DIY Solar Heating with the Heat Grabber

Build this DIY solar heating collector, the Heat Grabber is a "window box" solar collector you can fabricate in under an hour.
By the MOTHER EARTH NEWS editors
September/October 1977
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DIY solar heating using the Heat Grabber. Once built, the MOTHER EARTH NEWS heat grabber should give years of dependable service.
Photo by J. Weiland

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You can build this super-simple and super-effective DIY solar heating collector in just one hour!

Some of the climatologists are predicting that the coming winter could well be colder than the last one. But even if that forecast comes true, you'll be a lot warmer during the clear-but-below-zero sieges ahead than you were during the frigid weather of last January and February, if your house or apartment has one or more unshaded south facing windows and if you outfit those windows with the Heat Grabber. (See the Image Gallery for Heat Grabber plans or click here for larger plans you can order.)

Believe it or not, this simple and effective "window box" DIY solar heating collector can be fabricated in just under an hour by an experienced home craftsman (or in less than two hours by the more fumble-fingered among us) for the astonishingly low price of $32.18 (see materials breakdown on next page, prices are from 1977). And once constructed, this sturdy unit should give years of dependable service.

The secret of the Heat Grabber's quick assembly and low cost is a new rigid foam insulation board manufactured by Celotex. This board, trade-named "Thermax TF-610," is impregnated with glass fibers for strength, faced on both sides with heavy aluminum foil, and available in thicknesses ranging from 3/8 inches to 1-7/8 inches. Celotex actually markets the material as a replacement for the pressed fiber sheathing or "blackboard" now used by contractors in the construction of wood framed houses and does not recommend it for any other purpose. MOTHER EARTH NEWS researchers, however, have run heat and other tests on the insulation board and found it near-ideal for use in quick, easy and low-cost solar collectors such as the Heat Grabber.

Yes, the basic Thermax TF-610 sheet does have a slight disadvantage. Its aluminum foil surfaces can be punctured relatively easily by anyone intent on doing just that. There are, however, at least two remedies for this problem: [1] Substitute Thermax-610/.019 — which is the same foam, but faced on one side with a much heavier layer of aluminum — for the Thermax-610 specified here, or [2] use the Thermax-610 called for in our plans and protect the sides and bottom of the finished collector with a casing of scrap lumber. The second alternative will be less expensive than the first, but, really, neither course of action should be necessary unless you live in a high-vandalism area.

The ideal angle at which to position a south facing solar collector (in the Northern Hemisphere) or a north-facing collector (in the Southern Hemisphere) is your latitude plus 10 degrees. This adds up to 45 degrees for MOTHER EARTH NEWS North Carolina offices (which are situated 35 degrees north of the equator) and that's the angle shown on the following plans. Please take this into consideration when making the cuts called for in Steps 3 and 6 in the diagrams in the Image Gallery.

(Miami, for instance, is located about 25 degrees north, which means that collectors there should be angled up at 35 degrees to the horizon, which, in turn, means that the 67.5 degree cuts specified in the following plans should be 72.5 degrees for Miami. Likewise, the cuts should be 65.75 degrees for Washington, D.C; 61.5 degrees for Seattle; and 54.5 degrees for Anchorage. You can calculate the specific angle for your own location (subtract your latitude plus 10 from 180 and divide by two) or just average it out from the figures given here. (The angle is critical, but not that critical.)

Remember that all the dimensions given in the plans are for a collector specifically tailored to fit the windows in one particular house. If your windows are wider or not as wide, feel free to build your Heat Grabber(s) accordingly. And don't get unnecessarily hung up on trying to keep the upper and lower air chambers in the collector exactly as deep as shown here either. A half-inch or more variation is fine. As a matter of fact, it's awfully hard to keep this little BTU-grabber from working, as long as its passages are deep enough for air to circulate through them at all.

One final caution: Although the single-strength glass used to cover the prototype Heat Grabber is no more nor less safe than the single-strength glass currently in use in millions of storm doors and windows throughout the continent. It can break and possibly cut you or a child if, for any reason, either of you falls into it. Take whatever measures you deem necessary so that such an accident never happens.

How the Heat Grabber Works

The Heat Grabber is nothing but a weathertight box that's insulated on the bottom and sides and topped with glass. An insulated divider is positioned inside this box and brought out its top to form an open "lip" at the box's upper end. This lip is designed to hook over a windowsill so that the window itself can be pulled down snugly onto the glass which covers the top of the Heat Grabber, leaving the main body of the solar collector "leaning against" the south side of the house at a 45 degree-or-better angle. (See illustration in the Image Gallery — How it Works.)

