Instructables
This is a solar air convection heater for my garage that is powered by the low angle winter sun. It's easy to build and it works very well. With enough building insulation, this can be a primary heat source with a secondary needed for cloudy days. The idea and design is from Mother Earth News December/January 2007

Ingredients:
2x8 lumber
2x6 lumber
2x4 lumber
2x2 lumber
glass, plexiglass, or some kind of clear material.
black aluminum window screen
caulking, paint, screws, lag screws, staple gun + other tools

I have some results on temperature differences from the first weeks of operation. Just like when you put up your first wind generator, the wind won't blow for days/weeks, I had overcast and mostly cloudy weather the first few days operating this thing. The heater puts out 87 to 104 degrees F at the vents on partly cloudy days to sunny days. The uninsulated garage hits about 17 to 30 degrees F above outside temps, very comfortable for me. It seems the colder it is outside, the bigger the temperature difference between the garage and outside. If you want more heat in an uninsulated area I would recommend possibly 1/3 more collector area than the ratio I give in the instructions. Seal up any drafts, it will do a lot to hold the heat. This thing works great.
 
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MDW.VisualArt made it!8 days ago

Have any of you heard about this new fan control innovation for Solar Heaters, called Okapi Systems?

We have another Kickstarter Project on the go: Okapi Bluetooth. Very hot stuff!

You gotta check it out!

https://www.kickstarter.com/projects/2107309041/okapi-bluetooth-the-solar-air-heater-control-syste

Okapi Bluetooth Kickstarter pic..jpg
La_beachrose7 months ago

Hey I've been wanting to do something like this. Since I was originally inspired by the soda-can designs (but I don't drink any sodas), I've saved a ton of clean cat food cans to use as a heat medium. It seems the most efficient way to use them is sideways however, vs the usual vertical arrangement used in the soda-can designs. Maybe arrange them along the face of the screen used hear, and paint that whole interior surface black. Feedback?

sbadri8 months ago
What about night time ? Doesn't it get drafty ?
robbtoberfest (author)  sbadri8 months ago
Its a sealed window.
batonas10 months ago
why don't people use reflectors with these kind of systems? made from plywood and aluminium foil it would remain cheep and would improve the performance quite a bit.
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That is a small 2x2; screws attach to it to support the glazing. It doesn't block flow, maybe a tiny bit.
nddell2 years ago
Hello All,

I have had success with a design similar to this. I used black spray-painted aluminum sheets, polystyrene (i think) insulation, scrap 2x4's, and a ton of caulk. It was by no means perfect and some of the glass is even cracked, yet just after peak sunlight today (about 60 degrees in January, weird) it was pushing around 130 degrees consistently. I was surprised. It is a small unit, between 6 and 7 ft tall and 1 and 1/2 ft wide. I was expecting less, honestly.

My question is, then, how would it perform if the box was taller or wider and if it was perfectly sealed. Would either of these modifications make a large difference? I would like to build a non-prototype model of this for actual use, but was wondering if anyone would be able to assist me by answering these couple of questions.

Thanks

ND Dell.
robbtoberfest (author)  nddell2 years ago
Yup, I felt exactly the same way. How can something solar and so simple put out so much heat? I'm amazed these aren't installed on most cold weather region buildings.

Yes, more box area gathers more energy, so bigger box = more heat energy collected. Perfectly sealed helps with heat loss of the building itself, like leaving the window cracked open a bit; this hurts most on windy days.

Good luck Nddell
I did have one more question.

What is the difference between using plexiglass (perhaps glazed, or double glazed) and using two panes of actual glass? Does one conduct heat better than the other?

Thanks

ND Dell
robbtoberfest (author)  nddell2 years ago
I would think a layer of plastic type glazing is better than a single layer of glass for insulation value. But if money weren't too big an issue, double pane glass might be even better because of the insulating value and the clarity to let in light. Double glazed plexiglass might not let as much light inside as regular glass.

Here's a great link for more home build solar ideas: http://www.builditsolar.com/
The only problem I have with finding newer glass is that a lot of it includes some kind of UV protection or reflective surface. I don't know if I am completely wrong on this issue, but how would using two separate panes of single pane glass with a small gap of space (insulated of course) affect the heat collection? For money reasons, that is how I arranged the last project that I did and it worked well.

What about plexiglass used for greenhouses? I feel like that would be a good choice as it is made for the purpose of creating an environment hot enough to start plants in late winter and spring.

