The heater will not shut down whither exceeding set temperature or being turned off. I replaced the thermostat and plugged the heater in again. It started immediately even in the heat off position. Could it be the ignition control or a thermo coupler? I'm not talking about the fan blower but the actual heat. Thanks, Sherri

Try unhooking the thermostat wires from the unit itself(not from the actuall thermostat, but the unit). If the burners don't ignite when power is restored after you do this, then I would guess there is a short in the thermostat WIRES. Let me know what happens, and if it doesn't work, please post some pics of your unit(wiring, burners, gas valve, the more the better).