How do I set the fan to turne off when the burner goes out

There is a fan switch on the heat exchanger that is meant to keep the fan on a while after the burner turns off. That switch is rectangular and has a dial with two sliding switch adjusters. There is a cut-in and cut-out adjustment. You have to be careful when adjusting it or it will not work right. It will always run some after the burner goes off unless you wire in a relay to break the circuit on the wire leaving the fan switch described above. Some of the new units have electronic fan switches that adjust differently. A relay can be wired to any type unit to work by breaking the low speed wire used for heat. The problem is that you will be wasting heat unless there is some sort of time delay wired in. DrTom4444