I have a gas heater and a Hunter thermostat (Model #44300).
With thermostat set to HEAT, fan on AUTO, the heater turns on at programmed low temperature and off at programmed high as expected. Fan doesn't come on at all.
With thermostat set to HEAT, fan on ON, the heater turns on at programmed low temperature and off at programmed high as expected. Fan stays on, as expected.

And is there a way to change Fahrenheit to celsius or reverse? I can remove the thermostat and find a manual switch on the back, but I did not change it originally unless it was some random mis-key. While attempting to alter the weekday programming, the display changed. The Owners Manual does not include this information in the instructions.

Sorry, you are lucky your hunter ever worked at all or has lasted as long as it did