120v 1,500w heater: is the temperature control knob most likely a standard potentiometer?
I don't think so. I think they are usually more like a thermostat for central heating and air conditioning. They mechanically adjust the operation of a bi-metallic device that opens and closes an electromechanical switch.

Edit
In order to have more than some conflicting opinions, you might post a picture of the type of device that you are asking about. Can you determine if there is some kind of electronic power control circuitry. Is this a plug-in portable heater of some kind? It seems to me that any heater that is controlled by a "high-low" knob is unlikely to have anything but an on-off control scheme.