Propane gas space heaters are either direct-vented or vent-free. Direct-vent heaters require outside air to support combustion. They are typically installed on outside walls. Check the outside vent from time to time and keep it free of debris. Vent-free space heaters use inside air to support combustion and vent directly into a room. Newer models have an oxygen-depletion sensor that automatically turns the unit off if room oxygen levels drop below a pre-set level. All space heaters should be checked periodically by your local propane delivery company. A trained technician will inspect the unit, clean the burner and controls and check for proper operation.