ABOUT THIS SYSTEM: Mix & Match Wall Mount and/or Ceiling Cassette for your custom system. Please select from below each of the indoor units. It is not required to use all four zones. The sum of the total indoor BTUs can not exceed 51K. Example: 15+18+18=51K BTU (suitable), 6+6+6+6+24=48K BTU (suitable), 9+12+15+18=54K BTU (NOT suitable).