A dual fuel vent free gas heater. In one implementation the heater includes a gas burner adapted to receive one of a first type of gas or of a second type of gas with a first pilot burner intended to receive the first type of gas and a second pilot burner intended to receive the second type of gas. A first temperature sensor is located adjacent the first pilot burner and a second temperature sensor is located adjacent the second pilot burner. A normally closed thermal switch is coupled to the first temperature sensor and located in the electrical flow path between a voltage source and a gas control valve actuator, the thermal switch configured to open when the temperature detected by the first temperature sensor exceeds a predetermined temperature indicative that the second type of gas is being delivered to the first pilot burner.