Window Mounted Solar Hot Air Furnace (Aluminum Soffit Based)

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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
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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.

Idontknow519 months ago

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
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.
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.
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
lbjmorris1 year 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
rlfulton1 year 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.
djohnson631 year 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.
EcoMotive (author)  n4nln2 years ago
Hello, I think you're on to something with the tilt of the absorber plate. When I built this, I simply followed a design that was featured on and did not put much thought into the benefit of tilting it the other way. The only thing is that air that is warmed by passing through the soffit would end up on the glazing side of the absorber plate rather than behind it. Maybe this would increase heat loss through the glazing to the point that the improved tilt angle is no longer a benefit. I don't know, just a thought. Thanks for your input.
Hi -- nice job!
The idea of tilting the absorber such that its closer to the glazing at the top of the collector is that at the bottom, all the flow is on the glazing side of the absorber, and then as the air rises and passes through the absorber more and more of the flow is on the back side of the screen -- so tilting the absorber just makes for a larger flow passage where the flow is large.

Another alternative for the absorber is to use two layers of metal insect screen separated by a half inch or so. In our tests, the vented soffit and the 2 screen collectors were tied for performance.


EcoMotive (author)  GaryGary2 years ago
Hi Gary, I must say it's quite an honor to have your attention on this project. This window box collector is not much more than a hybrid of the solar "heat grabber" featured your site and the aluminum soffit collector you built for testing alongside the other designs.
I was planning on building the "heat grabber" with the corrugated metal roofing absorber exactly as it showed in the plans until I seen the results of your aluminum soffit absorber testing and decided to put the two together. I'm glad I did because I think it worked out really well.
I learned everything I know on your site. Thank you for providing all of the free information and thank you for your comment.
Nice workmanship, nice project. Right out of the "Mother Earth News" somewhere I have the issue where thy make these.

however, you need the "sofit" material to be solid as in no holesYou could just get a lenght of aluminum trim metal or from a metal place the right size. By the sheet being non perforated, you will get a better air flow no fan needed, especially if you bring the input side down to floor level (much much more complicated) you do need to physicaly block them at night/at end of sunlight hour, or as posted by another they will go in reverse and cool your house. If you can find temperature activated louvres you need not be there to do it.

color simple fact darker colors absorb more energy, and black is better followed closely by red and blue.

I looked over their website and could not find the article (solar heat grabbers or heat grabbers) but did find this:

http ://
bfarm2 years ago
Look up Trombe wall...
wobbler2 years ago
Interesting design.

Regarding the "angle towards the sun" comments you can easily work out the gains to be made and decide if it's worth it. To work out the gain from angling the collector towards the sun at the winter equinox, get the angle of the sun to the horizon from your location. In the northern hemisphere, you can calculate the lowest angle the sun gets to from 66.5 – Latitude.. (If this goes negative, it means you're above the arctic circle. Good luck in getting any heat from this in the middle of winter! there)

Then use a scientific calculator to get the cosine of this angle. That then gives you the ratio of placing the collector vertical to pointing towards the sun.

e.g. I live at 53 degrees N, so the sun drops down to 66.5-53=13.5 degrees in winter. Cos(13.5)=0.97. This means the pointing the collector toward the sun would make about 3% difference. However, if the sun was higher, the difference would be greater. Taking the cos of the angle to the horizon, with the sun at 30 degrees to the horizon with the collector vertical, the difference is approximately 14%.

Whether these would make a significant difference in a real situation, I'm not sure. Bothering about the angle in higher latitudes may not be worth it and in aesthetics and simplicity, I'd probably find it easier to make the collector 14% larger (as this is an area increase, it's approx 7% larger in each direction).

There's a more comprehensive analysis of the effects of angle here:
earlyflyer2 years ago
Great idea and instructable. What is the reason for the slope on absorber plate?

Looking forward to your updates!
The angel of the absorberplate is to maximise aircontact if iam not mistaken.
Back in the late 70's we were building these 'dragon tongue' passive solar heat units.Instead of a ninety degree angle as you have,here in the USA it is more effective at thirty degrees.We also use a solid plate collector with the cool air having to run all the way to the bottom and back up across the black plate.The forced air is not needed as they are an efficient passive solar heat pump.OH and they are NOT self damping.At night the heat in the house can and will be pumped out if you do not block the intake hole.The one improvement I have made on mine is to extend the interior portion down to near the floor making a larger separation between inflow cool air and outflow hot.This can be done with a wood box or several PVC pipes with elbows on the floor end.Nothing to permanent cause these suckers got to come OUT in the summer time and stack quietly in the barn/garage ^_^
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