[busman]

Hi,
I am almost finnished building a 1450 sq/f house with a full basement. I have a 20KW Nortron Electric furnace(which I was told was over kill) with a 1/4 horse blower motor,68240 BTU per hour,1065 CFM max at 0.2 static presure. The plenum is appr. 17"x17"x5'. The main supply trunk runs off the plenum in one direction right across the house. The branch ducts are either 4",5",or 6" (round duct) depending on room size.
My problem is that I have sized my supply main trunk too small (curently starts at 18"x8" and reduces by two inches after every two or three branches all the way down to 10"x8"). I realized this when my Hi-limits kept trigering due to inadequate air flow. Increasing the blower speed or disconecting an ellement seems to solve the problem,but this seems like a band aid fix.
I would like to correct the problem properly by installing the right size duct work off the plenum. Can anyone help me out?