Solar Furnace – thermostat

Old wire thermostat

Earlier this week, I posted about my solar furnace project.  I can’t take all the credit for the idea of a solar furnace as they have been around for a long time.  Most of them seem to be passive – relying on natural motion from the warm air rising through the system.  This sort of air flow is not typically very strong (though it can be), so cannot open a louver or check valve.  Just leaving the hot air pipe open is an option though physics will bite back at night or when it is not sunny outside.  Just as hot air rises, cool air sinks so at night, warm air will syphon from the house backwards through the system.  Some folks drape a piece of plastic over the warm air output so that the warm flow blows the plastic open a little.   Cool air cannot flow back through the plastic, supposedly.

Thermostat for solar furnace

All that sounds nice, but I cannot be satisfied with simplicity when I can further complicate things with technology.  Being interested in saving a buck and doing the right thing energy-wise, I replaced our old thermostat with a digital programmable one a couple of years ago.  I noticed that the old thermostat had a mercury switch so I didn’t want to just throw it in the trash – instead I kept it in my stash of junk…and lucky I did.  This project is prime for my sort of junk.  I stripped the mercury switch and the bi-metallic temperature wire from the thermostat and connected it to an old computer fan.  I am building a box that will contain the thermostat and the computer fan.  The fan will pull warm air from the furnace across the thermostat.  While the air is warm, the mercury switch will turn on the fan which will blow open the dryer vent (that will prevent cold-air backflow) and send warm air into my room.

Thermostat for solar furnace

Initially, I had planned to power all of this with a solar panel.  My fan is a 12 volt, 0.62 amp fan (though I ran it just fine with a 9-volt battery).  To drive the fan directly from the solar panel (at 9 volts), I would need a 5.58 watt panel (watts = volts * amps).  I may be able to get away with a little less but the cost of a 4-6 watt solar cell would still cost somewhere around $50.  I have various 9 volt wall-wart transformers from old gadgets that I no longer need.

Louver for solar furnace

For now, I will just drive this system from wall power.  At 9 volts, when the fan is on, it will draw around 4-5 watts – about the same as a nightlight.  I can live with that.

My plan is to get all of this hooked up and running this weekend.  I will post again with the final project.  It is supposed to be cool this weekend so I guess the timing will be right!

Spammers have been attacking this page so I have turned off comments.  If you would like to have a discussion, please post on one of the other solar furnace posts or send me an email message.  My contact info is in the “about/contact page”.  Sorry for the inconvenience.


10 thoughts on “Solar Furnace – thermostat

  1. “I cannot be satisfied with simplicity when I can further complicate things with technology.”

    I’ve always considered over-engineering to be a virtue.

    Seriously, however, looking forward to hearing how this turns out!

  2. I shy away from complexity because it seems to kill so many worthy projects and leave basic needs unmet…. BUT I admire one – such as yourself – who wades into the murky waters of technology, cobbles all sorts of stuff together, and ends up with something actually useful and cost-effective. That’s sheer genius, man. 🙂

    We’re pretty well set in the heating department (passive solar, wood stove, ridiculous insulation), but I’m anxious to see how your project goes. I love the dryer vent idea for closing the backloop.


  3. Warren, will the computer fan have enough force to open the dryer vent? Maybe I am just thinking about it being in a different location. I supposed with both the fan and the pressure of the warm air rising the vent will open. Look very forward to reading more. I love it when junk comes together to make such a useful project.


  4. I always plan to keep things simple but I get to messing around and it becomes more than I started with. I think this one will pretty well work out but I will know more later today. Ron and Chris – I am anxious to see if the vent works also. The fan won’t blow it full open but it will open it. The question in my mind is whether it will be enough to really make a difference. I have also found some smaller fans that could run on a 1.5 watt solar panel. I am not sure whether it would be as forceful as what I have now. I htink this will be a work in progress…

  5. How very neat! Are you like a genius? My husband’s good at tinkering around with things too-I’m going to make sure he sees this.

  6. I’m just wondering when you’re going to install one at my house??? As cold as it gets here anything would help our heating bill.

  7. tipper – as a matter of fact I am a genius. I asked my wife and she confirmed it…she said, “you must be…you married me!”

    pa grandma – It’s too cold there…I won’t be back until spring!

  8. you are having some serious fun with this project! I was happy to insulate one of my chicken coop walls, and here you are solving backflow air problems!

    Way to go!

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