Saturday, December 10, 2011

Heat your house for free. Yes that's right, I said free heat. This is how we built a solar air heater

Everybody likes free stuff. At least I do. How about some free heat? Sound good? Well, what if I told you that if you did it right you could heat your house for free this winter, at least during the daytime? Would you like that? If you would, then read on. If you wouldn't, well, take up belly dancing, reload some ammunition, build a mortar, learn to make moonshine or do something else useful to mankind.

See, they have these things called a solar air heater. What they do is they take cold air from outside, it goes inside this wooden box like contraption where there are cans, pipes, pieces of guttering or even flexible tubing, whatever kind of metal you want to stick inside. The sun shines through the window on the box, heats the air up to 150, maybe even 200°and then you simply send this hot air into your house where it keeps your tootsies toasty warm while you watch Gold Rush, Rocket City Rednecks, Nights of Mayhem, Sons Of Guns, Baywatch or some other quality programming. Or whatever it is you like to watch. But I gotta say, if you don't think Knights of Mayhem is cool, then you have a screw loose. But could this possibly be true?  It sounds pretty simple. Yes it does. Because it is.
There are tons of youtube videos out there of guys showing off their solar heaters and a lot of them also show the results they are getting, the temperature of the air coming out of their solar heater and some of them even show you how to make them. Here is one of the simplest that I found.

If you have any type of desire whatsoever to build one of these and you can't do what this guy did, then I just don't know what to tell you.  Take up knitting, I guess.
I liked it for the fact that it was simple, he didn't spend a lot of time cutting out the bottom of his cans, cutting little spirals into the top of them or anything else, it was easy to make, and we already had most of the stuff lying around that it would take to build it. But why? Why even bother to build one?

This same sort of question must have been asked of Columbus before he set out on his quest. Why, Columbus? Why do you want to go out into the big scary ocean in search of a new world when we have a perfectly adequate one right here? And do you know what his answer was? Because it is there. That was his answer. Or at least that should have been his answer. His actual answer was probably something like because I am broke and desperate and I will do anything to become famous, just like that Kim Kardashian chick, but I don't have a video camera so that I can make a sex tape like she did. But I digress. At least I think I have. Yes. Yes I have. Just a little.

Why not build one? So off we went. Time to scrounge up materials.

I wanted to try something like the guy in the video did I had a couple of my own ideas I wanted to incorporate.  What if we used a wooden box?  And what if we had more than one layer of cans?  Why use wood? Because it is an excellent insulator. And because we didn't have any of that rigid board type of insulation that the guy in the video had. So I decided that we needed a wooden box.  No problem. Somebody left one in one of my dad's rental buildings so we claimed it. All we had to do to it was to remove a few nails that the person who built it couldn't drive straight and had left them sticking out the side of the boards. The wooden box measured 22 inches by 34 inches on the outside, three sides of it and the back of it are made from ¾ inch plywood and one side of it is a two by eight. Why it was built like this, I don't know.

This is our box.  Note the stylish carpet on the top.  Note the white caulking already installed along inside seams

We used caulking to seal up all of the cracks on the inside of the box. Have you ever used a caulking gun? If you haven't, it's pretty neat. You squeeze the trigger, kinda like a regular gun, and glue squirts out the pointy end of the tube. It looks like an endless worm crawling out. I guess if you're drinking beer while you're using a caulking gun it could resemble an endless dog turd coming out or even something else, depending on how much beer you have had.

Next we needed some insulation. Again, no problem. We found some in my dad's shop and promptly rescued it. There were two different kinds and I don't know why exactly but we ended up with fiberglass type insulation on the back of the box and on two sides, and on the other two sides are pieces of Styrofoam insulation. Don't ask me how I got that way, it just did. Then we spray painted the inside of the box with Rustoleum high heat spray paint that we got at Walmart for $5.00.

This is called insulation and we have cut it to fit our box.

See?  I told you it fits.

