From the archive, originally posted by: [ spectre ]


almost free garage heat – just drink a lot of soda

solar furnace

I’ve had a few days during the HMX build while I’m either waiting for
parts or waiting for something to dry and had some free time. I’m not
exactly one to sit and watch TV when I have nothing planned, so I set
out on another project.

While I have electricity out to the garage now, heat has been an issue
all winter long. Mattar graciously lent me his kerosene heater, which
did an okay job of taking the bite off the chill. Insulating the
garage would go a long way to help keep the bitter Vermont cold out,
but that’s a project for another day. I decided instead to take
advantage of the south-facing side of the garage and build a solar
furnace to collect some of that sunshine just bouncing straight off my
garage. My dad built one years ago and said he recorded a 110-degree
temperature differential between inlet and outlet. And I had enough
scrap materials around the basement to do something similar to what my
dad built.

my carpentry skills suck

I started with some 2×4s and plywood to build a simple box. I’m no
carpenter, but I learned that if it’s wobbly, just add more nails.

that’s a lotta soda

I actually built the box to certain dimensions, based on what scrap
materials I had and on the dimensions of my heat collection method –
aluminum cans. That sure was a lot of Sprite. Fifty cans in five
columns of 10 will funnel the air upward.

caulk fills all holes

Sealed the box using adhesive caulk, just to keep any heated air from
escaping the box.

45 cans of soda on the drill, 45 cans of soda…

So you may have already thought, “How can air climb the columns of
cans when there’s no hole at the bottom of the can?” Answer: drill
press and 3/4-inch bit. Times 45.

doesn’t have to be a clean cut

The last five cans, the bases of each column, will sit on the bottom
of the box and thus will be unable to draw air from underneath, so I
poked holes in the sides of each of the five.

use more caulk than that

Stack the cans with liberal doses of adhesive caulk. Give them enough
time to dry.

make sure you’re only drilling through wood, not the nails holding the
wood together

Once they’re dry, I painted each column with black BBQ paint. Black to
best absorb the sun’s heat, BBQ paint to keep from flaking off the
cans. At the top, I drilled an outlet hole. I left an inch or two of
space between the tops of the columns and the top of the box to permit
air to flow out of the columns.

wet-dry vacuum hose – I did buy that

I drilled the outlet hole based on the diameter of some wet-dry vacuum
hose I picked up, about 1-1/2 inches in diameter.

don’t put it here

At the bottom, I used another wet-dry vacuum attachement that would
more evenly disperse the incoming air. Screwed it in at each end, then
caulked the seal.

keep your columns straight

Then started to caulk the columns in place. At the bottom, you can see
the inlet hole I drilled. At about this point, I realized that a
better place for the inlet would have been through the plywood at the
bases of each column. In this location, the air can simply pass over
the cans (there’s about 1/2 to 3/4 of an inch between the cans and the
upper edge of the 2×4 frame) and not really pick up that much heat. If
I were to relocate the inlet, it would force all the air to pass
through the cans and pick up the absorbed heat. Next time.

I see a red door…

Had some red paint left over from one of Heather’s previous projects,
so slapped on a couple coats to at least keep the weather off the bare

not a trick shot

The caulk is pretty strong. Kept the cans from falling out while I had
the box inverted.

finally put to good use

Also had some 3/4-inch PVC pipe from another previous project. Bought
a couple elbows and T-fittings and whipped up a simple frame to keep
the box off the ground and to angle it upward toward the sun. Didn’t
give the exact angle too much thought.

sittin’ pretty

Caulked a clear plexiglas cover on the front and sat the furnace out
in the sun for a full day over the weekend to see how it would work.

need to clean out those leaves

Using some advanced technological equipment, such as this precisely
calibrated pyrometer, I determined the intake air temperature, which
should have been the same as the ambient air temperature, to be about
80 degrees.

could be better

Using the same equipment and methods, I determined the outlet
temperature to be about 95 degrees – thus a 15 degree temperature
differential. Not 110 degrees, but not bad , considering I didn’t even
break $50 in materials – most of that being the plexiglas window.

