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
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.
Christopher A. Young
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