> If you want to not have to change modes when the season changes, it gets more complicated
There's an additional complication for multi-story houses that I didn't mention before. In multi-story houses, when you're cooling, you want most of the ducted airflow to be directed to the upstairs outlets, to take advantage of natural convection (cooler air sinks); similarly, when you're heating, you want most of the ducted airflow to be directed to the downstairs outlets (warmer air rises). That requires adjusting dampers in the ducts, usually just downstream of the output of your HVAC unit. Unless that adjustment can be automated (which would require automatically controlled motors on the dampers, something I haven't seen marketed, though again I'm not sure why), your airflow will be suboptimal for either heating or cooling if the system automatically switches between modes.
Switch controlled open/close dampers are common enough.
A typical suggested application is controlling the airflow to a room using a thermostat to open or close the damper, but they can be hooked up to anything with a switched output.
Can these dampers stop somewhere short of full open/close? Typically one wants full open for one season and mostly closed but not fully closed for the other.
There's an additional complication for multi-story houses that I didn't mention before. In multi-story houses, when you're cooling, you want most of the ducted airflow to be directed to the upstairs outlets, to take advantage of natural convection (cooler air sinks); similarly, when you're heating, you want most of the ducted airflow to be directed to the downstairs outlets (warmer air rises). That requires adjusting dampers in the ducts, usually just downstream of the output of your HVAC unit. Unless that adjustment can be automated (which would require automatically controlled motors on the dampers, something I haven't seen marketed, though again I'm not sure why), your airflow will be suboptimal for either heating or cooling if the system automatically switches between modes.