The operation of the unit is just as simple. When the sun shines, its rays pass through the glass on top of the Heat Grabber, strike the upper surface of the divider (which is painted black), and warm the aluminum foil covering on that divider. As the foil heats up it, in turn, warms the air next to it. And that air, as might be expected, rises up the face of the divider and begins to pour out the opening at the Heat Grabber's top.

But, of course, that hot air can't move up the face of the divider unless it pulls cool air around the divider's foot to take its place. Which pulls even more cool air in through the lower opening at the collector's top (the only place that cool air can enter the otherwise airtight unit) and down under the central divider.

What we have, then, is a "convective loop" solar room heater that operates automatically on nothing but the sun's energy. Whenever the sun shines, this clever little unit (which, as near as we can tell, seems to be an old Steve Baer design modified by William A. Shurcliff and further refined by some of MOTHER EARTH NEWS' research staff) just sits there happily pumping thousands of BTU's of heat into the house. And when the sun quits shining? The air in the box cools and tries to sink to the collector's foot, which "shuts off" the whole convective loop. (The Heat Grabber, in other words, will spew heat into the room when the sun shines, but it won't pull heat from the room when the sun doesn't shine.)

Tools to Build the Heat Grabber

Thermax is so easy to work with that you won't need any saws, hammers or other "conventional" carpentry tools to build this solar collector. The Heat Grabber, in fact, was constructed with little more than a protractor, tape measure, paint brush and two little "we built 'em ourselves" knives. (See illustration in the Image Gallery — Tools.)

These knives are nothing but blocks of 1 inch by 2-1/2 inch hardwood cut to fit the hand comfortably. The pieces of wood were then slotted and rigged with 10-32 bolts and wing nuts to grip Stanley 1992-5 utility knife blades at either a 45 degree (for "V" cuts) or a 90 degree (square cuts) angle to the blocks' faces.

All cuts on the Thermax used in the collector were made straight and accurate by sliding one or the other of the two knives along a board or other straightedge that had been clamped to the rigid sheets of foam. For "V" cuts, the blade in the 45 degree knife was set to slice only to within about 1/32" of the aluminum facing on the "far" side of the sheet (not all the way through either the facing or the foam). Since the foam varies slightly in thickness, this setting (for the most part) kept the blade from cutting too deeply. Two such cuts (with the straightedge reset between them), of course, were necessary for the completion of each "V".

And if you don't want to make "V" cuts and fold up the box of your solar collector? Then just build your "heat grabber" from separate pieces of Thermax, all made with right angle cuts; peel back the aluminum skin from the butted face of each joint; and glue the sections — foam to foam — together.

Heat Grabber Materials

Quantity Material Our Unit Cost Cost of Materials Used
1 sheet 1 inch by 4 feet by 8 feet Celeotex Thermax TF-610 $10.75 $10.75
1/2 sheet + 3/4 inch by 4 feet by 8 feet Celotex Thermax TF-610     8.85     4.60
1 tube Liquid Nails panel adhesive     1.00     1.00
1/2 tube silicone caulking compound     3.50     1.75
16 No. 8 finishing nails (scrounged)     0.00     0.00
3 pieces single strength glass cut t fit (all inclusive order)   10.49
1/4 roll all-metal aluminum foil duct tape     4.00     1.00
1 quart Rustoleum flat black paint     2.59     2.59

Total cost of materials used in construction of window collector: $32.18

Collector's size: 12.6 square feet                                                                       

Cost per square foot: $2.56

Note: All materials were purchased at retail from local outlets in Hendersonville, NC (1977). Expect minor differences in the prices quoted above in your area, due to variances in freight charges, dealer policies, etc. Thermax TF-610, for instance, is manufactured in Tampa, Florida, and the further you live from Florida, the more your dealer will probably pay for his shipment of the panels. Celotex is opening up several new Thermaz manufacturing plants across the country, however, and this particular pricing difference should soon disappear.

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Post a comment below.


1/22/2015 8:08:34 AM
Wow, what a great idea. I've been reading about electric underfloor heating mats on I wonder if this would generate enough power to run some matting in small bathroom? Either way, this is a fantastic way to get into solar and start saving on your energy bills - without the initial expense. Brilliant, keep up the good work!

