2 Answers

If it is producing soot, the fuel air mixture is not set collectedly.   Soot in unburned carbon.

Fire places are set at a LOWER air setting to produce the YELLOW flame that is more attractive to view, but the heat given off is lower as well.

If you want the most efficient use of the fuel,  set the fuel/air mixture so the flame is BLUE, like you see on a gas stove burner, WBMS

This above mentioned answer was very good but let me emphasize one should have their furnace checked anually for safety sake. A major concern is carbon monoxide posioning is one and a malfunctiioning furnace can overheat and cause a fire. If I scared you ?good. CO2 is oderless and by the time your in trouble its too late. Always check the exhaust vent for cleaniness and no holes,tears or ruptures. As for as getting to hot it could be a thermal limiter switch not shutting off the combustion when the upper heat limit is satisfied shutting off the fuel source. In any case this is not a item for a shade tree mechanic to try and save money on by resulting in multple deaths from gas posioning or setting the domicile on fire. Usually this happens at night when everyone is asleep and might not wake up from Co2 or a blazing inferno. Do what right and safe have it looked at by someone that knows what their doing...