Need help sizing ductwork

[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?

[Hube]

How many Supply runs and what size are being fed off this main? (at plenum)and how long is this main line..? Also what size is the R/A line coming in to the unit? Hube

[imeduc]

20Kw that's 68300Btu for just a 1450 sq ft home even up there thats a lot .Id pull some of the strips so they dont turn on. you want the furnace to run longer at a time not on and off. Like hube said what all duct work C/A and runs do you have on here . Furnace at one end of the home put the blower on high .[8D]

My mistakes dont define me they inform me.

[busman]

quote:Originally posted by Hube

How many Supply runs and what size are being fed off this main? (at plenum)and how long is this main line..? Also what size is the R/A line coming in to the unit? Hube

Hi,
I only have one main line off the plenum, which is 37'long. In addition to this there are three short runs of round ducts off the plenum, a 4,5 and 6 inch. I have 16 runs that branch off the main line including 6 in the basement. One 6"(living room),two 4"(Bathrooms), the rest are all 5".
The return air duct that enters the furnace is 10"x20" . This duct also reduces in size the further it get from the furnace. The length of the R/A main line is about 34'.There are a total of 9 R/A vents throughout the house, one in each bedroom, two in the living room area ,one in the dinning area and three in the basement.
I hope this helps
Thanks

[busman]

quote:Originally posted by imeduc

20Kw that's 68300Btu for just a 1450 sq ft home even up there thats a lot .Id pull some of the strips so they dont turn on. you want the furnace to run longer at a time not on and off. Like hube said what all duct work C/A and runs do you have on here . Furnace at one end of the home put the blower on high .[8D]

My mistakes dont define me they inform me.

Hi,
I have disconected one ellement already,which makes it a 15 KW funace(about 51000 Btu I think),but the rooms at the end of the supply run(both bedrooms) are not getting enough air as they are colder than the others. We have also tried turning the blower speed to high, but we find it too noisy and find there to be a lot of vibration in the ductwork closer to the furnace.
Thanks

[imeduc]

When you dont see the set up its hard to say. I cant see that many runs in 1450sq ft up and down. old rule of thumb for 16 runs off furnace you need a duct 8"X32" at the start. Thats 256sq " and your cold air is only 200"sq . Thats 19 runs tot.on a furnace with a 1/4 hr blower and 1065cfm.A 6" pipe is 100cfm a 5" pipe is 50 cfm.WE dont use any 4" not worth the time .If the duct where bigger it wouldnt be as noisy.

ED

My mistakes dont define me they inform me.

[Hube]

For a home of 1450 sq ft,normally a total of 8 runs would be sufficient to serve this upper area, using 5" pipe-4x10 regs.. and approx 4 or 5 more 5" runs to the Basement area would do it. A grand total of 12 or 13, 5 " Supply runs, together with various Return air grilles in individual rooms wuold suffice nicely. Your duct work size is"way out" of line. You'll be better off to get an Pro Hvac Tech in on this and RECALCULATE the sizes properly taking into consideration as to the R value of the existing insulation, type of windows, exposure, etc, etc. . This main duct run of 37' will certainly need some size adjustments to deliver the right cfm. Proper fan air speed is necessary not only to provide the suitable CFM, but for quietness too. Good luck...Hube