Hi, I have a 1980's 100,000 BTU Heil Gas Furnace (Spark Ignitor type). The problem that I am having is that the Blower fan runs continuously after the heating cycle. I know that it should run for awhile after the heating cycle to cool the Burner etc to prevent warping, but the Blower just keeps running continuously, even if I switch the "Fan" switch on my Thermostat to the "Off" position, it just keeps running. The model # of my Furnace is Heil NUGK100DH05. I assume that there is a Thermostatic Sensor that detects when the Burner Temperature is cool enough to safely shut off the fan, but I am not 100% sure if that is the problem, and if it is, where it is located or what it looks like. there is a small "Relay" looking part mounted on an "L" bracket on the inside, right above the Burner (silver can with 2 wires coming out of the top) Is this what controls that, or am I off base? Also, if that is the right part, what is it called & about how much do they usually cost? Thank you for any help you can give! :)

On a older unit you probably have a fan and limit control that is stuck or out of range defective.