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Pipe adapters ( reducers ) are required when installing indoor units larger than 9000 BTU to your multi zone outdoor unit. 

Most multi zone outdoor units have 1/4” & 3/8” connections, so in order to connect a 12000/18000 BTU indoor unit (which requires a 1/4” & 1/2” copper line), will need to use (1) 3/8” to 1/2” reducer (Changes the 3/8″ flare connection to 1/2” ). 

If you are connecting a larger indoor unit, like a 24000 BTU you would need 2 reducers: (1) 1/4” to 3/8” Reducer, and (1) 3/8” to 5/8” reducer.