Picture of Window Mounted Solar Hot Air Furnace (Aluminum Soffit Based)
Good evening welcome to my entry for the "Off the Grid Contest". I present to you a product aimed at lowering your winter heating bill and carbon footprint by generating heat for free using the power of the sun!
Projects that involve warming air for space heating using the sun are plentiful. However, most of them involve permantly installed flat plate collectors made out of soda cans or aluminum downspout. Installing a permanant collector usually means drilling two large holes through the side of your house in order to route the ductwork. My collector mounts just outside a window and can be taken down when the heating season is over. The most invasive part of the installation is the removal of the window's flyscreen. Furthermore, the aluminum soffit based absorber plate is much more efficient than soda pop cans or aluminum downspout; you can harvest more heat for a given size of collector. The aluminum soffit based collector is more expensive than a soda can collector but less expensive than an aluminum downspout collector.
As an added bonus, this collector does not require electricity or fans or forced air of any kind. The current of air through the collector is driven solely by natural convection. As the sun heats the air in the collector, it rises and escapes through the output vent. As a consequence, cold air is drawn into the collector through the input vent to replace the warmed air. The whole loop continues without the need for fans.
If you want to make one of these I suggest you use some better quality (more expensive) materials than I did but for this particular project, the cost was about $60
1-40 of 84Next »

Great idea. My question is this; "So what do you do, with the unit, when the sun isn't shining or at night? Wouldn't cold air come through?"

Canoeist1 year ago
Would these unit be more efficient, if the roof venting is on a angle facing up instead of down.
To absorb more of the suns rays more directly
EcoMotive (author)  Canoeist1 year ago
Hi, When a solar thermal collector is to be used solely for wintertime space heating there is a large benefit to having the collector mounted vertically rather than at an angle. A vertical collector will produce a maximum amount of heat in the winter and a minimum amount of heat in the summer. Please see the attached image for an explanation. As for the aluminum soffit absorber itself, it's necessary to tilt the top outward to ensure the warmed air travels behind the absorber rather than against the glazing, where a large amount of heat will be lost.
Thank you for your question.
Space Heating.JPG

Hope you don't mind but this is a bit off topic. I've designed a passive solar house using thermal mass. I am using south facing sloped windows. What are your thoughts on vertical vs. sloped glass for solar gain. John Hait and others say sloped is better. Just wondering what you think.

EcoMotive (author)  mikewinfrey7 months ago

Hi. This is an interesting question since I too designed a direct gain passive solar house and am in the process of having it built. Sloped glass will certainly allow more solar gain but that's not always a good thing. In order for a house to be truly passive it should also prevent overheating in the summer. Sloped glass creates a risk of letting in too much light in the warmer months. Sloped glass would be of most benefit in climates that don't get much clear sunny weather in the winter where it would take better advantage of diffuse light coming in from an overcast sky. On the other hand, sloped glass will likely accumulate snow preventing light from entering the house at all. I find that even a slight tilt on my solar thermal collectors causes snow to build up on the glazing whereas a vertical collector will gather virtually none. A tilted wall may cause special structural and weather sealing concerns where a vertical wall wouldn't. I personally would recommend a vertical glazed wall and worry more about the proper amount of glazing, the glazing to thermal mass ratio and most importantly, super-insulation. Hope this helps. Thank you for your question.

Thanks for the reply. All the things you mention have already been accounted for in my design. I always other people who may have some experience besides the ones I based my design on. Again, thanks for your reply.

Very nicely thought through. It took a minute for the back tilt on the collector to soak in. I read all the comments I think. I didn't see anyone say anything about building the collector say 2' wider than the window then ducting down to fit window width. More collector sq ft = more btu and I don't think heat build up would be an issue even without a fan,but especially not if a fan was used.

