The boiler stack temperature is considered to be excessive if it’s more than 400°F above room temperature. Some remedial measures that can be done are:
1. Be sure the proper amount of combustion air is available in the furnace room.
2. Ensure that the furnace has the proper air-to-fuel ratio (purchase a kit for flue-gas analysis if testing is required often, to verify operating conditions).
3. If the furnace is over-firing, the nozzles may be oversized. Also, fuel pressures may be too high.
__________
Excerpt from Energy Conservation Projects.