In this day and age, any free heat is a good thing, and these may help keep your forge or garage a little warmer in the winter. With some adaption, they might even be able to help heat your living space as well at the cost of loosing light coming in the windows they sit in.
Essentially, all these systems are is a large, shallow wooden box, with a labyrinth of wood inside to make the air travel a longer path through them. The entire interior is painted flat black to absorb as much infrared heat from the sun as possible. As the inside of the box warms, the air will begin to rise through it, creating draft at the inlet hole which will draw cooler air into the box, where it warms and rises continuing the cycle. The only real problem is the heat will come out the upper vent in the box and rise to heat the ceiling of your work space, so some kind of ceiling fan or other air circulation unit will be needed (and we will cover a clever one as well in another post).
Measure your south facing windows, and subtract 1/2” from that measurement for the outer dimensions of your box. Keep in mind that these boxes completely block most of the light coming in, so you may want to only cover a few windows, depending on how your shop is lit.
The outer frame is made of 1x3 or 1x4 pine with the uprights cut the the length you came up with above, and the cross members cut 1 1/2” shorter than your measurement to account for the thickness of the side rails. This box is assembled with glue and screws, making sure it is square at the corners. Measure and cut your back panel and glue and screw it onto the box. With that done, cut several 1 by sections about 90% as long as your side to side measurement. These will make up your labyrinth slats.
Lay these out on the inside face of your unit, mark the positions and drill small pilot holes for the screws that will mount them. In our illustrations, the holes are large round openings, and either this shape or rectangular holes will work effectively. If you want to use rectangular holes, drill four holes in a rectangle at the top and bottom of the back corners as shown in the illustrations to cut out the intake and exit holes. The exit hole should be a bit bigger than the intake hole, perhaps as much as 50% bigger.
Once the labyrinth cross bars are in place, the entire inside should be painted with flat black paint and allowed to dry. It may take a couple coats of paint to get an even, complete coating. You may want to paint the back of the unit white to reflect any ambient light in the room back to help make up for the lack of light your covered window will have.
The purpose of the labyrinth is to make the path from the intake to exit longer. The cross laths can be mounted flat or slightly inclined upward. Both work, but in theory the slight inclines should cause the air to move through the box a bit more easily.
Once the paint is dry, place them in the windows they are made for, and secure them so the box doesn't suddenly fall out. The glass in the window frame acts as the front of the box, and a bit of foam weather stripping around the outer edge of the box should help seal the box into the window frame, hopefully incresing your amount of heat output.
That's all there is to it! These systems may not produce a lot of heat, but any free heat is still free heat. In closing, be aware that some glass restricts infrared more than others. The more infrared that gets into your collector, the more heat it will put out. These systems can be adapted for stand alone use, but that is a whole other topic. Have fun, be safe, stay warm!
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