Fan/ Limit switch

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A fan limits function is: to bring the fan on when the heat exchanger reaches a certain temperature to prevent cold air blowing at startup to shut the fan off after the burner is shut off to prevent cold air and to extract the maximun heat from the heat exchanger while cooling down the heat exchanger and to shut the burner off at a safe maximum if the blower does not come on.
You should not be playing with the fan limit. What you may want to check is the heat rise between the incoming air and the discharge air after the unit has been on about 5 to 10 minetes and compare that to the manufacturers rating. The heat rise should be in the middle, to low means to much air flow or cold air infiltration, to high means not enough air flow. Either one will affect the system proformance and life. Also have the gas pressure checked at the regulator each year when the unit is inspected, the heat exchanger is checked and the burners are cleaned.
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