Window Mounted Solar Hot Air Furnace (Aluminum Soffit Based)

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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

Step 1: How the Vented Aluminum Soffit Collector Works

I've drawn a diagram for you that I hope is self explanitory but nevertheless I will go through it starting from the cold air intake.

1) Colder air from the room is drawn in from the intake vent at the bottom of the unit. From there it travels inside the cold air plenum until it reaches the chamber at the front of the unit that is exposed to the sun's rays.
2) Inside the chamber behind the polycarbonate glazing the sun's rays shine on the black aluminum soffit and heat it up. When the cool air encounters the soffit, it is warmed as it rises through the perforations. This rising air is continually replaced by cooler air being drawn into the cool air intake.
3)The warmed air travels through the hot air plenum until it is released out into the room through the hot ait outlet.

The collector unit is ment to hang on a windowsill with the intake and exhaust vents inside the house with the rest of it on the outside of the house. The window must be the single hung type; where the pane of glass slides vertically to open and close the window. The collector is built to be the same width as the window opening. When the collector is hung on the windowsill, the window can be closed down onto the collector to sort of "clamp" it into place. After some weatherstripping is added to seal up the small gaps, installation is complete.

Step 2: The Outer Shell

The exact dimensions of this box are not important, as you will have to customize this design to fit your own window. If you are fortunate enough to be able to use a south facing window near the ground then all I can say is the bigger, the better. The reason for that is you can use the ground to support your collector and you dont have to worry about weight. You should also use thicker materials, such as 1/2 inch sheeting instead of the 1/4 inch sheeting I used. But beware, collectors mounted close to the ground are at increased risk of shading. You should do the solar site survey (link here) before building the collector.

My collector is hung from a second floor window. There is no support from below. Therefore I had to try to reduce it's weight as much as I could. I opted for a shell of 1/4 inch OSB with very little supporting framing. The only framing are small 3/4'' by 3/4'' strips of wood that were ripped down from 1'' by 6'' board. They are installed everywhere along the corners where two sheets of OSB meet.

The two "L" shaped sides of the collector are cut out of OSB in one piece. This is very important. If the sides of the collector were cut out of two pieces there would be nothing to keep it in shape and it would fold like a book as soon as you picked it up.

The whole thing was assembled using wood glue and 18 gauge 5/8'' brad nails. When the foamboard insulation was installed later in construction, I nailed everything over again with 1 1/2'' crown staples.

Step 3: The Foamboard Insulation

The insulation used was 1'' polystyrene. Some of you who are reading this who are familiar with solar collectors may be thinking, "WTF is he doing, he should have used polyisocyanurate!". I am aware that I should have used polyiso insulation because the polystyrene has a tendancy to melt at high temperatures. However, I scoured the city for days in search of polyiso and couldn't find it. I had to settle for polystyrene and who knows, maybe it will work fine.

The foamboard was simple to install with little more than a knife and a tube of foamboard adhesive. I feel inclined to tell you that you're supposed to pull the board off of the wood after you first glue it and let it air out for five minutes before you stick it back on. However, when I insulated my basement with this stuff years back I purposely didn't air out a few panels after I glued them and they're stuck on just as good as the rest.

After insulating I sealed up all the joints with tuck tape in order to prevent drafts.

The last picture shows what was left over from one 4' by 8' sheet of foamboard. I was cutting it a little close there!

Step 4: The Divider Between the Hot and Cold Air Plenums

The divider splits up the space in the unit into two seperate channels for airflow. It allows cooler air from the intake to travel along the back of the unit and down into the chamber where it will be warmed. It also allows warm air to travel up along the top of the unit and be exhausted into the room without mixing with cooler air.

The divider consists of 1/4'' OSB with a spacer on each side. The spacer was made by ripping down 1 by 6 board into 1 1/2'' by 3/4'' strips. The strips were glued with foamboard adhesive to the insulation  and then secured with 2'' 16 gauge brad nails shot in from the outside shell. They run along both sides of the collector and stop 1 1/2 inches from the bottom.

Next, two sheets of OSB were cut to fit across the spacers and then glued and nailed in place. At the bottom of the collector, an 1 1/2 space was left out to allow the cool air to enter the warming chamber.