RickS37 months ago

I have 2 pair of double doors exiting to my large deck, and several windows, all facing west with lots of direct sun and setting sun. This amount of glass perform quite well in using solar heat to warm the house. I see no need for building this type of panel. I do have a question about the effect of the computer fan used on the design. You did not include how that fan is powered, and how the power supply is incorporated into the unit.

but how will this work in the north east (north America) where common winter temps are well below 32 F (0 C). I was using a large kerosene heater in my work shop (which is insulated) and it had a hard time keeping up. I had to upgrade to a propane hat to be comfortable without the stink of kerosene

ntense991 year ago
Nice build man. Here are my units (I built several of them ). These press into the interior part of the window frame and are thermostatically controlled with 12v fans

and I wrapped the units with fabric, included decorative directable vents, and a 1-way flow control air dampener.
These keep my entire first floor temperature at 70 degrees (f) when the outdoor temp is in the low 15s (f). Although the windows are facing EAST, the units DO get sun from early morning 8:30 until 13:30. Since my house is extremely insulated, the heat is retained for several hours more and THEN the real household (Hydronic) heating system engages.

High temp paint, a little bit extreme I would say.

EcoMotive (author)  ntense991 year ago
They look great except they're not going to do you much good unless they're on the outside of the window. The collectors you built are going to be blocking the direct solar gain that would come in from the window. You see, a window provides just as much heat as a solar collector of the same size would as long as it's letting direct sunlight into the house. You're going to get this solar gain weather or not there's a collector in the window. If for some reason you want the view out the window to be blocked and you still want the solar gain then these units you have built will come in handy.
A solar collector mounted to the outside of the house allows you to have the extra solar gain without the incredible amount of heat loss in the nighttime. In some cases the area that the collector occupies on the side of your house will have extra insulation. That extra insulation will save you money day or night, rain or shine, heating or cooling. Thank you for sharing this.
Hey Lance, thanks for the reply. You are correct, in typical circumstances, in theory the solar gain would be the same. However in my case, I made optimizations that will capitalize on the solar radiation - for example, the metal collectors that are engineered to coincide with that time of year's sun angle so that the plates are intercepted by solar radiation at a 90 degree angle; the collectors are black; and all of the solar radiation gets focused on one area. This controls the heat to generate in this entire area and gets circulated in a fan-assisted convection loop. That's right, you also mentioned the other benefit from this: the insulation. In the past, I would go to work and leave the window shades completely open and the room temp would increase - but now the room temp increases by a lot more! The other thing is: These units essentially seal off when no heat is being generated and immediately stop any thermal loss through the windows; as soon as the unit's internal temp threshold is achieved (through a simple attic fan thermostat set at a specific temp) that heat is blown into the room. That's right, one of the variables that affect the worthiness of any solar heater is the surface area used to collect heat. It was nice reading your instructable man, keep up the good work!
EcoMotive (author)  ntense991 year ago
I'm glad to hear of your success on your build, You certainly thought everything through. All the best with your project this heating season.
Thanks dude! Yeah, Over the years, I have built quite a few various solar heat collectors.
CHECK THIS OUT! Here is an idea I have been toying with. It is a little on the CRAZY side :) You know the fresnel lens used to melt steel and concrete?

Imagine... building a variation of the solar air heaters that you and I have, BUT, it would look a little like one of those solar ovens. It would consist of an enclosure whose sides are made from probably plywood AND highly insulated AND even fire-resistant! The cover of this enclosure would be the FRESNEL LENS! The interior of the enclosure would have a large and THICK steel plate that is positioned and highly secured and suspended so that it is struck by the FOCAL POINT of the fresnel lens. The STEEL's mass would be greater than the lens' ability to melt it (because the heat would dissipate from the steel) AND the enclosure would have an air INLET and OUTLET, with forced air circulating to the interior of your living space. Now THAT'S SOMETHING I'd LOVE TO BUILD! Imagine getting like 400 degree oven temps inside the unit. Also imagine if the high volume FAN failing and the STEEL PLATE melting!
EcoMotive (author)  ntense991 year ago
It shouldn't create any more heat than a collector with conventional glazing. The amount of heat energy that a solar collector will produce is limited by the insolation and the square footage of glazing. All the Fresnel lens will do is concentrate the energy on a small surface. The net amount of energy entering the collector will be the same. The only way to make a collector produce more heat is to make it bigger.
ntense998 months ago

Nice man - so you too can remove yours when the warm weather comes. I made solar air heaters using a slightly different approach. I have a blog post about it in detail here if you are interested in it. Works really great!

