When we built our house, we designed it to need very little extra heat in winter. It makes excellent use of thermal gain (passive solar heating on the south), has good insulation in the roofs, and is partially buried on the north side to increase thermal mass. We have an RMH stove for the cloudy days, but we very rarely have to use it.
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The town house, on the other hand, was not as well designed. Nor is it terrible. It has large windows on the south, which helps, and the walls are about 18" thick. In summer, it stays cool and pleasant. In winter, it seems to hold a constant temperature of about 60 degrees, even when it's 30 outside.
So, seeing as we didn't need to raise the temperature by much, we figured a solar heater would be perfect.
Abe made it in a day or so, and it cost about $90. It's an 8' by 4' box, with insulation on the back, greenhouse plastic on the front and black screen on the inside. It stands almost upright on the southern wall outside the living room. There are two holes through the wall. The bottom draws air from inside the house. That air passes through the heater and is then blown down into the room with a small fan.
The air inside the box is about 160 degrees, but it blows into the house at 130, so there are a few inefficiencies in our prototype. Abe has already figured out several ways of improving the design for the next one, which we'll put on the kitchen wall.
Even with its imperfections, we love it. The concept works great and uses almost no power. Needless to say, the sofa by the fan has become a favorite spot on cold days!
A full howto will be coming soon.