Solar furnace installation

Solar furnace thermostat

I last posted a week or so ago about my solar furnace.  I mentioned that I needed to install the system in my window and get the fan/thermostat working.  Well, my friends, wait no longer!  This weekend I finished the installation of the solar furnace and even powered it with a small solar panel…but I am getting ahead of myself.

Solar furnace entry to house

My downstairs family room has 5 windows, 3 of which face southward.  A south facing panel is optimum because it gets sun almost all day as the sun progresses across the sky.  I read that 20 degrees from due south results in a 5% decrease in performance.  Luckily I have south windows but if I didn’t have due south, I would still try the furnace.  A 5% decrease would still make things interesting.   In addition to the direction, you have to consider the angle from horizontal of the panel so that it gets optimum sunlight in the winter when the sun is lower in the sky.  I have read various thoughts on the optimum angle but the easy one for me to remember is “latitude + 10”.  Here in Charleston, our latitude is 38 degrees.

Solar furnace entry to house

According to this calculation then, the optimum angle from horizontal is 48 degrees.  Using my trusty eyeball, I leaned the solar panel along the south wall of my house and tilted it at precisely 48 degrees.  To be more exact, you should consider doing a site survey to make sure you really have things right. I will probably just mess with it until it gets the best sun. In a more permanent installation, you would want this to be much more precise. Mine is easily movable.

Anyhow, I built a box (which I insulated) to run a warm pipe and a cool pipe through my window.  The thermostat and fan are inside the warm pipe inlet so the warm air should automatically rise and slowly flow across the thermostat.  The rise in temperature should kick on the thermostat/fan and move the air a little better through the system.  As the warm air evacuates, it will cool the thermostat and turn off the fan.

Solar furnace entry to house

I mentioned in the last post on this topic that I could not drive the fan with a super cheap solar cell.  I was rummaging through the junk bin at the office when I found a pair of fans (actually, quite a few fans) that were lower amperage than my original one.  Anyhow, my new fan is rated at 12 volts and  0.16 amps.  I know it will run at 9 volts so I figured I needed a solar panel that would produce 1.44 watts (i.e. 9 * 0.16).  Harbor Freight sells a solar panel used for trickle charging auto batteries for $19.99.  Actually, the website has it for $14.99 but in the store it is more.  If I would have taken the printout to the store, they would have reduced the price.  Apparently the store and website are somewhat independent.  I didn’t have a printout and it wasn’t worth driving back for it so I paid $19.99.  Ok, back to the story – it will produce 1.5 watts which will run my new fan.  I don’t know if it always just produces 12 volts and the amps vary (because it produces differently depending on the strength of the sun) or if it produces 0-12 volts and constant amps, etc.  All that is to say, I don’t know exactly what wattage is driving my fan but it turns it.  Solar furnace entry to house

It occurs to me as I write this that I should measure the output with my mulitmeter…but really, for now, I don’t care.  The fan turns fine in sun and that is my main goal.  Unfortunately, the fan will not blow the dryer vent louver open. I had planned to use a dryer vent to close the inflow when not in use.  I will have to resort to the “plastic over the hole” method I mentioned in the earlier post, to prevent back-siphoning.  And let me tell you, back-siphoning is real and a problem if you don’t deal with it…I quickly learned a lesson on that topic!

Ok, lots of words to describe all this.  The only problem is, I have not had a sunny day since I got this thing installed.  I wanted to post about this progress but I don’t have any real results yet.  I will have to post again with results.  I know it will make a difference, but I don’t know the extent.  Stay tuned!

6 thoughts on “Solar furnace installation

  1. My husband is avidly following your progress on this project. He can’t wait to do the same.


    Thank you for your kind words on the adoption post. I can’t tell you how the comments touched my heart.

    In friendship,

  2. Alright, Ms. Ignorant chiming in here. Isn’t there something like passive solar power? I’m thinking that your solar panels should be getting *some* kind of charge from the sun through the clouds? Or am I completely off base here?

  3. Warren, thant’s great man. I’m praying for sunshine so you can test the results. I can’t wait to see them. Looks neat and clean, great job.


  4. I am excied to see how this thing does as well. I had planned to have results by now but it has been raining, snowing or just plain overcast for a long time (it seems like it anyhow).

    Farmgirl, you are right…the panel would make some power in overcast/indirect conditions. The panel I got is amorphous (one of the three main types of cells) and tends to work better in lower/indirect light than the other two types (though my type is generally regarded as not as good for other reasons). I am not storing the power from the panel in a battery. It is a direct power source for the fan. The mercury switch keeps it from just running all the time, even if it is making electricity in the indirect sun. Does that answer your question/make sense?

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