My heat reaches set temperature and turns off, but the fan keeps running... does anybody know what's the problem?? I can't turn fan off...

The fan should run after the flame goes out. This is to extract all the heat from the exchanger. If the fan runs for more than 1-2 minutes and won't shut off, your limit switch may be bad, not that uncommon. Take off the front of the heater and above the burner area you should see a box. Some have 1 dial, some 2. The 2 dials may be marked fan on and fan off. The 1 dial has fan off on the left side and fan on on the right side. Fan on sets at 125-130 and off at 95-100. Sometimes the contacts get a bit dirty. Use a screwdriver to spin the fan off switch back and forth, or slide it forward and back, set it at 95 and then run it through a cycle. If it still doesn't work you probably need a new fan switch. They are usually easy to replace. Draw a wiring diagram, mark the wires, take out the screws holding the unit in place, replace with a new unit and connect the wires. Should take 20 minutes or so. Good luck!