Duct flow problems

[ElvisCat59]

Our house is a 1000 sq foot ranch with a 500 ft Family Room Addition built on a slab.
to get supply heat and a/c into the room, I had to run an 8"supply duct in my crawl space upt to my attic, a distance of 25 feet.
Off of that I have 4- 6x10" vents on the perimeter walls that are fed by a 6" supply.
Problem #1- My first two vents that are fed by 2- 6" 90Degree elbows get very little flow. they are not the end of the run but in the middle.
Problem #2- I have trouble keeping the room warm even with the new windows and the attic insulated. Average Temperature inside is 62 degrees when outside air temp is 20 degrees.
I tried a duct booster and it doesn't solve the problem doesn't help at all.

Trane Furnace XE80, 90,000 BTU Downflow.

 

[glennjanie]

Welcome ElvisCat:
I see you have a downflow furnace. Is the new room at a lower elevation? Are you using metal pipe? Flex duct wears the air out and drags it down. Each elbow adds 10' to the effective length of the duct. Are the 6" vents taken off with tees or just cut into the 8" pipe? The tee, take off will encourage more air to divert into the 6" pipe.
If the new room is at a lower elevation; it would be best to run the new duct under the floor of the house and come out in the wall of the new room, even if it has to go through concrete block.
It would also help to have 2 8" pipes to feed the 4 6" ones. An 8" is just not enough for more than 2 6" pipes.
Please let us know how you come out with it and we love pictures too.
Glenn

 

[ductworksystem]

provides the capability to easily model onedimensional fluid flow. In this unit, you'll learnhow to: Represent duct flow networks with curves, Define beam sections and beam, elements to model fluid flow, Define duct flow boundary conditions.
thanks ..ductworks.net

 

[Daryl in Nanoose]

Glenn has hit the nail on the nose.
I would of went with at least a 10" main with 5" take offs and 4x10 vents.