If you do go look on youtube at some of the solar air heaters you'll notice that there are lots of different kinds of them, built of lots of different materials and lots of different ways. I took most of the ideas from the video I already posted and I also wanted to change one thing about it. Since our box was 8 inches deep, I wanted to have two layers of aluminum cans, giving us more heating capacity. So we did. There are 46 cans on each layer I believe, two layers deep. We didn't even have to glue them in, they all fit in there fairly snugly. We simply stacked them on top of one another, cramming in however many we could fit. Then we spray painted the first layer a couple of times, added second layer and spray painted it a couple of times. Presto. Done deal. Now we need to move some air through this box. How to do so? Maybe we could shoot holes in it?

Here you can see the insulation added, one layer cans installed and painted, and you can see the intake and exhaust holes.

Yes we could. And it would have been fun. But we needed some neatly trimmed holes.
You need an intake hole and an exhaust hole so that the air can circulate through your solar heater. So we set a can on top of the box, drew a circle around it, then used a drill bit to drill a few holes through the wood, making a hole large enough to get a jigsaw blade through. Then the jigsaw cut a circular hole in the top of the box. Turn the box over on the bottom and repeat. Cold air could now come in through the bottom of the box, heat up, and  in the process of heating up it will rise to the top where it will exit through the exhaust hole and then go wherever you send it.

Here it is inside getting ready to get fitted with Plexiglas.  As you can tell, we used a coke can for the exhaust pipe.

OK, now came the hard part. We needed a piece of glass for the lid. No problem. My Buddy Jim was helping me and he happened to have a piece of tempered glass. Perfect. Then we started trying to cut the tempered glass to the size that we needed. At this time we noticed that it was not quite so perfect. We could not cut it. How were we supposed to get it to the right size if we couldn't cut it? hmmmmmmmmm. Maybe we could trim it to our specifications by shooting it?

Heads were scratched. Thoughts were pondered. Ideas were hatched. Perhaps a butt or two was scratched as well. Then came the grand solution.


Plexiglas would be perfect.  It was such a grand solution that we didn't have any.

A quick online search commenced where we discovered that a piece of Plexiglas 30 inches by 36 inches is the closest size commonly available and it would cost $22.00. Yikes! $22.00? For a thin piece of Plexiglas? We checked on a piece of glass also and a single pane glass would be $15.00 and the first hailstorm that came along would likely wipe it out. It takes lots of money to achieve the advancement of mankind.  Just look how much it cost for the space program.  We decided that mankind definitely needed advancing and coughed up the $22.00.

We laid out the Plexiglas on top of our box and drew an outline on to the Plexiglas. Then we placed the Plexiglas onto a table and used a jigsaw to cut it to the right size. Next it was placed back on top of the box, the metal molding that we were using to hold it down was placed on top of the Plexiglas and holes were drilled through the Plexiglas and into the wood.

You can see where the molding was laying while the holes were drilled.

Then we removed the Plexiglas and the molding and it was time to caulk again. Lay a bead of caulking all the way around the top of the box, place the Plexiglas down onto it, lay your molding on top. Now it was time to screw everything down. Screw, screw, screw. Then you screw some more. If you're not using a cordless drill you will discover disappointing and terrible things about yourself at this time. Things such as, gosh I thought I was stronger than this. Then you will discover that, gosh I am really out of shape. Screw some more. Get things nice and tight, tight enough that caulk squeezes out the edges. That is when you know you have a good seal.
This is Jim involved in a flurry of activity.  Never interrupt a man while he's screwing, it's simply bad juju. Note his speed.  Note his grace.  Note his stylish wardrobe and his cheerful and happy demeanor.  He has put down the bead of caulking around the top, the Plexiglas is lying on top of it and he is in the process of screwing down the molding to hold the Plexiglas tightly in place.  It needs to be airtight.

One piece of molding left to go.

You're done screwing.  Go rest.  Smoke a cigarette if you wish.

And that was it.

The total cost was approximately $28.00. The only question was, would it work? There was only one way to find out. Let's stick that sucker out in the sun and see what happens. Yes, let's. 


Well what? 

What happened? 

