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[Phys-L] Gas Heaters and Fans



The time lags between the burners and the fan in my heater is hard wired
into the heater control system and has nothing to do with the
thermostat. What happens on turn-on is the furnace heats up some of the
air column in the ducts so that when the fans turn on they don't blow
cold air into the home. When the burners turn off the air inside the
ducts is still warm and so the fan continues to run so that the warm air
is not "wasted" inside the ducts but is vented into the home.

A careful inspection of my furnace wiring diagram shows a thermistor in
the air duct that is wired to the fan control circuit. Clearly, the
system is sensing the temperature inside the ducts as it makes decisions
about when to turn on and off the fan.

John

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
John E. Sohl, Ph.D.
Professor of Physics
Weber State University
2508 University Circle
Ogden, UT 84408-2508

voice: (801) 626-7907, fax: (801) 626-7445
e-mail: jsohl@weber.edu
web: http://physics.weber.edu/sohl/