Have 2 separate but identical furnaces, one Tappan, one Frigidaire. The Tappan blower motor runs constant in cool setting. Blower AND flue fan run constant in heat or off setting, or when thermostat is completely removed. Circuit board flashes once signaling open limit switch or external load on quot;Wquot; (? No external load that I know of) Have replaced blower, flue, and rollout limit switches to no effect. Frigidaire model blower won't shut off. Circuit board doesn't flash a trouble signal. New thermostats and cleaned filters and ducts on both. Have tried to talk to anyone at Frigidaire/Electrolux/Tappan, but apparently no human beings are employed there
Sounds like your High Limit switch is open. These usually look like a black plastic rectangle about 2 wide by 3 tall with two wires going to it. If you have something that looks like this installed just above the burners and through the sheetmetel of the furnace, give it a couple quick taps to see what happens. These usually fail due to running the furnace with dirty filters. Over time, the constant swithing on and off of the limit switch due to excessive temperatures in the furnace will cause them to wear out.
