Usually it is a distribution verdict, not a size verdict. The original tract ducts were designed for the original footprint; the addition was tied in, often with a long, small, or uninsulated run, and it starves at the end of the line. A room by room airflow read settles it: if the system produces the cooling and the addition does not receive it, the remedies are duct correction or a ductless zone for that room, both cheaper than a larger condenser pushing harder into the same restriction.
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