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 three zones. The sum of the total indoor BTUs can not exceed 39K. Example: 18+18=36K BTU (suitable), 9+12+18=39K BTU (suitable), 12+15+18=45K BTU (NOT suitable).