"TAP" the Sun

Enlarge Image Image Gallery The integrated thermosiphoning air pane! (TAP) is the perfect solar collector for metal buildings because the side of the structure can be used as the absorber. LUKE ELLIOTT AND DALE STRODE

The integrated thermosiphon air panel (TAP for short) is a fresh approach to solar air heating — one that's perfectly suited to the 10 million or so folks who live in mobile homes and to owners of the countless metal-sided storage, industrial, and commercial buildings around the world. By making use of the building itself as, a part of the collector, the TAP keeps material costs down, simplifies construction, and maintains a low visual profile. And because it relies on thermosiphoning to move air past the absorber, the TAP avoids the complexity and cost of an electrically driven fan.

Steven McCarney, the developer of the TAP, says the idea evolved after he'd installed several standard collectors on metal-sided buildings. In his words, "It seemed redundant to put a collector with a separate absorber and an insulated back on a building that already had those elements." Hence, Steve decided to use the building's own insulated wall as the collector's absorber. Now, longtime readers may recall that the integrated collector idea goes back a little ways (remember the Waterman solar setup in MOTHER NO. 48?). But, regardless of the origin, McCarney and his co-workers can take credit for researching and refining the scheme to the point where just about anybody can build a durable TAP that will pay back his or her investment in only a few years.

What It Takes

What do you need to build your own TAP? The critical element, of course, is sunlight:

You need a metal-sided wall that faces within 25 degrees of south and that isn't shaded in the crucial winter months. You'll also need some hand tools, such as a drill, metal snips, screwdrivers, a caulking gun, a 1/4-inch nut driver and maybe a circular saw. Add to these items less than $100 worth of materials and a weekend's work, and you'll have solar heat flowing into your home.

How Big?

A rule of thumb for solar air heaters is to allow one square foot of collector for every 10 square feet of floor area you wish to heat. (The room should first be caulked, weather-stripped, and at least moderately insulated. Always take care of such energy-conserving fundamentals before adding solar heat.) Thus, a 180-square-foot room suggests an 18-square-foot collector. This, however, is only a rough estimate. The available wall space for mounting the collector and the limitation of standard tempered glass sizes (28-, 34- or 46-by-76-inch; 34 or 46-by-90-inch) will determine the actual dimensions of your collector.

Because the TAP relies on passive airflow to distribute the heat collected on its absorber, you'll get the best results by sizing the collector to warm the room directly adjacent to the wall on which it's mounted. Though heat will eventually be transferred to other rooms, oversizing the TAP to try to heat more than one room will make the first room too warm.