My 6" system has been running a little less than 600 (F) on the outside-top of the barrel; although that temp is just being measured with a cheap $10 or $15 magnetic flue thermometer, which are not highly accurate, so it may actually be running hotter without my knowing it. This is in the outdoor test build. I have just gotten my digital thermometer, so I can start using my thermocouplers, so I will be able to take more accurate temperature measurements in the future.
I have not yet built an 8" system, but I am guess that the 6" systems tend to run in the 500-700 (F) range at the top of the barrel, whereas the larger 8" systems run the higher temps we hear quoted (800-1000 degrees F).
With regard to clearances, I'd treat the barrel like a
wood burning stove. The more conservative code clearances I have seen are 48" (four feet) of open space between the
wood stove (barrel) and objects within the house (be that a wall or book case, tv, etc). If you use metal shielding with an one-inch air gap behind the metal shielding you can cut that in half. Personally, I also consider 4-feet a safe distance to run along the wall, extending beyond the edges of the stove/barrel. So, if the barrel is 2-feet wide, I'm going to have a minimum of 10-feet of heat shielding along the wall: the two feet immediately perpendicular to the edges of the barrel, and then and additional 4-feet farther in both directions (4+2+4 = 10 feet).
To get a feel for clearances, browse fireplace,
wood stove, and masonry heater building code specifications. Of
course, one ought to look up their own
local codes. Even if you don't observe them, it is nice knowing what is required by code. Not everything in the code is after all there just to annoy people
some of it really is directly related to safety. A very valid point someone made, was that when an inspector looks at your chimney, they are thinking in terms of safe building practices established over many decades. Individual home owners tend to have shorter time frames and reference points.
Bottom line
answer?
Build for 1000 (F) temps and use at least 4-foot clearances. I would expect that to be safe. But you are outdoors? So prevailing winds, and shifting winds would also have to be taken into consideration. I'm uncertain how that would effect the above suggestions, other than to say, it is by definition a more erratic set of conditions, so I'd think you'd want larger margins for error and unforeseen situations/weather.
