MAIL: Micro-Solar Project

Using a 15-watt solar-electric module and four small “computer” fans, I built a simple exhaust system to keep the crawl space under my house dry. Projects that are simple, low-cost, and easy are the kind I like, and this could not be less complicated. The goal was to exchange damp, stale air with dry, fresh air. When it is dark and rainy, or damp outside at night, you don’t want the fans to operate; when it is bright and sunny, you want the fans to operate, exchanging the air in the space. This is a perfect application for solar electricity.

If you have experimented with solar projects, you probably have a fair amount of “stuff” you may not be using. Better crawl space ventilation was what I needed. Using an old 12-volt, 15-watt PV module was the perfect start. It didn’t put out a lot of power, so low-power fans were required. Four 12-volt DC muffin fans ($3.50 each) fit the bill. Each 4-inch-diameter, 0.1 amp fan can move about 50 cubic feet per minute. I used four fans for 200 cfm. My 1,200-square-foot house has a 3-foot-high crawl space—that’s 3,600 cubic feet of air. So when the fans are running, the air exchanges every 18 minutes. Larger (120 mm) computer fans would work, but you’d need to be mindful of the total amperage—they need to be less than 0.8 A for a 15-watt module.

The 4-inch fans are mounted on two 1-by-6 boards with 4-inch holes. The two boards were then mounted inside the crawl space at the existing air inlets. I covered other air inlets and left two exhaust outlets, since you want the air to flow completely through the space.

I originally disconnected the fans in the depth of winter, since you don’t want freezing air in the crawl space. What I needed to make this a perfect project was to add a thermostat. I built a Cana Kit (CK112; $25) electronic thermostat, which is also available assembled (UK112, $30). This thermostat uses only 0.1 A and has a built-in voltage regulator and a 3 A SPDT relay. I mounted the thermostat on one of the fan boards. When the temperature drops below 40°F, the fans don’t operate.

I have had this in operation for about two years, and the fans and temperature control are working fine.

Roger Clery • Murfreesboro, Tennessee

Great idea, Roger. All Electronics has an assembly of four muffin fans with guards, already mounted on a plate, which could be attached directly to a vent screen.