12/22/2014 5:14:53 PM
Does this not leave the window easy to open by intruders? Anyway, our windows here in Denmark are hinged at the top and open out. So not sliding onto this construction, but leaving an opening at the sides.

8/23/2014 10:25:29 PM
Hello! I'm in process of building a window mounted solar heat grabber as per MotherEarth news... I'm adding a middle heat collector plate on top of my 'foamboard' divider and I'm planning on putting 'baffles' in... These baffles would go on top of the divider and raise the metal heat collector (aluminum printing plate (0.008" thick)) about 3/4"-1" (another plan using this metal plate uses (3/4") but I was thinking of using a (1") space - not sure what the difference would cause)... My question is: my box is going to be about 27" wide and about 66" long - with a 4" space at the bottom the metal plate will be about 27" wide and about 62" long... I'm not sure how wide to make each baffle, how many baffles to use, how long across the face to make each baffle (which would say - how long an opening at the end of each baffle)... I am also thinking of putting baffles on top of the metal plate and making them the opposite of the baffles under the plate and then using your idea of putting weather stripping on top of these to seal better with the glass... Also, I've read where you can attach a Solar Fan to the hot-air outlet to increase the consistency of hot-air output flow... I'm wondering what 'fan' is the better/best to use for this purpose and where to find one, ie. Walmart, etc (Or make you own from a computer fan - how would this be done)...?? Any Suggestions/Ideas would be greatly appreciated...

6/11/2014 10:29:16 AM
I use solar heating in my home and it's done wonders in helping to reduce my annual energy bill. People think that these methods are large expendatures that you don't see a return back on, but I have benefited wonders. I also use throughout my home, in replacement of a conventional central heating system. I have found that the heat transfer is much more even and also has considerably lower running costs.

5/16/2014 12:51:03 PM
@Amber: If your office has a south facing wall you can still do this. There are a couple alternatives to think about. 1. Place your heater on a south facing wall and run the ducting through your foundation and up into the room. Keep in mind that cool air sinks. So have it so the cool air draws in to the heater from the floor level goes too the heater and as it rises... have the heat come back up through a different duct. 2. Route the ducting all the way around to the North facing window. This would require much more duct, and insulation because you'll have to insulate the entire length of the duct work very well to prevent heat loss. 3. attach your panel on the roof and make it face south...use insulated ducting to get the heat where you want it.

3/7/2014 7:41:30 AM
So what if your South-facing windows are shaded OR you don't need heat in those rooms. We have an office that we'd like to make a bedroom but there's no heat in that room (we didn't add the addition, don't ask me why they didn't put heat in there!). We've been using space heaters but this seems like a MUCH better alternative. It's a North-facing window though. How can we make that work?

1/28/2014 3:33:24 PM
I made one of these for my home in Johnson City Tn. in 1980. It was to heat my two car garage in the winter. My wife and the neighbors decided to have a garage sale in the winter (Temp 20 degrees) she called me at work and wanted to know how to turn it off. She said she had both doors open and they were burning up. The inside "Draft" was 4 inches off the floor, this was the air on the bottom of the outside unit which was made of a 2X6 48" Long X 32" Wide frame it was split in the middle with plywood with a 3 inch slot at the bottom end. Everything on the outside was painted black and covered with a piece of Plexiglas. The 32 inch window closed down on the heater frame. Simple and inexpensive.

11/14/2013 10:03:30 PM
Im studying plans for How to Build a heat grabber, I'm left with questions. 1. What's to keep outside-cold from cooling air inside the Grabber when the sun isn't shinning? 2. Is it not possible to make a fat but short Grabber that extends beyond the window width? The bottom of my two south-facing windows is 27" above the brick floor of the covered front porch. (I know it gets hot there) The opening for house-air of course would have to fit the window opening. In fact I would think my Grabber (as described in ME) could be 8' long and only 3' tall. It would collect more heat. The size of the window opening may have some restrictive effect on the volume of air flow through the Grabber, but I believe the added heater-size would cause the air to circulate faster. What do you think? 3. Why is shiny foil important if everything is to be painted black. I understand the insulation board, but what's with the foil? Robert

11/13/2013 10:39:37 AM
Very convincing post. Nice informations here.

10/9/2013 10:44:47 AM
Has anyone ever tried to build this on a smaller scale. I have a bay window (the side panels crank open)in my dining room that ices up during the winter. I have to keep a small heater fan circulating air to stop this from happening. I was wondering if something like this could be done to heat the window & stop it from freezing?

