Alas! The political campaign season came to an end right before the cold weather set in. Just when you could actually use some of that free hot air generated by the candidates to warm your house, it stopped flowing.

Politicians! They huff and they puff, but the changes they promise can take years to deliver — if they ever are delivered.

So if you now discover that you’re not likely to be one of the beneficiaries of the new order’s largesse before your utility company puts a lien on your house for unpaid bills, fret not. There’s still plenty of free hot air available, if you know how to harness the sun’s energy.

Enter the solar air heater — a simple, inexpensive panel that costs nothing to operate. Similar in appearance to the solar photovoltaic panels seen on many roofs, a solar air heater is a thin, hollow box with “glazing” (glass or clear plastic) on one side. The other side is insulated, and a piece of light-gauge sheet metal, usually painted black for better heat absorption, is suspended between it and the glazing. The sun heats the sheet metal, and the air in the box is heated by contact with the metal. The warm air rises through a vent at the top of the box, to be replaced instantly by cold air entering through the bottom.

Unlike photovoltaic panels, solar air heaters are generally mounted vertically on the south-facing outside wall of a house (for maximum solar exposure) rather than on the roof, so they take advantage of the fact that hot air rises to move it into your house. Roof-mounted heaters need fans to force their rising hot air down into the living space.

However, wall-mounted heaters might need small, solar-assisted or standard plug-in fans to help draw the air from the heater and distribute it throughout a room. Otherwise, the heat will just rise and “pile up” at the ceiling where it enters your house.

Depending on the design of the heater, the inlet can take in outside air or recycle pre-warmed air from inside the house through a return-air vent similar to that of a standard furnace. Outside air, of course, can be much colder than the air in your living space, and, after moving through the solar air heater, might enter your house at a lower temperature than recycled inside air would.

But you don’t need to install a vent through your wall to collect outside air; you can bring the heated air into your house by venting it through a slightly open window that can be closed when the sun goes down. The outside-air heater, then, can be both less expensive and more portable than the inside-air models; renters could use them to cut their heating costs in successive rentals.

How effective are solar air heaters? One manufacturer says its 3-by-7-foot heater can provide 25 to 50 percent of the heat for a 400-to 800-square-foot space — depending on available sunlight, outside temperature, the building’s insulation and so on. One Web site claims that a homemade model raises the owner’s living room temperature by 6 to 10 degrees, which could reduce the operating time for a typical furnace by as much as 20 percent.

You can buy lightweight solar air heaters with welded metal frames and high-tech acrylic glazing for $400 to $1,750, depending on their size and application. A reader of this column has one on his roof and says it works so well he plans to install a second unit to harvest more free energy.

Alternatively, you can buy a prefabricated kit and assemble your own solar air heater for less than $400. Kits can be found on the Internet, but I recommend doing some research and getting some references before purchasing a kit online.

If you’re handy, you can scrounge some local materials and build your own heater for $50 or less. (Use safety glass from an old shower door or high-strength acrylic sheet for your glazing.) Instructions are available on several Web sites, and there are few projects that will harvest more energy for your effort at your ecological house.

Philip S. (Skip) Wenz is a freelance writer specializing in ecological design issues. He was a general contractor, residential designer, teacher and writer in the in the San Francisco Bay Area.