I'm at a loss here. I'm installing a 150,000 btu furnace with 4 90 degree elbows and have forgot the calculation when you have more than two. Do you multiple .90 by 150,000 btu or is is by 10%. Also i have a room 20x30x8= 4800cu.ft AND 150,000 FURNACE 150,000/1,000x50=7,5000 CUFT. So I need to bring in 2,700 cuft of fresh air and can not remember the calculation. Also when furnace is derated and orifice size is not on chart do you go up or down? Example the orfice size comes out to 27.30 and there is a 26.78 and a 28.23 which would you choose.