Obviously don’t have the inlet and outlet attached to the garage –
figures that the day I finish the furnace, it’s 80 degrees and sunny
and it looks like we’re finally done with winter. Dad recommends
wiring a pusher fan at the end of the inlet tube to keep the air
circulating through the furnace.

Were I to do this again, I’d first make the furnace larger. As I
recall, Dad’s measured something like four feet on each side.
Obviously, the more surface area, the more heat you’ll pick up.
Second, as mentioned above, I’d relocate the inlet to the back of the
box to direct all the air through the cans. Or at least I’d cut a
piece of aluminum to act as a baffle and prevent the air from rising
straight up. Third, I might use those small soda cans I’ve seen in the
grocery stores lately, just to get more surface area.

Fourth, I’d finish the build at the beginning of winter, not the end.

UPDATE: Welcome, MAKErs. I appreciate your comments and suggestions on
improving the design of the box. I also appreciated the comments over
at a similar project page on Instructables. Version 2.0 will be a lot
better, so thank you all.

UPDATE UPDATE: The response on this has been fantastic. Thank you all
for your comments and feedback. If I didn’t have the HMX to finish,
I’d already be working on the next version of this box. By the way,
I’m no engineer and only have the vaguest understanding of
thermodynamics. I know how old cars work, that’s about it. But common
sense tells me to build this thing bigger, to insulate it, to add a
fan and to snake the air sideways as a few of you have suggested. Keep
sharing your ideas and your successes in building your own boxes.

This entry was posted on Thursday, April 26th, 2007 at 7:15 am and is
filed under Daniel Strohl, tech, tools and garages. You can follow any
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45 Responses to “almost free garage heat – just drink a lot of soda”

3. Alan Says:
April 26th, 2007 at 8:19 pm

I’m wondering if it would perform better if all the cans were
connected in series, in a single long snaking chain.

8. Daniel Strohl Says:
April 27th, 2007 at 7:10 am

Possibly. A long, snaking chain would definitely allow the air
in the cans to build more heat, but consider that this whole thing
operated on the convection of heat and you’re thus talking about a lot
of energy required to force the hot air from the tops of the columns
back down to the bottoms of the next columns.

10. Jim Says:
April 27th, 2007 at 8:35 am

I don’t think the air actually has to go in the cans. You could
cut the cans flat and paint them black and tack them to the back wall.
The trick is to get enough airflow so the heat leaves the exhaust port
and not back out thru the plex. Keep it up!

11. Brian Says:
April 27th, 2007 at 9:00 am

Another good addition to this would be to insulate the outlet
hose to keep the cold air from cooling your heated air going back into
your garage. As well as possibly insulating the back and sides to keep
heat from escaping through the wood.

12. zabolyx Says:
April 27th, 2007 at 9:36 am

this would take a bit more work but. cutting the top and bottom
off of each can then unrolling the remaining cylinder would result in
a ton of moldable material.

Simply fold the flat sheets to make a sort of heatsink design
then mount them in the furnace. Figuring each tine being about 2
inches high from the back of the furnace and with a 1/2 inch spacing
in between. This would give you a magnitude more surface area and the
channeling effect would be natural.

Of course this would require a need of plenty more soda to be
drank. I’d mix it up a little with some different types so you
wouldn’t burn out. Also with the current system you will have better
luck making multiple panels to link together for portability reasons

13. GenDis Says:
April 27th, 2007 at 10:17 am

Cool Idea.

Here is an idea to get more surface area and longer period of
heating the air.
Try isolating each column of cans that would make a maze.

Start from the bottom right, go up to the top then go left one
column and then down to the bottom. Continue until you make your it
all the way until the left top. Use a divider of wood and caulk and
then drill the can that would follow the divider path.

This will make the air travel through each column of can and
increase the amount of time exposed to the sun.

Just make sure the inlet is drilled on the bottom right and the
outlet is on the top left.

14. Green Alien Says:
April 27th, 2007 at 11:35 am

Great idea,

For those suggesting to maximize the air travel through each
column, the actual method would be to make the air travel from side to
side as it moves up (as opposed to making it go up and down as
suggested by some people earlier).

