I removed an old wall mounted ac a couple months ago and have been trying to figure out what to do with the space. Its on the south facing side of the house so I thought about just putting a window in which would have given me some more solar heat. Windows on the south side of the house can help but even the energy efficient ones will lose heat when the sun isn’t shining. I finally made the decision to built a passive solar heater. It was something I have been thinking about for some time. The idea was for an indirect gain system, where a space outside of the living area is used to warm air with the sun. The air rises and enters the living space through a vent or duct while cooler air from the house replaces it. There are a number of designs on the internet but I decided to build my own version of one using what I learned about how they can work. Its fairly simple in concept, an inlet near the floor and an outlet near the ceiling and a collector in between. I built a small framed chamber with 2×3’s on the outside of the house reaching from the sill to the ceiling. The thing that really makes it work is the solar collector. Surfaces when they heat up create a thin layer of hot air. As the air heats, it becomes viscous so the warmed air moving up the chamber never comes into contact with the surface and most of the heat stays in the heater. Heaters with continuous surfaces are very poor at exchanging the heat collected to the air. The material used needs to have the right properties of heat absorption and conduction. One of the really expensive versions had thin metal strips but most of the cost effective designs used black screen as the absorber.
The important thing was a material which could heat up fast and then release that heat so that it didn’t hang on the surface. Well I happen to have a roll of 10 inch aluminum flashing which seemed like it might have the right characteristics. I bought a can of matte black spray paint to help it absorb and not reflect. I built a flimsy frame with some wood I had around and then cut pieces of flashing that would be nailed onto the frame at 30 degree angles so that at mid-winter the sun would be almost perpendicular (the sun is about 25 degrees above the horizon at winter solstice). The walls, floor and roof were heavily insulated with recycled styrofoam. Mostly from containers we get at work. Apparently, most of those are just remelted when recycled so the real value is lost. For the south facing side, I have some windows removed from the barn which I have been planning on taking to the dump but just couldn’t quite bring myself to do it. I cleaned up two of them and while they needed to be turned on their side they ended up with an almost perfect fit. In order to not have cold air back flow into the house at night, I created a trap. The hole for the inlet is about 3 feet above the floor but on both sides, inside and outside, I made wide narrow plywood ducts so the air is pulled near the floor where it is coolest and then goes up and then through the wall and then down again where it comes out just above the floor of the chamber. Because the chamber is not heated at night, cold air will collect at the bottom. If the vent was at at floor level, this air would flow back into the house. Making the actual hole well above the floor still allow air to flow into the chamber when the sun is heating it but prevents this passive back flow at night.
I’ll try to add some measured drawings when I get a chance.