If it is the best option (or most reasonable costwise) then I will probably go with plexiglass of some sort. I have also considered the idea of making a really giant collector and using insulated piping to transfer the heat from the collector to the house. This, I'm sure will create a whole new set of issues, though.
robbtoberfest (author)  nddell2 years ago
Good point, no Low E glass or anything like that. Personally I think you'll get plenty of heat out of a single pain of glass or plexiglass without too much loss.
Fogged glass might be an issue with the homemade double pain. Anything clear for greenhouses would probably work here.

Let me know if you run duct work or pipes off of the collector or make an instructable; I'm thinking the same for my house. I'd like to see your version; maybe it'll need small booster fans.

Yes, we were seriously considering the fan idea. we have been running with a crazy amount of ideas lately (like removable units that blow air into the floor, then let it rise slowly, hoping that it would help with heat storage). I'm sure what we end up doing will in no way resemble what we have been talking about now, but I will do my best to document it well enough to make an instructable page.

On that note, this might be quite a long time in coming, since I am trying to balance this with College and job (note what is capitalized and what is not).

And yes, fogged glass has been a problem with the small prototype unit I built with homemade double pane. On one hand, it works just fine (130 degrees on any day in direct sun) given that it is cracked and very very small. On the other hand I do wonder how much less efficient it is (that drives me insane!).

Also, does anybody know how to theoretically calculate the amount of heat produced by any given system using specific heat and physical properties?

Thanks,

N.D. Dell
tricker692 years ago
After reading about these heaters, here and on the web, I built a mini version ( 2' x 1' ) out of some scrap wood and window screen laying around the shop. Of course it was cloudy and snowy the last couple days, but I got to check it out today. Sitting in the window sill for an hour, with the house at 68 F the air coming out of the contraption is approaching 90 F. Time to go full size on one of these. Thanks for posting this idea here.
CamoBedding2 years ago
so cool, great job and thanks for sharing.
wow, such a great idea.
cararta2 years ago
What a great idea. Non technical.
Have seen one made with gutter drains but a lot more work.
There is a site that is just solar, they make all kinds, the hot air collectors, solar cells and water heaters.

They have experimented with plastic and metal screen...make a 1x2 frame and wrap one long piece front and back.
They said the results seem as good from the plastic screen as the metal. Some of them used 4 inch pvc to run an air vent from the top and one from the bottom. on the inside of the house they ran 4 inch pvc down to the floor to collect cold air.
..
seems if you want a fan there is a small one that you can connect to the hot side of the pipe (fits inside) with a thermo switch that cuts on when air gets to 90 degrees and off at 80.
Also had a vent flap on the hot side so that at night hot air didn't go back into the collector. The vent only opened when the fan runs...like a dryer vent...
As for the summer, they suggested covering collector with one of those reflective "survival" blankets and it would stop the collector from collecting heat. No wiring required, just plug the fan into a regular wall socket.

I have a video of them building the screen part of the collector at
http://solarplanet1.blogspot.com/
May need to scroll down. If you go look there shoud be a link to the simply solar site also.

Cararta
dropkick7 years ago
I have one important warning and one reason why you wouldn't want to do this on your house.

First the warning:
In the summer block off the sunlight to the collector (cover it) don't just close off the vents.

I bought a house with a homemade passive solar heater attached. It was mounted wrong (bad airflow) and didn't work well. I decides to remove it.
While the house was vacant it had been "turned off" by shutting off the air vents into the house.
The outside frame work was wood. It had gotten so hot that the interior had caught fire. The only thing that saved the house was the lack of oxygen. The fire smothered itself.

P.S. This was during the winter - ouside temperature doesn't mean the collector isn't going to get very warm.

Okay now why you wouldn't want this on your house.

A passive (no fan) solar panel should be mounted lower than the area you are heating. Otherwise it turns into what is called a "heat siphon" on cloudy days or when the sun goes down and will cool instead of heating.

The reason for this is simple: heat rises. If the panel is on the same level or higher with the area you are cooling once the temperature inside it gets lower than your inside temperature the airflow reverses and it draws the warm air from your house instead of heating it.

This can be defeated by mounting it lower than the area being heated or by having the vents shut off at night or in cloudy weather. (see earlier warning).
etlerd dropkick3 years ago
As to it causing inadvertent cooling because of the heat siphon effect, isnt that the reason for the plastic flaps--to act as valves preventing the siphon effect?

