Fan thermostat

[Harold Kestenholz – Hydronic Network]

There is usually a snap switch near the heat exchanger that permits the fan to come on when the heat exchanger is warm enough. This switch may have changed settings, or the fan is blowing too much air away from the exchanger to keep it warm. Janitrol still exists, so ask them for a manual.

http://www.janitrol-goodman-gmc.com/

http://www.furnaceparts.com/furnace_parts.htm#Fan%20&%20Limit%20Controls