up to SEER 21
Many contractors told us we are short on return. We have a 16' and a 10' return duct per unit.
According to the contractor increasing the 16' to a 18' will be enough. We have flexible ducts. Do you agree or is it better to increase the 16 to a 20' or a new third duct ? What size should the 3rd return duct be if needed? I tryed to find answers online but just got confused since I didn't find a site that offers complete and clear informations on how to calculate the duct (return and supply) size. On one site I read that first we need a load calculation, than the right equipment, then the right supply ducts and at the end the right return ducts needed to supply the unit and the supply ducts...is this correct or how should be the correct sequence if you wanted to fix the existing system and get a well balanced one?
Thank you for your help!