Step 5: The Aluminum Soffit Absorber Plate

Now we get down to business. Now we get down to the real meat and potatoes of this whole thing. The absorber plate is a flat plate-like thing that absorbs the sun's rays, gets hot and passes the heat onto the working medium flowing through it. In our case the working medium is air but it can also be water. There are several types of absorbers that you can use including, solid soffit in a backpass collector, soda pop cans, aluminum downspout, vented soffit and aluminum window screen. I think that vented soffit is the best choice because it's one of the most efficient in terms of how much heat you get, it's easy to build and it's cheaper than window screen and downspout.

The vented aluminum soffit is pockmarked with hundreds of little holes called perforations. When the sun shines on the soffit it gets hot. Air is drawn through the perforations and is warmed up in the process. When you as an occupant of the home feels the free heat on your skin it warms you up and makes you happy in the process.

Anyways, the first step in installing the soffit is to make the spacers. They run the whole length of the OSB divider and are sloped from 1/2 inch at the bottom to 3 inches at the top. I just took some 1 by 6 board, cut it to length on the miter saw, ripped it down to 3 1/2 inches and then ripped that piece down on a diagonal. The two resulting pieces should be near identical with 1/2 inch thickness on one end and 3 inch thickness on the other. Next, they were glued in place with foamboard adhesive and secured with 2 inch 16 gauge brad nails shot in from the outside sheeting.

The first piece of soffit was cut to length and then a 3/4'' lip was bent into the tail at the bottom. The lip was made by clamping the soffit to my workbench and using the egde of the workbench as a guide as I went tapping along the piece with a hammer. When the first piece of soffit is installed, the lip covers up the air gap between the soffit and the plenum divider.

Two more pieces of soffit are cut and installed. They fit together they same way as you would if installing them on a house. Each piece has sort of a "tongue" one one edge and "groove" on the other. The groove of the first piece accepts the tongue of the second.

Three pieces were used in all. A 10 foot piece from the hardware store was enough to do it all.. About $17 worth.

After all of the soffit was in place, a foamboard cap was made to fit inside the remaining space and glued in place with foamboard adhesive. A piece of 1/4'' OSB was cut to fit over the styrofoam cap leaving an 1 1/4'' of the bottom of the cap exposed. Now the absorber plate has an 1 1/4'' border on all four sides. This is an important feature for the installation of the polycarbonate glazing and the batten trim.

FInally, the whole absorber plate was painted flat black with a couple cans of spray paint. I should have used the high heat barbecue paint but I couldn't find any.

Step 6: Installing the Polycarbonate Glazing

The first thing I did was make a frame on the face of the collector out of 3/8'' by 1/2'' strips. They are 3/8'' thick to match the thickness of the twin wall polycarbonate glazing and they are 1/2'' wide to cover up the OSB sides while leaving 3/4'' of foamboard exposed all the way around. The twin wall glazing is known to have good thermal insulation properties. The glazing rests only on foamboard all the way around in order to keep the presence of insulation seamless between the foamboard and the glazing.

The glazing was cut with my circular saw with a special "plastic panel" cutting blade. It was cut 1/4'' by 1/4'' smaller than the opening of the frame to allow for thermal expansion. While cutting, it is a good idea to leave the plastic film on both sides on, in order to prevent burrs from forming. Make sure you cut so that the cells are oriented vertically. There is one side only that can be facing the sun's rays. It is usually marked by the plastic film that covers it. After cutting, peel away one corner of the film on the UV protected side, mark the panel with a piece of masking tape and peel off the rest of the film.

After cutting, take the blow gun from you air compressor and blow out all of the dust in each cell of the sheeting. Then seal the two ends with dust tape (also called vent tape or ventilated tape).

The polycarbonate glazing can then be laid inside the frame and then secured with the batten trim. The batten trim is 1 1/4'' by 3/4'' strips ripped down from 1 by 6 board. It is cut to fit around the polycarbonate glazing in a frame with miterd corners. The batten trim is secured down with wood glue and brad nails.

FInally, a bead of exterior caulking is run around the inside of the batten trim between the trim and the glazing.

