Rheem RHG PRO50-40F Pro Series Natural Gas Water Heater, 50 Gallon One-of-a-kind air/fuel shut-off device offers double protection Guardian System - Engineered to exceed ANSI standards for flammable vapor ignition resistance - a Rheem exclusive Meets Low Nitrigen Oxide emmission standards High altitude compliant Check local code in The South Coast Air Quality Management District for possible shipping restrictions