So here's the story...bought a house built in the 1950s in southeast Wisconsin. Furnace is an 80%er. I saw the paperwork for the installment of said furnace that the previous owner left and I believe it was 67,000 BTUs. The supply registers are all on the inside of the house and I plan on changing that by running new round duct to the outside of all rooms. The house is 2 bedrooms and approximately 868 sq. ft. All the ducts right now are rectangular and measure 5" x 7". Here's the breakdown of the house right now...
East Bed (108 sq ft) 1 supply East Bed 7" round
West Bed (132 sq ft) 1 supply West Bed 7" round
Bath (50 sq ft) 1 supply Bath 5" round
Living Room (232 sq ft) 2 supplies Living Room 7" round (2)
Kitchen (154 sq ft) 1 supply Kitchen 8" round
(all supplies 5" x 7" duct)
I plan on insulating this round duct. Here's where my worries come into play...a few of these runs will have quite a few elbows on them. The living room in particular. A run for the living room will come off the plenum running parallel (between) the floor joists, then it will have to make a 90 down, then a 90 to run perpendicular to the floor joists, then a 90 up to get between the floor joists...then run to the register boot. I'm wondering if all these turns are going to slow down the air flow enough that I should increase the size of the living room ducts to 8" round? Anyone have any advice for me?
Also I should tell you that I'm trying to keep as much headroom as possible and thus i don't want to use a 45 to complete the run...so the 90s are necessary.