Q: Assuming that the dial on the fan/ limit switch is an accurate measurement of temperature, what's the most efficient range of temps that the dial should rest at when the blower is running and the burners are on? Thanks
A: When the furnace is running, if the fan limit switch indicates a lower temp, then the flue gasses will be cooler, also. This is more efficient (efficiency being defined as heat going into the house, not up the chimney). In order to maintain higher temps into the house, the flue gasses will retain more of the heat. So, the most efficient temps (efficiency being measured as heat going into the house) are the lowest ones on the dial. Note, please, the original poster said "rest at when the blower is running". He's asking for the efficiency of the furnace at one specific moment of the run cycle.