As to the risk of fire, I'm surprised. Wood doesnt combust, I think, until about 540 F. Maybe the paint caught fire, though?
robbtoberfest (author)  etlerd3 years ago
Yes the plastic flaps are for the siphon effect. Fire doesn't happen with passive solar. Combustion of paper is like 412 F; I belive what the above guy is talking about is creosote in the siding of an old house. My interior siding looks black and burned, but it has never been burned. Concentrating solar with mirrors and magnifiers is what becomes hot and causes fire in a good way.
Fahrenheit 451. A book and the temperature at which they will ignite.
Hi I'm puzzled by your experience with the passive solar collector. I have measured the hot day temperatures in mine with the air vents shut off, and never seen more than 185F. This is a long long way from what is needed for combustion, and not that much different that occurs in window cavities with insulated shades or even in hot attics under dark roofs. On my collector, I included a set of outside vents that could be opened to allow air to circulate through the collector to take the stagnation temperature down when the vents to the insider were closed -- I have since closed them up permanently because they do not appear to be necessary. The reverse flow cooling at night or on cloudy days is not an issue on this design because the collector has poly film back draft dampers that prevent reverse flow. They are quite effective. These simple little 2 cent dampers were invented many years ago, and should get the Nobel prize for simple things that work :) Gary
GaryGary, Sorry I took so long to get back to you (haven't visited here for a while and didn't get notification about peoples posts till today -?-). As far as the heat build up goes there are many factors that could be keeping your passive heater from reaching combustion temperatures. Where it's positioned, air leaks, shadows, the insulation in the sides of the heater, sun angle, etc. Also you may not have let it sit closed up on a sunny day long enough to build up the temperature. I hope it never reaches combustion temperature. Robberfest has made a very good suggestion with the deciduous trees. As far as it working as a heat siphon, unless you close off the vents at night there isn't any way that it wouldn't function as one - this is basic thermodynamic law. It may not be enough that you physically notice it but it is still happening.
Hi Dropkick,

The flapper valves that stop reverse flow are very very effective. The pressure differential that the reverse flow causes sucks the light plastic film against the hardware cloth that backs them up tightly enogh that you can see the pattern of the wire mesh through the film. They are actually more effective than the commercial motorized dampers that are used in some systems because they don't have any leakage around the edges. Have a look at the pictures in the Home Power article at this link, and I think you will be convinced:
http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

On the issue of collectors causing a fire: I run a solar website that gets 4000 visitors a day -- including many that own collectors. Yours is the first case of a non-concentrating collector causing a fire that I have ever heard of or read about.
Gary
Hi GaryGary, This reply is a long time in coming (over a year) but I got tired of defending my comments to people who obviously didn't know what they were talking about and quit reading the thread. However I've gotten 2 notices in a short while that people were posting to my comments and decided to look again. Flapper valves are one way valves. In order for the air to move through a passive solar panel you would need an inlet and an outlet . Therefore if you used flapper valves you would need one valve to let the air in (lower) and one valve to let it out (upper). Also the valvescouldn't be very strong or the air wouldn't be able to open them on a passive system. So when the temperature outside cooled below the indoor temperature the warm air in your house would go into the lower valve and travel upward while cooling. Some of the cooler air would be forced out the upper valve by the increase in pressure from the warm air's entrance while some of it would exit every time the lower valve opened (most likely the lower valve would either flap or stay half open). And you would have a thermal siphon. I just have one question: Where's your solar website that get's 4000 hits a day? Please provide a link. I'm sorry to say this but I don't believe it exists. I haven't seen from your posts where you have enough real knowledge to actually have such a popular solar site. I'll apologize here if you prove me wrong.
Hi dropkick,
My website is www.BuildItSolar.com -- it now averages about 7000 visits a day, and it most certainly exists. I put up a new page that shows a picture of the flapper vents -- the page name is dropkick, so (hopefully) this will convince the site is real and is mine -- you are a tough sell :)
http://www.builditsolar.com/Projects/SpaceHeating/DropKick.htm

The flapper valves work quite well. Its true that there is an air inlet from the room to the collector at near floor level, and an air outlet from the collector to the room at near ceiling level. The flapper valve is installed on the outlet vent only because if you stop air from entering the collector at the top (as it would do at night when the collector is cold), you stop flow through the collector and there is no need to put a flapper valve on the entry and exit vents. If you look at the collector at night from the inside, the upper flapper valve is pulled tightly against the hardware cloth so that you can see the pattern of the hardware cloth through the plastic -- it seals quite well. If you put your hand near the floor vent you can't feel any flow at all from the lower vent. This does not mean there is absolutely no flow from the lower vent, but it is very small -- one could certainly add a 2nd set of flapper valves on the lower . The flapper valves for my collector are made from about 2 mil plastic film. During collection periods, the warm air from the collector easily blows the flapper out of the way. There is a half inch mesh hardware cloth across the exit vent opening, and at night, when the collector wants to backflow, the the flapper valve is pushed against the hardware cloth and very effectively seals the opening and prevents airflow through the collector.
I'm not the inventor of these flapper valves -- the have been used on this type of solar collector since the 70's and probably before -- a number of books on solar collectors include a description of them.

