Oil furnace no.2 fuel oil. What would a normal stack temperature be, what would be considered too high or too low? If too high what would be causes and if too low, causes. I have been told between 450-600 but i had a furnace today pushing 675, is that normal. I had the correct nozzle in the unit, draft.04"wc, co2-12.9%, o2-4.6%, draft overfire-.03"wc, smoke-trace. Seems to me unit was running fine but stack temp seemed to be high. Is that high or is it ok?