Gas furnace burner runs no fan

If your gas furnace burners come on, but the circulating fan does not, the problem is either the fan motor or the controls that turn it on.
Older furnaces used the heat generated by the burning gas to actuate a heat operated fan switch as on the left.

The best way to start is to remove the blower door (lower door on upflow, opposite on downflow).

Find the fan motor.  If it is hot, the problem is in the motor or its start components.  The easiest part to check is the bearings.  If the shaft of the motor rotates easily and does not drag, 

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