MAIL: Micro-Solar Project

A Muffin Fan
A muffin fan used in solar ventilation project.
A Muffin Fan

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. 

Comments (6)

ken reid's picture

I use a 20 watt panel and 24 volt fans, they move more air and run direct from solar panel

peter's picture

Peter K, I got offers to seal my crawlspace for about $4000, and I've talked to others who have had offers up to $10000. The little solar fan and plastic took everything from damp to dust dry for a little over $200. While condensing conditions may be possible from time to time, I've never experienced them, and over the long term, they bring the dryer outdoor moisture-evaporating air in, and the wet air out. Good results without a lot of cost and hassle.

Peter Kulakosky's picture

Under some conditions, ventilation of a crawlspace will put more moisture there than if it were sealed. If the area under the house is below the dew point while the ambient air temp is above, you will have a condensing environment under the house so the flowing air is a better source of water than the ground.

peter's picture

Your can buy a complete, matched, engineered CSF kit with warranty at I've been running one for 2+years. Just mount it inside an existing foundation vent. Cover the ground with 4 mil plastic to act as a vapor barrier. Covering vents for exhaust-only can be dangerous if you have fired appliances like furnaces or water heaters, as you could back draft the exhaust CO. If you have temp adjusting foundation vents, you don't need a thermostat. You may want to keep some airflow in the winter, too, because it'll keep the underfloor insulation dry and increase R value.

Lucubrare's picture

I tried a similar project, and ended up burning up two former-computer fans. Thanks for the specs, I'll try this again.

Michael Welch's picture

The assembly of four muffin fans from All Electronics:

Michael, Home Power

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