I'm not sure about max temperatures but it just reminded me of this horrific accident a few years ago. http://news.bbc.co.uk/1/hi/uk/1443939.stm maybe if you do a little research into the investigation then, you could find some learning points from that.

Lucy ExDeeps has it right, lots of things to factor in, but an industry guide if its any use? I was a shift operations supervisor in a power station. Staff had to regularly access boiler furnace spaces for maintenance, some of this emergency stuff on burner rings or jammed steam lances. I had to prepare the safe system of work and the threshold for staff entering the furnace was 45'Celsius. This was never compromised and even at that we rotated people out every 30 minutes as the furnace cooled, allowing longer work periods. Hope that helps a little?

I did some research on this a few years ago and wrote a set rules and guidance for entry into a furnace (hot environment) for repair / maintenance. PM me and I will send you the 5 page word doc as a rough guide; but do your own research and discuss with the staff / contractors that are familiar with this type of work. regards Steve

Did you manage to sort out heat stress calcs? Sorry I didn't have time to explain in more detail the other day, I am working at home a bit over the next few days so post if you need clarification.