Delivering 83% thermal efficiency, this separated combustion tubular gas fired direct spark propeller unit heater uses outside air for combustion, enabling operation in areas having slight negative pressures (near exhaust fans, etc.), with no adverse effect upon combustion or venting. The burners are isolated from dust, humidity, and nonhazardous chemicals, to allow heating of special environments. The LED diagnostic control board allows easy maintenance. A 24V thermostat, available separately, is required. For applications in altitudes above 2000 ft., the burner orifices need to be derated. Consult a qualified contractor to order orifices. Do not use this heater in areas requiring explosionproof appliances.