Can I replace three 6” flex duct runs about 5ft long in two bedrooms, and one 10ft run on the third 3 bedroom (3 bedroom, 2 bath house). This is toward the opposite side of the fan end of the run. The house doesn’t cool properly on that side—I think the flow rate isn’t high enough. Main starts 12”x 25ft long from the fan box with two 4” and two 6”, then a reducer junction box with 3 connections: 6” to 10” main and another junction box with three 6” and one 4”. Question is: can I change the last three 6” to four 4” using a reducer at the box and at the register?

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    Is the issue one end of the home doesn't get cool? If so where is the return in relation to the rooms that don't cool. Often improving the cooling in "hot" rooms is achieved by adding returns to pull out the warm air which is replaced with cool air.
    – RMDman
    Commented May 18 at 14:29
  • Agree with @RMDman. If return air is adequate, another possibility is restricting the near points to push more to the far points. Commented May 18 at 14:42


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