Hello there. I am new to this site and is currently adding to our house in california, bay area. I am not an HVAC expert but do need some feeback in regards to our HVAC ducting.
We are adding on a second floor on top of our garage and an office space next to the garage. The second floor consists of a bedroom and bathroom. The furnace is located in the garage right under the bedroom. The HVAC contractor ran 14 inch circular main truck duct above the garage. There is a 10 inch flex duct to service the second floor and the 14 inch steps down to a 9 inch then to a 6 inch servicing the office. My husband wanted to increase the ceiling height of the garage by using rectangular duct between the floor joists. Our HVAC contractor refuses to do that mentioning that the sizes for the rectangular duct needed to fit between the floor joists will be too small and will not provide the amount of air needed for the spaces. My husband claim instead of one main truck line, he can run multiple rectangular lines that service each space, which will provide enough air supply as well. He insisted that the HVAC contractor just don't want to do hard work and just looking for an easy way out. Any feedback and ideas are most certainly welcome.