The older FWC had a radiant heater with a pilot light and no fan. They were not hooked up to the battery and didn't have a separate thermostat. If this is the case, The thermostat is on the bottom part of the heater behind the bottom part of the metal grate. There's a knob on the right to light the pilot and turn on the gas, and a knob on the left (thermostat) to adjust the temperature. I'm not aware of any way to add a separate thermostat if this is the kind of heater you have.