Begin with the rooms, not the equipment catalog
Describe which rooms run hot or cold, when the problem appears, and whether the house has changed since the current system was installed. Finished basements, additions, new windows, insulation work, and changes in how rooms are used can affect the assessment. These conditions belong in the assessment because they help define the loads and distribution problems the replacement is meant to address.
How capacity is selected
ENERGY STAR advises sizing heating and cooling equipment from the actual characteristics of the home instead of a rule of thumb. A load calculation accounts for the building envelope and design conditions. Equipment selection then applies that load to manufacturer performance data; the nominal number in a model name is not the calculation itself.
Follow the air path
The outdoor unit relies on indoor equipment and the distribution system. Return ducts bring room air to the blower; the blower moves it across the indoor coil; supply ducts return conditioned air to the rooms. Filters, undersized transitions, closed dampers and duct leakage can affect that path. Replacing the outdoor cabinet does not by itself correct a distribution problem in one room.
Photograph before the visit
Useful starting photos include the complete indoor equipment and surrounding room, the outdoor unit and adjacent wall, the electrical panel directory, and readable model labels. Keep covers in place; the contractor can open equipment during the visit.