I am currently installing a Rheem RGRL 95%,75k btu upflow gas furnace.The installation manual says to jump up to 3" on the PVC intake and exhaust venting.I am going horizontal on my run ,up about 4' and over about 5' to the outside wall,total run about 9-10'.The maunual say go to 3" then choke down the last 30" to 18" of 2" then 12' of 1 1/2" on the vent.On the intake the last 30" down to 2".Why choke it?What about all 2" or 3",anybody have the answer???
