Probably should never see much more than 150* The closer you get to 200*, the more you are in the range of pyrolosis (sp?) If you look at a furnace limit switch, most have the burner safety cut off temp set to 200* max, cause you be officially in pyrolosis range by that point! Mine is set at 150* blower on, 110* blower off, pretty typical.
For anybody that is not familiar with pryrolosis, it is (for this application) the slow baking process of your houses wooden framework, that is next to the duct work (or flue pipe or chimney) until its combustible temp is lowered to a VERY low point. A point to where it could light up just from one lil ole overfire of the furnace. This process usually takes years, but can be a lot less.
EDIT: I probably go to far with this stuff, but hey jack, me n my fam are sleepin on top o' dis "indoor camp fire", safety first. That's why I sprayed all the wood framing in my furnace room with No Burn fire retardant...
http://www.amazon.com/No-Burn-1102A...ref=sr_1_2?s=hi&ie=UTF8&qid=1390163598&sr=1-2 and installed a safety heat dump door
http://www.yukon-eagle.com/LinkClick.aspx?link=200&tabid=201 on my supply plenum. Heat dump has a fusible link that melts at 212* to let the 12" x 12" door open, allows enough air circulation without the furnace blower so as to not overheat the firebox if some clown knocks off a power pole right after I chuck the ole girl full of wood for the night, allows me to sleep better, or more peace of mind when I go off to work with my fam still sleeping there...
