There are no pre-engineered duct paths built into the C-320 contrary to some widely held beliefs. Some HVAC upgrades are very simple and minimize ducting and attendant carpentry associated with concealing the ducting. Others like the ones pictured here are more complex and involve a great deal of carpentry.
In the simplest upgrade, a self-contained unit is located on a shelf in the port locker, butting up against the interior bulkhead. The return is cut out at the foot of the aft berth, covered by a grill and a plenum crafted to confine the airflow to the air to be removed from the cabin. A short duct is opened into the galley area with a single vent supplying the entire cabin area. Clearly this does not meet the design criteria of any of the self-contained marine AC units on the market, but owners utilizing this simple installation report satisfactory performance. One would surmise that a properly sized unit would operate at less than marginal efficiency at the high and low temperature ranges.
Kent Nelson with Catalina Yachts Engineering Department supplied these pictures and drawings of a well-designed installation. Note that all four living areas of the C-320 have a supply vent and all ducts in the living areas are concealed behind extensions of the existing shelves. While this more elaborate version of the project may be beyond the skills of the average do-it-yourselfer, it is well within the ability of a decent cabinet maker. The photographs and drawings contain sufficient information and measurements to recreate the original work.