I have a screened porch with one side brick and three walls screens. The ceiling is 1/2 inch plywood. The window guys from window world are coming out in a week or two to install windows and remove all of the screens . I plan on adding r30 insulation on top of the plywood. I also plan on adding a supply and return duct, tapping off my existing hvac. I'd like to control it with a separate thermostat and dampers. I would add a 24vac transformer to power the thermostat and the dampers. Really I just need a switch but the thermostat would keep it from opening the dampers if I forget to turn the switch off. I'd really like some simple ideas or other suggestions.
Your idea is full of holes as we like to say. As @Ecnerwal stated in his response, you have an area with a different heat loss and gain from the rest of the house. Just adding a supply and return duct will not provide a good result. It would be great if this idea would solve your problem but I doubt it will. If you live in a warm climate, this will not add enough cooling to be comfortable. If you live in a colder climate the reverse is true. To get the best results, you will need a dedicated unit to heat and cool this area unless you will be OK with what you get otherwise.