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 there is no need to change the orifices. If you are using propane make sure to also order the LP conversion kit BG11K50. 

You will also need this pressure switch if you have a BG801 series furnace that is rated at 110,000 input BTUs and you are between 7501-10,000 feet above sea level. If you are using natural gas make sure to also order the high altitude natural gas orifice kit B51W01. If you are using propane make sure to order high altitude LP kit BG11K45.