I like the design. I'm building a workshop/garage and will not have any electric available except a temporary generator so electric heat is out of the question. I will try to make louvers similar to the house crawl space vents and that way I won't have to mount them at an angle but can angle the louvers at the optimum angle towards the winter sun. Possibly with a way to adjust each month for the changing angle from October to February. The louvers should be able to catch the warmed rising air and force it towards the back of the collector, away from the glass and up and out the exhaust duct. Possibly a slight slope on the top of the collector so melted snow or rain cannot run inside the window.

This has got to be the best design I've seen yet ! And it beat's the ....heck out of sawing pop cans and gluing them together . As usual I'll be building one of these ,just in time for summer! (and ,yes I'm the guy that moves to Fla. in the spring and comes back in the FALL!!) But maybe it'll still be useful in the fall ? Thanks for a very good design !! It will be used to heat my future home ---a shed , I mean a cabin !! Lol !

doo da do1 year ago
Just a thought, you could use a solar fan less electricity. You could have a thermostat for air on and off. Doodado
lbjmorris2 years ago
I have been planning on trying something like this but all I have seen cut holes in walls. This is a really great idea with no commitment like permanent holes. Thankyou
rlfulton2 years ago
Ok folks, after reading all this, why can't I create a tunnel between my two south facing 1st floor windows and one second floor (attic room) as the exhaust with sheathing (canvas or tent material) hung on the wall upstairs to flap closed and block reverse flow heat siphoning at night? I have about 15 feet between windows and the tunnel would set into the window wells. Opening the three windows would activate the system. It would appear that this inverted V design (decause of the position of my windows would be efficient, extend the collector space based on the distance between windows in use, and create "whole garage" heating since heated air would be drawn into the first floor through my stair well on the north end of the building.
djohnson632 years ago
How did you come up with this? Did you have any prior experience with something like this? Its fantastic work, its been running smoothly in my attic room for a couple days now. In fact I might use it for all of my windows in my Chicago shop. Thanks!
It looks like you did a very good job designing and constructing this project. Please give us an update on how this design performs. I have a very similar one worked out in my head so I'm curious to hear if it was worth the effort!
Wepwopper2 years ago
How about this design? 
The second diagram shows the cold air is forced  through a piece of solid soffit, then through the main panel of solid soffit to then lastly, pass between the glass and the solid soffit. 
Green = solid soffit 
Grey = glass 
Black = walls and partitions
Solar Hot Air Collector.jpg
valveman2 years ago
Before you paint that chip board, apply a thin coat of drywall plaster on all areas you want to paint. Lit it dry and lightly sand. Then paint. It will look great I assure you.
Great idea simple and buildable by anybody
i wonder if it will work for solar water heater
with suitable modifications of course!
can anybody help
Pojeros2 years ago
I am a member of the Simply Solar Yahoo group, which discusses this type of D.I.Y. solar heating. I would try fixing black screen door screen over the soffit pieces shown here to see if the screen, with its much smaller holes, transfers more heat into the air flow.
Th SS group engages in a lot of discussion of various aspects of D.I.Y. solar. Check it out at:

Orngrimm2 years ago
Great idea with the soffit and letting the air move thru.

Simple and smart. Thats like an idea has to be! :)
chuckyd2 years ago
That appears to work well, even if only for the southern exposure.

A few suggestions. If the box is well insulated, what is the point of painting it black?