Nice job

15. shackleton Says:
April 27th, 2007 at 11:59 am

A shiny back would also help, giving the cans all around

16. imagol4 Says:
April 27th, 2007 at 8:10 pm

Alan’s reference to “a long snaking chain”, as you mention,
would give the air more time to absorb heat, thus creating a larger
temperature differential between inlet and outlet. When I first read
Alan’s comment I thought about it exactly the way you did, the air is
rising from convection, and it would take more force than the
convection alone to get it back down the next column….then I thought
about snaking it ’sideways’.

To wit:
Put your inlet on the side of the box through the 2×4, drill
holes in the sides of the cans and caulk them together, the last can
and it’s mate on top will be set up like you currently have them, the
air will rise there, and proceed in reverse back to the other side,
where it again rises and proceeds back across the width of the box,
and so forth. The top row of cans can be set up as you currently have
them, and the heated air will then be exhausted into your outlet hose.

This method would take much longer to get the hot air out the
top, the convection movement would be slowed by the sideways movement,
but I bet it would generate a significantly greater temperature at the

Oh, and I’d have to use pop cans, I guess I’m a beer snob too.
Only bottles, and only “the good stuff”…

17. Sven Says:
April 28th, 2007 at 1:26 am

Hey, awesome project man, that’s something that would be a smash
hit in cold Norway, too!

What I’m wondering is the metal used, and how capable it is for
this job. In my work with normal aluminum I have found that bars of
the stuff, after lying in the sun all day will feel really cold. I
know that you probably counteract most of this effect by the black
paint, but do you guys think that another metal would improve the

Also, just a wild idea, how about making a deeper box in which
you put mirrors in angles such that you could have a “double machine”.
The mirrors would of course be in the sides. Not really sure how to do
this, just something that struck my mind.

And finally: Would it not be better if you used a black “uneven”
plate for your receiver? In this way you would, according to black
body radiation at least, have more area – which is proportional to
what you get out.

But hey, I’m just a noob anyways..;)

18. jayce Says:
April 28th, 2007 at 10:43 am

try using two pieces of plexiglass in a convex set up and
magnify the sun on the box to increase heat draw from the sun

20. father-52 Says:
April 28th, 2007 at 2:30 pm

Look at this website: or this:
The last is better. I already built that in Hungary.
The “Cansolair” has the primary idea in Canada.

21. father-52 Says:
April 29th, 2007 at 2:13 am

Working experience indicates that comfortable room temperature
can be maintained in a 1000 square foot dwelling with 15 minutes of
sunlight per hour. (Cansolair)

22. nikos from greece Says:
April 29th, 2007 at 5:44 am

hi my englesh is no good sorry for that but change the
plexi glass with real glass .

23. Hip2B2 Says:
April 29th, 2007 at 3:58 pm

Has anyone found a reasonable translation (or site to translate)
the Hungarian site
(Info looks really interesting, I’m just missing the subtle details

24. Daniel Strohl Says:
April 30th, 2007 at 7:38 am

Well, now, that makes sense. Hadn’t thought of it that way…

I used aluminum soda cans simply because they’re a simple item I
had laying around. I’m no scientist, so I can’t say whether they’re
ideal for the purpose (thermal transfer and all), but it’s a lot
better use than sending them to a landfill somewhere.

25. Craig Fleming Says:
April 30th, 2007 at 10:18 am

The long connected snake is ideal, just lie the rad on it’s
side, with the intake at the bottom and exhaust at the top. The heated
air will rise through the rad, no additional force (fan) required.

26. Will Says:
April 30th, 2007 at 10:42 am

We built this sort of heater in high school; they’re
surprisingly effective. Using a cardboard box, foam for insulation,
the same blacked soda cans, and plastic wrap we were able to get the
temps to around 185 F on a moderately sunny day. That was a closed
system and after about 90 min.

