If the draft is too weak, it may not vent combustion products properly. If it's too strong, valuable heat will be pulled out of your home. This measures how much heat is going up through the chimney instead of into your home. Optimum Between 350 and 600 . Smoke Smoke indicates whether or not all of the fuel is being turned into heat when burned in the combustion process.
Smoke also forms a layer of buildup, which reduces efficiency.
Optimum 0 or trace amounts. This rating combines the above readings to give an overall efficiency rating of your unit. If the reading is too low, it may be time to replace your system. There is no best time to have it done. The important thing is to have it done properly and on a regular basis.