Would it be less obtrusive if it were painted to match the house?

Could it also be detailed to look like a part of the house, instead of a window air conditioner?

Could the design be altered to fit in the wall below the window?

Would it make more sense to reveres the slope of the vented collector plate so that it is more nearly at a 90 degree angle with the sun?

Would triple wall or greater glazing provide better efficiency?
StuNutt chuckyd2 years ago
The reason for the collector plate being fitted that way is so that the hot air is kept away from the glazing. Even with twin-wall polycarbonate glazing, some heat can escape back out through the "glass".

If you wanted to keep the collector more perpendicular to the sun's rays, the answer would be to angle the whole assembly away from the wall at the bottom, but remember that, when we need heating most is when the sun is low in the winter.
Sabata StuNutt2 years ago
I'm just thinking out loud here...

Even though the sun is lower in winter, the optimal angle from vertical for a solar panel in St. John's, NF, is 20° in December, 27° in January and 35° in February. So if the collector is 10° past vertical, the sun will be hitting it at a 45° angle in February. It would be neat to do a test of three different versions--one as is, one with the collector vertical and a third with the collector angled backward--to see which one works best in real-world usage.

Props to the OP for a job well done, even if it is only a "beta" version. I'll be looking forward to seeing your updates during the winter.
EcoMotive (author)  Sabata2 years ago
Hello, you are correct about the optimal tilt angles for St.John's, NL. However, the vertical face of the collector makes it much easier to construct and I've read that a vertical collector is perfectly acceptable for northern climates bacause of the low tracking sun and because it can absorb light reflected from the snow on the ground. Thanks for sharing your thoughts.
EcoMotive (author)  chuckyd2 years ago
Hi, you are correct about the choice of paint color. My next collector will most likely be white or yellow. There's no performance benefit from painting it black.
I dont want to fit it in the wall below the window because the whole point of this perticular design is to do no permanant change to the house. This will make this instructable useful to anyone who rents their home. The rest of your inquiry is taken care of by "StuNutt" below. Thanks for your questions.
There is a strong performance benefit to painting it a dark color.

The polycarbonate is transparent to the solar spectrum (enabling the collector to work). A white absorber will reflect about 90% of the incoming solar energy. The insulation does not factor into this loss. For a shallow cavity, such as your collector, nearly all this reflected energy will leave via the polycarbonate. A black absorber will only reflect about 10% of the incoming solar energy. Thus, a black absorber is about 9 times as effective as a white absorber. This is why all commercial collectors (insulated or uninsulated) But I encourage you to experiment. Just post the data =) !
EcoMotive (author)  thejaq2 years ago
I think you misunderstood. We were talking about the outside skin of the collector, not the actual absorber plate. There is an obvious benefit to painting the inside of the collector and the absorber plate black but anything on the outside of the collector wouldn't matter, would it? Thanks for your comment.
thejaq thejaq2 years ago
This is why all commercial collectors (insulated or uninsulated) .... are dark colored :)
n4nln2 years ago
Great project!

I am, however, a bit unclear as to why you oriented the collector panel tilted out at the top rather than tilted out at the bottom. The systems I'm familiar with absorb the most energy when the surface is normal to the incident rays. Maybe the difference isn't material in this case and it results in easier fabrication?

When considering the alternatives for improving V2, be sure to include  using closed-cell insulation with a reflective foil surface. It performs significantly better than a bare foam surface."blue foam board" is a possible candidate although my familiarity is in refrigeration applications and don't know the high-temp behavior without further research. Marine supply outlets are a good source to pursue for foams and such.

I do like the idea of constructing the entire structure of rigid foam with the overall structural stiffness provided by an external plywood skin uniformly bonded to the foam. In boats this is done using epoxy resin bonding a marine plywood skin to a foam core. This results in a strong and stiff structure which is also very light.

Again, this is a great project and wish you success in the competition.
1-40 of 84Next »