A Heat Recovery Ventilation (HRV) system would be a good investment for your house in Napa, but a whole-house fan with louvers that open up into the attic and draw the air out of the house would not be a good investment in California because these systems are notoriously leaky and don't really provide what they advertise.

The mixed climate in Napa, where you might have cold winters and hot summers, would benefit from having more ventilation from an HRV unit in addition to saving the energy normally lost to the outside. I would strongly suggest having a building performance test done first to determine the effectiveness of the existing insulation, the overall leakiness of the building itself as well as the safety of all the combustible appliances currently installed.

John Rising
Rising Design & Construction
Certified Building Performance Contractor