I have an older Carrier downdraft furnace and the blower motor will not stop blowing after the burners have turned off on the heat. The thermostat is set to auto and I have replace the limit switch. It will not turn off when it hasn't been running in a while but when on during the day it will turn off and act normally. The problem is usually when it has been off during the night and comes on early in the morning, house comes up to desired temp and the blower will not stop until the breaker is turned off. Then after a while the breaker is turned on the fan starts running, heat comes on and from that point on during the day as it runs it will turn off.