This is a pressure switch designed for the BG801 series of furnaces in high altitude applications. Select this pressure switch if your application is between 4501 - 7500 feet above sea level and you have a 45,000, 70,000, or 90,000 input BTU furnace. If you are using natural gas then all you need is this switch. If you are using propane make sure to order BG801-LPHAK 1 which contains this pressure switch and the matching propane conversion kit.