Unique 2-stage system furnace conserves energy. Automatically switches from high BTU for quick warm ups to low BTU to maintain set temperature. Built-in self diagnosis flashing LED indicates troubleshooting diagnostics. Digital thermostat provides 13 heating/cooling functions controlling both the furnace and roof top air conditioner. Fits the same cutout size as all 85 series furnaces  Cutout: 7-1/2H x 19-1/2W. Access door sold separately. 1/box.