Step 7: The Initial Test

When I got home from work late the next sunny evening I decided to prop the collector up in the sun and see how well it would do. I was very pleased with the initial results.

First of all, it was late in the evening so the sun was hanging low in the sky very far to the west. The collector recieved very little direct sunlight because of a lot of low lying clouds hanging around the sun. Most of the sunlight that reached the collector was partially filtered through the thin cloud cover. In addition, it was hard to avoid the shadows of my neighbor's shed as well as a nearby tree and telephone pole. Needless to say, conditions were less than ideal.

Nevertheless, the thing worked. Even without a fan there was a decent amount of warm air rising up out of the exhaust opening of the collector. However, I read that forced air dramatically increases the output of these collectors so with the rapidly setting sun, I scrambled to tear apart an old junk computer to get out the (very large) cooling fan. I quickly made up a piece of OSB that was larger than the exhaust opening of the collector and drilled a hole in it the same size as the fan. With the sheet of OSB over the exhaust opening and the fan over that drawing air through the collector and out the top, I was amazed with the results.

For a short period of time thereafter, the collector was recieveing full direct sunlight and I must say, the heat was bailing out of that thing. It felt like just as much output (in terms of both temperature and flow rate) as a 240 volt forced air electric heater such as the ones that are installed in my house for space heating. In other words, it felt like I had my hand in front of one of those heaters while it was running.

Before I could get my thermometers out it clouded over again. The output temperature dropped my a large margin. With the thermometers in place I measured a steady 12oC at the intake and 20oC at the exhaust. All with a good airflow; about the same as what comes out of a clothes dryer vent. Sorry, but I don't have any way of measuring and quantifing air flow yet. As a result, I also cannot calculate the power of the unit in terms of the rate that it is collecting heat energy from the sun.

All that being said, I am confident that this collector will provide %100 of the heat required to keep a room warm during a sunny, clear winter day. Given that I cost approximately $60 in materials, it should pay for itself about three times each heating season depending on the weather.

As soon as I get this thing mounted in the window, I will see how it does with heating the room. I actually expect it to perform better in December than now in September because the sun still tracks fairly high in the sky and makes a rather high incidence angle with the vertical collector. In December, the low tracking sun will shine almost straight onto the collector, increasing the output.

Unfortunately, I will not be home while the sun is shining on the collector until a while after the dedline for this contest. I will update this insructable with my results the minute I get them.

Step 8: Painting

Before I had this thing hanging out my window for all of my neighborhood to see I wanted it to look a little more "finished" than an OSB box. So I took a black marker and colored in all of the parts that would be exposed. Just kidding, I used paint but the paint job is so horrible that you probably would think that I was two years old and that I did use a black marker. The scary thing is it actually looks half decent from a distance of 5 meters or so.
When I build the "Window Box Collector Mk.2" I will use nice good-one-side sanded plywood and paint it really nice. I'll even put a clear coat on it, I swear.

Step 9: Profiling the Bottom to Match the Window and Installing the Sealing Flange

Profiling the bottom for my window was nothing more than adding a strip of wood of the appropriate size under the corner of the unit. The strip transfers the weight of the unit to the windowsill, rather than stressing the vinyl trim around the sash of the window.

The sealing flange is the ring of wood that circles the unit in the second picture. A double ring of foam draft sealing tape will go around the outside face of the flange. This should create an airtight seal between the flange and the inside frame of the window.

Step 10: The Second Test

I'll be a little more brief about the results of the second test. I left the collector out with the fan running for about an hour and checked on it every five minutes or so.

This time it was much earlier in the afternoon than the previous test. The sun was higher in the sky and there was no shadows from the nearby trees and telephone pole to worry about. However the sun was just barely poking through the dense cloud cover for most of the test. The results were still good. The heat output held steady at 29oC with a 17oC input and a pretty strong current of air. I made a makeshift streamer out of a sales reciept and taped it to the fan to try and give a visual of just how much air is coming out of this thing. It really is a lot for a fan that size.

I'm very happy with the results of this second test too. What it tells me is that I can still get plenty of useable heat even on a clear, cloudy day. By that I mean a cloudy day without rain, fog, or snow.

