My total length is 27' of 5" duct work in my basement. I would like to reduce 15' of it to 4". This will eliminate 3 elbows and make it a straight run. This will only supply my basement and I am wondering if this can be done? Their are waterlines near the duct work and this is why I am using 4 inches. Would appreciate any advice from you professionals? Thank you for your time.