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6) Measure the draft at the point where the vent connector attaches to the
heat exchanger flue pipe. With the burner operating, the stack draft
should not exceed 0.05 in. W.G. If the stack draft has been adjusted
above this value to give the proper overfire draft, the heat exchanger
will require cleaning.
If there is little or no stack draft, the chimney flue way may require
cleaning, the chimney is too restrictive, or a downdraft condition exists.
E. Symptom: Furnace blower will not start.
Items to check:
1) Confirm there is 120 VAC at the blower motor terminal block.
2) If there is not 120 VAC at the blower terminal block, measure the
voltage at the fan center relay. If the fan can be activated by itself from
the room thermostat subbase, confirm it will operate. If so, the fan
center relay contacts may be burnt, or damaged, or the fan limit control
may be defective. Test these components and replace the defective
component, as required.
3) If there is 120 VAC at the blower terminal block, either the run
capacitor or blower motor may be damaged. Test and replace the
capacitor or motor, as required.
F. Symptom: Blower cycles on and off after the burner has shutdown.
Item to check:
1) Increase the fan limit control differential. Typically, the fan “off” setting
should be 30 degrees F. below the fan “on” setting. Adjust the setting
as required.
NOTICE: If the fan and high limit control is faulty, it should be
replaced. However, it must only be replaced by the same make
and model as the original. All the original temperature settings
should be duplicated on the replacement model as well. REMOVE
THE LOW VOLTAGE JUMPER IN THE CONTROL, IF EQUIPPED.
Refer to the electrical diagram for proper electrical connections.