At one point, the collector did get a blast of direct sunlight for a few seconds. The output temperature quickly rose to 35oC and then quickly fell again.

Step 11: Final Thoughts

This project was build mainly as a "Proof of Concept". I tried to save as much money and time as I could by buying cheap materials, using redily available materials that might not have been the best choice and taking little care in my workmanship. If this window box collector seems to be a worthwhile investment after a full heating season, I will build another one that is much prettier and more durable.

Upgrades in materials would include:
  • Using sanded "good one side" plywood for the outer shell
  • Using glue and screws instead of glue and brad nails
  • Using polyisocyanurate insulation instead of polystyrene
  • Using foil tape instead of "Tuck Tape" for draft sealing

Upgrades in quality of workmanship would include:

  • Taking time with my cuts instead of sawing like a madman
  • Taking the time to clamp pieces together and wating for the glue to dry between steps
  • Sanding and rounding the outside edges of the shell or maybe some decorative trim
  • A really nice paint job with a weatherproofing clear coat

Well I guess thats it for now, I want to update this instructable at least every month in an attempt to compile a complete heating season worth of observations and data. I'm going to try to get my hands on one of those little devices that measure airflow too, then I can calculate the power output and try to optimize the airflow to get the most heat.

If you liked this instructable I would really appreciate your vote for the "Off the Grid Contest". As always, thank you for reading, please stay tuned and please share your questions, comments and constructive criticism. And please do your part to make our earth a little greener. Cheers.
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    90 Discussions

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    robkabob147

    1 year ago

    How do you prevent cold air from leaking in at night?

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    Lsteller

    1 year ago

    Very new to this site and very impressed! Not just with the project, although I am, but also the incredible conversation in the comments. Respectful, thoughtful, informative.

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    JulieC153

    1 year ago

    How would I do this if all I have is horizontal windows?

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    SteveM171

    2 years ago

    I built something similar back in the 80s using plywood and storm windows. On a particularly sunny day in early spring, I came home to find the house full of smoke. Investigation revealed that the temps has gotten so hot in the plywood box, it was actually charring inside. A few more hours of sun and I probably would have lost the house. so BE CAREFUL! I installed a small solar fan to constantly circulate the air, and that seemed to fix the immediate problem.

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    prince-of-weasels

    2 years ago

    Way back in 1979 we were building 'dragon tongue' window units in Hannibal,

    Missouri; with tin roofing painted flat black and plexiglass tops.You could stand one in the middle of a parking lot in six inches of snow and gather a crowd wanting to warm hands LOL It was amazing how hot it got with no fan.To stop the heat pump from sucking the house out after dark, we built wooden caps that slid over the intake and out flow. Energy goes from high to low and after sundown the inside of the house was much warmer than outside so out it had to go LOL The best improvement we made was covering the outside of the plywood with foam insulation and a 'collar' around the inside portion that fit it to the window like an airconditioner.

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    littlejohn1

    2 years ago

    Nice project,I've been wanting to build a few of these for my south facing windows for awhile now. Do you have a picture of it installed?

    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?"

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    Canoeist

    5 years ago on Introduction

    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

    4 replies
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    EcoMotiveCanoeist

    Reply 5 years ago on Introduction

    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
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    mikewinfreyEcoMotive

    Reply 3 years ago on Introduction

    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.

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    EcoMotivemikewinfrey

    Reply 3 years ago on Introduction

    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.

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    mikewinfreyEcoMotive

    Reply 3 years ago on Introduction

    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.

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    RickS3

    3 years ago on Introduction

    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

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    ntense99

    5 years ago on Introduction

    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

    https://scontent-a-iad.xx.fbcdn.net/hphotos-prn1/923513_509063625807346_1267312870_n.jpg

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    ntense99ntense99

    Reply 5 years ago on Introduction

    and I wrapped the units with fabric, included decorative directable vents, and a 1-way flow control air dampener.
    https://scontent-a-iad.xx.fbcdn.net/hphotos-ash3/544360_509063615807347_1033801988_n.jpg

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    ntense99ntense99

    Reply 5 years ago on Introduction

    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.

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    EcoMotiventense99

    Reply 5 years ago on Introduction

    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.