Have a rheem heater... won't shut off? What can the problem be? It has been like 100+ degrees... don't know why it would run... it seems to just blow air and I have the thermostat on off.

Open the furnace cabinet and see if there is a small black switch on top of the control box that says on/off. If there is, turn it to off and your problem will go away. The excessive heat outside is probably convecting down the flue pipe and heating the fan switch causing the fan to come on to cool off the htg chamber.