This is an important Technical Service Bulletin to be carried out by trained and certified service technicians.  The intent of this bulletin is to inform our customers of factory installed pressure switches as well as pressure switch changes necessary for higher elevations. For single-stage furnaces, “High Altitude” is now defined as 4,500 feet or higher. For two-stage furnaces, “High Altitude” is now defined as 2,000 Feet to 7,800 feet. Installing a 2-stage 80% furnace above 7,800 feet is not recommended. This information will be added to the Specification Sheets and Installation Manuals for 80% furnaces.