(by Quiet Earp and The Hun)
Last June, Father Mayhem posted about making a solar heater out of cans. We thought that sounded like a good idea, so we made one ourselves. With a little experimentation and a few mistakes, we came up with what we think is a good product. Ours works great when the sun shines: hot air flows into the house and it doesn’t draw any power. If you want to make one yourself, here’s everything we learned. Go for it.
You’re essentially making a solar oven. Make a box and line it with aluminum, painted black to create more heat. Add a glass panel and seal it off, and the air inside will quickly become very hot. Set it at an angle so the hot air will rise to the top. Add a duct at the top to let hot air escape, and one at the bottom to bring in cool air. Run the hot air duct into your house, and voila. Heat.
- Salvaged glass window or door, double paned if possible
- Window/door frame or lumber to build a frame
- Empty aluminum cans or flashing
- Tin snips
- Black spray paint
- Caulk & sealer for gaps
- 4″ dryer hose, long enough to reach from the heater to your window twice
- 4″ aluminum vent pipe, 1-2 feet long
- hose clamps to attach the dryer hose
- Various bits of scrap wood, screws etc.
How To Make a Solar Heater
1. Prepare the box
We got this salvaged door, with frame, from the Habitat ReStore. We sealed off the mail slot, removed the knob and sealed that off too. Caulked around all the edges to make a relatively airtight box.
If you don’t have a window or door frame, use 2×4′s and 1/2″ plywood to build a simple box that will fit your glass panel.
Line the back of the box with insulation board. We used 3/4″ board from the hardware store.
You want your box to be no deeper than 2 or 3 inches. Ours was a little too deep, so I added some extra pieces of insulation board to make the chamber more shallow.
Depending on your dimensions, you might need to line the sides of the box too. We lined ours with 1″ x 2 5/8″ pine boards, not insulation board.
3. Line With Aluminum
A lot of the sites out there say you should stack the cans on top of each other to make long tubes (see photo, right).
I did a bunch of research and couldn’t figure out why this would work. If anything it decreases the amount of available surface area, making the heater less effective.
(The Hun thinks this design is modeled after passive solar vacuum-tube heaters, but without the vacuum it won’t be very effective.)
I found this article by Mark Bower, who seems to really know what he’s talking about. He advocates a “snake” design where the air flows back and forth through channels, gradually warming as it rises. We decided to use that as the model for our heater.
Cut the ends off each can and slice it to make one flat piece. Attach these to the insulation board using glue or screws or whatever you want. It doesn’t have to be perfect, just generally covered. This is where the magic happens: the cans will get hot and radiate heat into the air.
Be prepared though, because cutting all those cans is a lot of work and you will get cuts all over your hands.
We got partway done with the cans, then I found a big piece of used aluminum flashing in our basement crawlspace. Forget the cans! I screwed the flashing directly to the insulation board and door frame.
4. Make Channel Dividers
Use scrap lumber or sheetrock to make dividers to guide the air through a series of channels. Depending on the orientation of your heater, the channels can be vertical or horizontal. The important thing is to keep the air in the chamber for longer, so it gets hotter. We decided to line our dividers with more aluminum to get the most bang for our $0.
Attach the dividers to the insulation board or to the frame. Make sure you can still set the glass on the box neatly and without gaps.
We followed Mark Bower’s instructions to calculate the size of the channels, but we used them knowing that we were using 4″ dryer hose and working from that . You could probably skip that part, but try to make your channels a couple of inches wide. If they’re too big, the air won’t heat up enough.
Also, because of the shape of the door, our channels go sideways instead of vertically.
5. Mount the Box
It’s time to figure out where you want this heater to go. You can mount it on the side of your house, but we are renting so that wasn’t an option. We found a spot that gets full sun most of the day, and oriented it approximately SSW. I made a simple frame to hold it at a 45 degree angle, which is pretty good for catching the winter sun.
6. Make Vent Connections
Cut two 4″ holes, one at the bottom where the air will enter the channel, and one at the top where it will exit.
You need these holes to be completely sealed, so you can’t just stick the dryer hose into them. We used a section of aluminum ducting, cut into two pieces. I cut petals at one end of each section, so they fit neatly onto the lining. Seal it off with duct tape so there are no leaks.
7. Paint it Black
Spray paint the interior of the box, including the sides and dividers, flat black. Now it looks cool and will get much hotter too.
8. Make Window Connections
Cut a piece of insulation board or thick plywood to fit in your window. Insulation board is better because you can get a tighter fit. Cut two 4″ holes in it, where the dryer hoses will go. I had this vent hood left over from the ducting, so I added it to the cold-air side in hopes that it’d pull cooler air from lower in the room. Who knows if that works, but it looks good. Kinda.
9. Attach Hoses
Pretty self explanatory. We used hose clamps. You want your hot air hose to be as short as possible so the air doesn’t get cold on the way to your house. We painted ours black to make it hotter; you could also add a layer of insulation if you thought you needed it. Don’t forget to check everything for leaks and seal off any gaps.
10. Mount the Glass
This was fun. NOT. But we did it. We ended up using long screws to attach the glass door to the frame. It wasn’t a great fit so we sealed off all the gaps with Great Stuff. Do whatever you have to do to make it airtight. Any drafts can make your heater much less effective.
That’s it. The next morning, the heater was covered in ice…
…but by noon, it was pumping hot air into our house. Since then, it just sits there and does its job, and the only thing we have to worry about is what we’re going to do with it the next time we move.
Here’s a photo from yesterday. We stuck a meat thermometer in the hot air duct and it stayed at 175° for a few hours at least. Outside it was in the 40s. The air coming in is not a blast, more like a draft. You could attach a duct fan to improve circulation but we didn’t want to spend the money or electricity on one.
originally posted Feb. 11, 2011