Build a Simple Solar Heater

Image Gallery Build this simple solar heater to tap a free source of heat. GARY REYSA

After walking into my workshop one December morning and feeling a bone-chilling 10 degrees, I decided to install a heating system. Given the rising costs of propane and my family’s environmental concerns about using nonrenewable fossil fuels, a solar solution seemed fitting.

I’m a retired aircraft engineer, but you don’t need a similar background to tackle this project. In fact, a solar hot-air collector built into new construction or added to an existing building can be an easy and inexpensive heating solution. Following the simple principles and plan outlined here, you can heat your workshop, barn or even your home with free heat from the sun. If it works here in Bozeman, Mont., it’s bound to work wherever you are.

I reviewed many solar collector concepts and decided to install a thermosiphon air collector on the south wall of the workshop. The elegant, simple thermosiphon design uses only the buoyancy of heated air to create circulation through the collector, eliminating the cost, maintenance and power consumption of fans, sensors and controllers commonly used in other collector designs. On a sunny day, in a cold climate like ours, this simple system can produce heat equivalent to burning about $2 worth of propane (equivalent to about $3 for natural gas).

To minimize costs, I integrated the collector with the structure and used readily available materials. It cost me about $350 and took only one trip to the hardware store. I built and installed it in about three working days. Follow the suggestions below, and you may be able to do it faster!!

How It Works

The thermosiphon collector consists of clear, corrugated polycarbonate panels fastened to 2-by-6-inch studs. The clear panels admit sunlight, and an absorber suspended inside the collector captures the sun’s heat energy. The air around the absorber warms, expands and rises, creating a convection current. Vents at the top and bottom of the collector allow air to circulate through it. Cool air enters the lower vents from the interior, is heated by the absorber, rises to the upper vents and returns to the interior. Air circulation continues as long as the sun shines on the collector.

At night, airflow reverses as air in the collector cools to outside temperatures. Simple flapper valves on the top vents stop this reverse circulation and keep the heat inside.

Performance and Economics

On sunny winter days, the collector raises interior temperatures to between 60 and 75 degrees. In my neck of the woods, that’s 25 to 35 degrees above outside temperatures. Overnight, the building typically cools to 8 to 15 degrees above the outside temperature. On heavily overcast days, the collector does little heating, but on partly cloudy days or with a thin overcast it provides useful heat. It usually takes about three hours to warm the workshop from 35 to 65 degrees—a good excuse to sleep in! If you’re determined to start work early, more insulation, more thermal mass or an early morning blast from a backup heater would be in order.