Our 'unheated' 32'x10' sunroom is one of the things Kathy and I did right when we designed our house thirty-odd years ago. I write this as I sit at the table in the said room and bask in the 80 degree sun-provided heat. This, despite the fact we've been harvesting the heat for the past hour or so and the temperature outside has barely made it to 20 degrees, up from the overnight low of 5 degrees.
The sunroom design allows us to take advantage of sunny days like today to circulate the heat through the house and to avoid cycling our central air system. Inside the house between the living room and the dining room is a rock divider, 200 cubic feet of stone and sand that acts as a massive heat flywheel absorbing energy during a sunny day then radiating it at night to delay the need for our central heat.
When we want the extra space for family gatherings, the sunroom can be heated or cooled from the house system. In the summer, the room is shaded from direct sunlight and any extra heat is vented to the outside.
I also enjoy the view of Kathy's greenry and flowers all winter