I'm in the process of getting quotes to replace a 4-ton package heat pump (Heil "horseshoe" 10 SEER unit). I have a 2300 sq. ft., single story house in northern Alabama. I'm going to be putting in a 16 SEER (probably 2-stage American Standard or Trane) heat pump.
So far, one of four estimators has said my 14" diameter return ducts are too small and he wants to put in 18" ducts. I have two return air grills, both are 20"x25". There is a 14" flex duct from one return grill that runs about 30 feet to a junction box. The second return has about 10 feet of 14" flex duct that runs to the junction box. Then there is an 18" flex duct that runs from the junction box to the unit, about 17 feet.
My question is, are 14" return ducts big enough, or should I pay the extra money to expand them to 18"? One estimator said a single 14" duct was good for 480 cfm, another said it was good for 700 cfm, and a third said it was good for 1000 cfm. What's the right answer?