The absorber plate is a prepainted piece of 4- by 10-foot aluminum, typically available from metal roofing suppliers. Prepainted steel is cheaper, but the aluminum transfers heat to the air more readily. Use gloves and eye protection when fabricating and handling metal parts, as the edges are sharp.
Cut the dimension of the metal about 1/4 inch less than the interior dimension of the 1 by 4s around the edges, so that there is room for expansion. Fit the metal into the box. If it comes with a plastic film, leave that in place for now to prevent scratching the metal.
Once the absorber plate is fit, cut the vent pipe holes. Cover the edge of the vent pipe with pencil graphite, then put the absorber in place, pressing it to the pipe to leave pencil outlines where the holes are to be cut. Now you’re ready to cut out the hole in the metal. Use a large bit to drill a pilot hole, and then use tin snips to carefully follow the marked circle on the metal.
Place the metal into the box and mark the center of the 1 by 2s that will support the baffles. All visible wood, including the baffles, will need to be painted with black, heat-resistant paint.
The installation begins. The sheet of metal has been removed from the box and placed on the wall to locate our two vent holes. Make sure the collector is positioned to avoid cutting any electrical wires in the wall. Studs also need to be avoided. In homes with exterior siding, you can determine stud position from the nailing pattern.
Nail a temporary board to support the heater while you position it. Shim it to the right spot (check by putting the vent pipes through the heater and into the office).
This wall had plywood sheathing and 1-inch-thick wood siding, so we felt comfortable using 3-inch lag screws and not worrying about penetrating the studs. If you have any doubt, however, go to the extra trouble of screwing into the studs. We put two lag screws near the top and two near the bottom.
After placing the fiberglass insulation, we installed the metal absorber plate. We sealed it to the frame with a heat-resistant, high-grade silicone caulk to prevent air leakage.
Stephen Hren is a builder, teacher, and author focusing on sustainable design. He is the author of the upcoming book Tales from the Sustainable Underground and coauthor of The Solar Buyer’s Guide for the Home and Office and The Carbon-Free Home.