I’d check the temperature of the box v. the ambient and see how
much the enclosure is radiating — it may be worth adding a layer of

27. Jed Says:
April 30th, 2007 at 12:20 pm

If you are considering adding a fan, some sort of simple fan
controller should also be added to make sure the exhaust temperature
from the solar heater is actually warmer than garage ambient. You
could probably whip it up with a PIC and a couple thermistors.

28. Paul Says:
April 30th, 2007 at 12:26 pm

@shackleton – I was thinking the same thing – perhaps using
aluminum foil on the back to reflect heat to the backside of the cans.

29. father-52 Says:
April 30th, 2007 at 12:30 pm

Hi, boys! The too high temperature of air flow is unnecessary.
As well as the long snake is unnecessary too. The parallel tubes are
the best with collector boxes downstairs and upstairs. That’s working.
If the temperature of tubes is very high the thermic radiation towards
the environment will be very high too. Output temperature 50-60
Celsiuses are sufficient. (I don’t know how many Fahrenheits.) In this
case the change temperature is 30-40 Celsiuses. You must use a fan to
push the air 90 cubic feet per minutes. This warm air includes
sufficient energy to heat for a 1000 cubic feet living space.

30. father-52 Says:
April 30th, 2007 at 12:44 pm

CORRECTION: 1000 square foot

32. Bill Says:
April 30th, 2007 at 1:28 pm

Great post, Dan. I made a passive solar panel similar to this
for a science project in school. I used soldiered copper pipe in a
snaking coil and poured water through it – top to bottom. I don’t
remember exactly what the temp difference was, but it was huge – more
like what your dad reported. Maybe it was the copper, maybe it was the
coil design, maybe the water provided a better heat-sink.

Some thoughts for version 1.5:
Paint the south side of your garage a very dark color, and
instead of insulating with fiberglass, try stacking black cans between
the studs (there may be a vastly better material for sinking the heat
– just off the top of my head). Of course, that’s a lot of cans 😉

Find a small photovoltiac cell to drive a blower fan if you wish
to use one – it’ll keep the unit self-sustainable.

I think Nikos is correct – try the next one with real glass
instead of the plexi – it will provide better insulation. Also – the
angle of your frame will make a significant difference.

33. father-52 Says:
April 30th, 2007 at 1:38 pm

Craig Fleming,
The gravitational air flow is very slow that is why it doesn’t
includes sufficiently energy to heating. The air swap in the room will
be very slow too.

34. Ash Says:
April 30th, 2007 at 2:05 pm

use an old domestic radiator, and just paint it black, put in
box of rockwool with glass front

35. Mike H Says:
April 30th, 2007 at 7:37 pm

This concept is great. I am going to make one for my wood shop,
cant have open flames and this is the answer to heating the shop. My
question is how big does my panel need to be for a 14′ X 25′ shop? Any
help would be great.

36. Chris McGuire Says:
April 30th, 2007 at 8:20 pm

I like the left-to-right then up and right-to-left then up
pattern. I am wondering if you nest in a row of full cans (not
blocking the air channel). This would heat up during sunlight and
continue to deliver a warm air a little longer in the day.

FFFFFFFFFFFFO O=Open cans for air
Figure 1

Another idea is to build arrays of these and put them at
slightly different anlge. I think I’ll build this with a bunch of
vegetable cans that I tend to collect for storing stuff. Great idea
for week end projoect. I intend to do this with my kids.

37. linkpub Says:
April 30th, 2007 at 8:48 pm

Nice design. You took into consideration that you didn’t have a
lot of window space and built the unit externally. It’s also
impressive the ability to move the unit anytime to face the sun.

My BeerCanSolarHeater (
) assumes (as was my situation) that I’ve got too much inefficient
window, and that my windows face the Southwest and Southeast (or just

Your design is VERY rugged for outdoor use and durability. I
chose a lightweight design (4 lbs 3 ounces) that could hang from
heavyduty curtain rods indoors.

Our form factors are very similar, in that you use 5 cans x 10
cans, and mine uses 7 cans x 8 cans.

I also took a different approach to putting holes in the cans
and avoided drilling. It was quick and clean for me.

