I have two conflicting quotes from contractors. I want to change my duct system from a radial system to an extended plenum system. The extended plenum would be 8X18X14 feet long and have 5-6"round ducts and 1-9"round duct installed off of it. The order the duct take-offs are 6", 6",9",6",6",6",at 16" spacing at the last 6 ft of the duct. Each of them running up between the floor joists to their destination. One contractor told me that there should be a reduction in size after the 9"duct, the other contractor told me there is no need because it is such a short run, and they were just trying to run the price up. Who is right??
I also asked to have the duct jog 1 foot to the side immediately after leaving the furnace, what is the best way to accomplish this? Curved ducts or some type of modified takeoff from the furnace plenum. I have a diagram that says it is from ASHRAE 1952 guide, Warm-air and return-air bonnet or plenum, figure D that I was thinking of. It angles from the furnace, and I thought I could have this made to eliminate adding 2 turns in the duct. I dont see it on the air duct calculator that I have unfortunately.
I have been doing some research to figure this out but don't know the best answer.
This work is being done in an OLD house so the system was retrofitted in the past to accomidate forced air, and is not set up the best. I would like to finish the basement and the extended plenum will clean up all the ducts running every where. Thanks for any help you can give!!