Oh. This is what happened.
We set it out on a Wednesday at 12:30 PM. It was a clear, sunny day, 37°outside temperature. Less than 5 minutes later, the temperature inside the box was around 100°. In 30 minutes the temperature inside the box was at 150°. After 1 hour the temperature was 160°. We had to leave at that time and when we returned later on it was after 4:00 PM, the sun was blocked by our garage and the temperature inside the box was less than 100°.

Here it is, doing what it is supposed to do.  Some of our cans are a little bit bent.  They still heat up just fine.

I was kind of happy about this. Certainly if we piped 160° air into the room it would heat things up. Wouldn't it? It should. But before we went to the trouble of doing so I wanted to test it for a couple of days first. So began our test and evaluation phase.

On Thursday I was sick and we didn't check it much but at noon it was 48°, there were scattered bits of high thin clouds around and the temp inside the box was 154°. One group of clouds came over and a couple minutes later the temperature in the box had dropped down to 140. Seems like clouds have a pretty dramatic effect on the temp. Once again, we had to leave and got back at 4:00 PM. The sun was behind the shop and the temperature inside the box was less than 100°.

Sunny day, although just slightly overcast. We put the box in the sun about 10:15 AM and took temperature readings every 5 minutes for the first half hour, then every 15 minutes. The starting outside air temperature was 31°.
10:22 AM 33.7°
10:27 AM 35°
10:32 AM 50°
10:37 AM 70°
10:42 AM 85°
10:47 AM 102°
10:52 AM 110°
11:10 AM 130°
11:25 AM 132°
11:40 AM 146°
11:55 AM 160°
12:10 PM 170°
12:25 PM 172°
12:37 PM 176°

At this point we pretty much stopped checking it but we did once more and at 1:30 PM the temp was 168 degrees. The outside air temperature rose to  41°this day. That is a 130° difference, if you measure from our highest temperature reading. I was impressed. And I am not easily impressed. Certainly impressed enough that we could come up with some sort of way to send this heat through the window and into our work room. Our tootsies deserved it.

That completes step one.  Step two is to rig up a ducting system and a blower motor to send the hot air into the house.  Stay tuned for step two.

Here are a few more videos that I thought were good enough to watch.  The more you watch the more you learn.

All temperature readings inside the box were taken with my mother's handy-dandy candy thermometer, guaranteed to be accurate down to + or-the nearest 247°.  Sound good?

Building tips and observations
If you want to build one of these for your own use, here are a couple of things to keep in mind.
1.  If you are going to use Plexiglas, you might as well buy the Plexiglas first and then build your box whatever size the Plexiglas is.  We ended up wasting part of our Plexiglas by trimming it to fit.  If I had known before hand how much it cost, we would have simply used some two by eights and built a 36 inch by 30 inch box.  The Plexiglas would have fit perfectly on top of it, making things even simpler.  If you have an old window lying around or even an old glass door, use it.  Build an even bigger box.
2.  Even if we had bought everything new at Home Depot or somewhere the cost would have been, I don't know, maybe $50.00 or so.
3. We screwed up.  We should have placed the air intake hole on the back of the box near the bottom.  We're going to need to put a fan on it to blow more air through the solar air heater and this would be a much easier place to put one, rather than on the bottom where ours is.
4.  Before you build one of these, consider where you're going to put it.  Ours is in a pretty good location, facing directly south but it still only receives sunlight from approximately 10:15 AM until 3:45 PM.  Before and after that time there are trees and buildings blocking the sun.  When you're dealing with solar, more sun is always better.
5.  It is tricky to cut Plexiglas with a jigsaw without ruining it.  You need to have it lying on a solid surface with something else lying on top of it holding it down securely.  And it also helps to have someone else holding the piece that you're trimming off so that it doesn't hang down or flop around..  The vibration will cause cracks and chips in the Plexiglas if you're not careful.  It is also best not to ingest questionable substances or liquids before cutting Plexiglas.


  1. ???
    Is there a way to make a coil to capture the heat kepp the heat even longer

  2. Oh, I am sure there is, but we did not try anything like that. This was basically just a simple little experiment to see how hot the air would get. Adding a coil would make it more expensive and complex. If we were going to do it and use it as a meaningful source of heat, we would probably go ahead and build a bigger box.