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 five zones. The sum of the total indoor BTUs can not exceed 72K BTU. Example: 9+12+12+12+12=57K BTU (suitable), 12+12+18+18=60K BTU (suitable), 9+12+12+18+24=75K BTU (NOT suitable).