Solar furnace

Passive solar furnace

Being a cheap-skate, I am incredibly excited about doing anything I can to put a little extra green in my pocket.  I have been looking online and thinking a lot about making a solar furnace to supplement our home heating.  We happen to have replaced 2 screen doors with full-length windows so I was presented with a couple of large pieces of tempered glass.

Insulation for passive solar furnace

You can look around online for solar furnace or passive solar heat and find tons of additional information, but basically, these things work by capturing heat and using the principle that hot air rises.  I snaked clothes dryer exhaust tube side-to-side through an insulated (with sheets of styrofoam insulation) wooden box.  I covered the insulation with roofing felt to make the inside of the box black (plus I knew roofing felt would live through high heat).  I painted the box and the dryer hose with flat black paint so it absorbs heat very well.

Air passage for passive solar furnace

Cool air from my family room floor is pulled in to the bottom of the system.  The sun heats it as it sits in the tube.  The heating causes it to move upward eventually running back into my family room at about waist-high level.

This version does not show a blower on it yet so air flow depends purely on the principle of hot air rising.  I am working on a version that uses a small fan powered by a solar panel to move the air.  I’ll post more on that later as I get the details worked out.

Passive solar furnace

Anyhow, for the results…I set this in the sun at about 4:30 one afternoon.  The input temp was stable at 62.8 degrees F.  I then measured the output temp.

Input air temp

I could not believe it but my digital thermometer maxed out when the temperature got over 160 deg F.  The last picture I took before it maxed out was at 157.3 deg F.  I have no idea what the temperature actually got to but I saw at least a 100 deg F temperature differential!

Output air temp

Maxed out my thermometer
I have some more info to post on this but it will have to wait until later this week…

15 thoughts on “Solar furnace

  1. the saurkraut that I am making is bubbling over. Apparently, letting it sit ferments it into saurkraut, and I’ve already had to add liquid to each jar (brine – 1 tsp. salt to 1 quart water) to keep the water level over the cabbage. Nasty stuff – but it tastes so much better than store brought. After 10 days, there could be “scum” on top of the cabbage – supposedly you just skim it off and keep on going – YUCK. It sure does smell good though (already it’s starting to smell like saurkraut).

  2. Warren, YOU ARE MY HERO !!!!!! I love this project. You are the Tinker Man for sure. I love any kind of solar project. Keep up the good work.


  3. I am going to show this to my hubby! I would really like to increase our use of renewable resources. Currently we heat our home 95% with wood. However, we still use Gas for heating our hot water and running the dryer (the 50% I don’t air dry). Thanks again for another inspiring post!

  4. Oh that is so cool! I am going to be showing this to my husband. I would love to build something like this and save some money on heat.

  5. Wow! I wouldn’t even begin to know how to do that. I’ve thought of doing some smaller-scale solar projects more to show the kids how it works (and myself too, I guess!). I’ll consider your project motivation to begin work on our little ones!

  6. Interesting… since I just told Sharon at Causobon’s Book that I was going to become more “handy”, perhaps I’ll make this, rather than asking Husband. (Sent it to YoungSon just in case… he’s fairly handy and could be my backup plan) Thanks for stopping by my place, btw.

  7. This looks so doable and useful. I wonder what it will do in the dead of winter? That is quite a jump in temperature!

  8. It’s taking me a bit longer than I had hoped to get the next post on this. I have found a cool way to rig up a thermostat and a fan to keep the air moving. Anyhow, this was easy to build and is a doable task for almost anyone. I am pretty curious what will happen in the winter too. Hopefully the insulation will manage the temp difference some. At the worst, I can just take it out of place if it makes things worse. I suspect it won’t though! We’ll see…

  9. Thanks for the information on solar furnace. I know that active solar heating takes the liquid or air it releases by using a pump but you added lot more information which every one should know about solar systems. Thanks for the info.

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