Your project and mine exemplify how we can take similar ideas
and principles and apply them to our unique situations (our garages or
sun-rooms). Newbies can take the ideas from our projects, and
customize their own soluations.

I really like what you’ve done. Once again, great job!

38. father-52 Says:
May 1st, 2007 at 2:19 am

You shouldn’t laugh. By this one you can save about 25-30% cost
of heating energy. The fan uses 20-30W but a 27 square feet collector
can push about 2 kW of heating energy. It’s an empirical data. (When
the sun is out. In late autumn or in winter too.)

regards: father-52

39. charon Says:
May 1st, 2007 at 7:35 am

Great idea.

I’m wondering if the air has been heated up at let say 5th row
of cans and there is no more temperature gain for stacking up more
cans. May be a horizontal array is better.

41. VJ Says:
May 1st, 2007 at 9:46 am


I don’t think Hip2B2 was laughing at the idea from that site, he
was laughing at the fact that he’s missing all the details of that
site because it’s in Hungarian and thus he can’t read it.

I too had a laugh when I looked at that site and tried to make
sense of it. Unfortunately due to my lack of skills in the Hungarian
language department, it was a lost cause.

42. JC Says:
May 1st, 2007 at 11:04 am

I thought this was one of the coolest things I’ve seen. I
started my google search to see how many people shared my interest in
such a thing and found this site too! Looks like they stepped it up
and spent way more money on it which is cool but I like the idea of
using stuff you had around the shop! KUDOS!

43. Nyle Says:
May 1st, 2007 at 12:42 pm

As you said make the box bigger and skip the cans and just paint
the entire inside of the box black. Larger surface area black and wood
absorbs more heat than aluminum anyway. ;^) I bet you get a bigger
gain than 15 degrees.

44. father-52 Says:
May 1st, 2007 at 12:53 pm

OK! I ask to writer to translate this though it’s long time a
bit. For that matter I think the pictures are telling all because it’s
a very simple thing. Should you read the Cansolair’s website then look
at the pictures on the Hungarian website then you can make a neat
panel similar to this. Well I’m going to have a supper. In Hungary it
is about 8 o’clock p. m.

regards: father-52

45. Super Dave Says:
May 1st, 2007 at 1:23 pm

This is a great idea. very good use of leftover materials. I
would recommend adjusting your angle per the season.
has a great explanation.

Generally (quoted):

– To calculate the best angle of tilt in the winter, take your
latitude, multiply by 0.9, and add 29 degrees
– The optimum angle of tilt for the spring and autumn is the
latitude minus 2.5°
– The optimum angle for summer is 52.5° less than the winter

I have a couple more suggestions. Create “swirl” in the air
stream. In your setup, the air flows (generally) evenly through the
cans (no swirling). This causes the column of air closest to the cans
to warm up and the inside column of air to not warm up as well. Like


If you make the air swirl (add turbulence), it should warm the
air more evenly. This is because the air will “take turns” touching
the sides of the cans. More air will touch the cans; therefore more
air will warm up. Adding fins inside the cans would work. More work,
but much better efficiency.

I’d also use two panes of plexi-glass (like a double paned
window). Leave an air gap between the two panes and make sure both are
air tight. That will help with heat leaking through the plexi-glass. I
would also use housing insulation around the entire box (not the glass
of course). Again, helps with heat losses.

I would also suggest copper instead of aluminum. As an
inexpensive solution, soda cans are great! Don’t get me wrong.
Aluminum has excellent heat transfer properties. Copper is better. So
if you want to spend a few extra bucks, you can buy copper tubing to

Right now, the front half of the can is facing the sun taking in
heat. The backside is not receiving any heat from the sun. If you can
find a way to get the sun to shine on the back as well as the front
(mirrors?), you will essentially double your heat intake.

Someone mentioned using water. Water has better heat transfer
properties than air (just like copper is better than aluminum). Water
will absorb heat better than air (water is more dense). Although
converting that heated water back to air (using another radiator for
example), increases the complexity and might negate any advantages of
using water in the first place. It’s a great way to heat water though!

Keep it going! This is a great project to work on.

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