An older furnace is kinda simple-thermostat calls for heat-red wire makes contact with white-furnace starts-if flame sensor don't see flame-shuts off furnace-furnace lights and when reaching a pre set temperature (stack control or heat control) turns on blower motor-when thermostat reaches a pre set temperature shuts off flame and fan runs till furnace cools down too a pre set temerature set on the (stack control) also there's a hight limit on the stack control (safety) in case furnace gets to hot. If someting does'nt go right just push the reset. The newer ones have a fan that blows out the combustion chamber before ignition and a board and othe high tech bull crap.
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