An inverter controller, a method of operating a controller and a field-installable kit. In one embodiment, the controller includes: (1) a thermostat interface configured to receive conventional thermostat signals providing for a ventilation mode, first- and second-stage cooling modes and at least one heating mode and provide relay control signals for an inverter forward start relay, first- and second-stage cooling speed select relays and at least one heating relay and (2) an inverter controller coupled to the thermostat interface and configured to receive the relay control signals and provide an inverter forward start signal, an inverter high speed set, an inverter medium speed set and an inverter ventilation speed set.