Hey John,
We've had below freezing temps the last few nights and heavy frost but no snow yet!
My external mass has been between 70F and 95F since I started it up a few weeks ago. The integral mass has run between 150F and 90F. Yesterday I let it run longer than usual and got the external mass up to 115F-75F (Hottest and coolest spots) and the integral mass up to 300-130F. The house temp at that point was 77F and went down to 68F this morning. It's an old house with plaster and lath construction and basically no insulation at all between the studs!
To
answer your questions....
So far we have not sat on the external mass but plan to cover it with a sheet of plywood and a cushion. No danger of combustion there! Until yesterday it was barely warm to the touch.
The radiation from the top of the bell (not enclosed in the integral mass of sand) can be felt a couple of yards away. That portion usually runs between 550F and 300F.
My system is basically a 6" diameter except for larger portions (a 15" diameter
water tank and a length of 8" ductwork) within the external mass. The stovepipe to the chimney is 6".
My entire core is steel. While I know the flame path exceeds 1225F (as it consistently melts aluminum cans) the core is prevented from glowing red as it is not insulated but rather air-cooled and embedded in sand (a heat conductor). The hottest spots on the core are around the junction of the
feed tube and burn tunnel where the oxygen is feeding combustion and radiation is most intense. The 1225F + flame path is measured inside the burn tube at the base of the heat riser.