The collector is now entering its 5th season, and it continues to work well, and has already paid its cost in fuel saving several times over. It has required zero maintenance, and fully heats the shop on sunny or part sunny days.

Gary
Gary, I apologize for doubting you about your website. It looks like a very well made and informative site. I also apologize for the way I framed the accusation, I got mad at Squishy's comments and even after more than a year I let that carry over into my comments to you. But with some disclaimers I still stand on what I said before. While your flapper valves stop some of the thermal siphoning, there still has to be some. --However I will allow that the flapper valves may make the amount negligible. Have you tried testing the temperature differences around the vents? Also as I have the proof of my own eyes that a passive solar system can get hot enough to cause a wood frame to catch on fire, nothing you might say is going to dissuade me from that truth. --But my experience was a homemade solar panel on a roof and it had much more insulation, so unlike yours, it probably built up more heat with no way bleed much of it off.
avid0g dropkick4 years ago
  Whooee!  This is the first post of yours where I noticed that you mentioned the overheating collector being on the roof.  I totally agree that a well insulated closed absorber facing a hot-or-reflective roof could overheat.  (Was that the case?)  Especially with home-made construction using exposed lumber, plywood adhesives, etc.  
  When I was designing my passive collector, I seriously considered avoiding polyisocyanurate insulation because of the high temperatures that could occur in a stagnant collector.  Instead, I specified a bi-metallic thermostat to open vents during overheating conditions.  
I'm just wondering also, Where is this proof you claim to have about fires. Please provide a link. I'm sorry to say that it probably doesn't exist either.
robbtoberfest (author)  dropkick7 years ago
I highly doubt this would cause a fire; you would need temps over 400 degress F. Also the siphon effect is controlled with a flexible valves. I'm sorry you had a bad experience with this system, but mine is working very well. Another consideration for summer, plant deciduous trees near it so the leaves shade it. The summer sun will not be at the same direct glaring low angle as in winter.
Mad props for posting this very good ,very green , instructable and answering so many queries ! So here is one more - if you were doing this same project on an insulated house wall do you think adding silver foil sheet (maybe even silver lined potato chip bags) to the back wall would boost air temp. ?
robbtoberfest (author)  svenbuss5 years ago
Thanks, I don't really know if there would be more loss or gain from that idea. Some of the energy would reflect out and some back onto the metal screen. I'm thinking there would be a net loss because some of the radiant heat from the screen would bounce out too.
These won't start a fire. Even the more efficient commercial units won't get anywhere near hot enough. These don't create a heat siphon unless you skip a few steps -- eg, not installing the flap valves that prevent that. I don't question that you had a fire -- only that a passive solar heater had anything to do with the fire. Unless your house is on Venus or Mercury.
Squishy, I won't argue your statement that it won't get hot enough to start a fire except to say two things: 1) the solar cooker and solar fire starter people are going to be really sad to hear that the products they've been pushing for the last 30 - 40 years don't work. 2) I've seen proof that solar heaters can actually get that hot (read my first post) Now as for your heat siphon comment: Flap valves won't work. They would have to be manually closing valves. -If the air can travel in and out when it is working and you don't close this path off it can travel in and out when it's not. The entire problem is the elevation of the panel compared to the area it is heating. Non passive systems can work around these problems, passive systems can't without human intervention (the opening and closing of air passages) I suggest you go to the library and get some books on passive solar heating. This is where I started learning what I know about them. p.s. I'm sorry if I got sarcastic, but I got a little p.o.ed when you got sarcastic and called my comments into doubt when you actually don't know enough about the subject.
You keep mentioning that you have proof. It would probably help to post that proof. A link, article, etc...You actually have to provide said proof to make proof, by definition. Otherwise, it's just your word.
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Putting in a heliostat mirror across the yard would help a log :) Basically doubling or more the solar intensity (depending on the size of the heliostat mirror!)
ktm200 dropkick6 years ago
solar heaters do not start fires. i dont believe that they could get hot enough to comment back
dropkick ktm2006 years ago
I'm sorry to all the detractors but I have proof (read my first post) that solar panels can get hot enough to start fires, as mine did.
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