9/25/2013 5:19:10 PM
I built one of the original heat grabbers in 1977. It took my brother-in-law and I about an hour to build it one Saturday night in December. I added a dial thermometer in the center of the device. I used a 4' by 4' pallet for the frame. I stuffed the area used by a fork truck to lift it with regular Pink Panther insulation, then wrapped the whole thing with tar paper. I added a 2" by 2" wood border on the top side then stapled construction plastic 1" up from the base where the product would have been placed. Then I wrapped the entire creation with more of the construction plastic. I put it in the back of my station wagon (remember those?)for the night. At 9:30AM Sunday morning the dial thermometer read 35 degrees. I took it out and rested it on the south face of my house with a rough guess as to the proper angle. I went inside and forgot about it. The thermometer's scale went from 40 below to 120 above. There was a pin beyond 120 at maybe 165. At noon or so I went out and to see what had happened and, to my surprise, the needle was resting on the pin. Admittedly, this was a static device, with no circulation involved, but I was a believer in alternate sources of energy ever since.

9/25/2013 12:38:29 PM
Although Iv'e followed Mother Earth for years i've never Im glad I did becuse I live on a desole farm in France and I have developed (with notes from Mother Earth) a solar heating array for virtually pennies. I was lucky in finding the glass, but I had to pay for thew copper pipes. I have used the system for about 3 years with really good results. This is for heating water. I now am in the prcess of building a warm air panel (I think this is an old ME idea) I have photos if needed. Performance is good tank temp at 10 a.m. 60 C at 11 am 80 c in summer (150 litre tank) in winter it takes an hour longer in sunshine...Cloudy drops the temp by 10 C. I have saved dog food cans to make the warm air heater and i hope to have this in place this winter... I anyone is interested I can upload photos of the manufacture of these units, and as I have so much help from contributers to ME I would like to offer advice and help....Im a newbie to ME thanks for reading

Catia Hirsch
1/6/2013 4:44:57 AM
my windows are sliding not raised. Is there someway to modify the design to work?

3/20/2011 7:03:14 PM
My grandfather built one of these units in the 1980's. It worked great until some carpenter ants got into it and made a steady stream into the house. I've painted the outside of mine with old engine oil. Nothing has chewed through that yet.

Mike Coyle
7/10/2010 3:32:14 PM
With the lower sash being in the open position, cold air will have a tendency to come in between the upper and lower sash. How do you seal off the top of the lower sash?

B Knight
11/28/2009 5:38:23 PM
This is a great project! Great in the 1970'S and still today. Security - Cut 2 pieces of wood the exact length of the opening (above) and insert one stick on either side. No one can force the window open with the sticks in place. Here's a site recommending this heat grabber, with some minor changes. Some ideas to try include: 1 - use a dead air design and only pull the air from the bottom of an aluminum sheet. 2 - ZigZag the air flow and add a fan to improve air flow through the zigzag maze. Have fun....

11/27/2009 11:44:48 AM
I recently built a heat-grabger using an old 24" X 72" window unit for my glass top. I used a 1"X 8" board for sides & bottom and the insulated foam-board for the bottom and divider. I put the foil side up on the top-side under the glass, lightly sanded it and painted it and the inside of sides flat-black. I installed it in the window of my shop. I'm thinking I may attatch foam board to the outside of the 1X8's to insulate them against the cold air but haven't yet. Yesterday it was clear and about 39 degrees in the morning and perhaps mid-50's after lunch. I checked the temperature of the air coming out about 2 p.m. and it was about 110 degrees. You can easily feel the cool air going in the bottom and the warm air coming out the top. Nice, I think I'll keep it! WmG

Suzanne Horvath
10/15/2009 9:06:19 PM
Henrietta, Re: security for the windows. There used to be small brass(?) safety gadgets for windows if you wanted to leave them open a little without anyone being able to open the window all the way. I don't know what they are called, but old-time, vintage stores should have them. You basically open your window to where you want it, screw one of these things on the upper part of the window (the wood around the glass)just above the lower window. To lock the lower window down, there is a little swivel knob that you pull perpendicular to the window. It creates a stop. The window can't be raised. If you push the little knob flush to the upper window, then you can push the lower window up as far as you want. Sorry I can't remember the name of these things, but I bet someone who's been around awhile fixing windows or installing the older type windows would know. Regarding the safety of having glass in this project - I too am concerned because I have 2 cats and their nosiness could be dangerous if they were to jump onto or against this. I like the clear polycarbonate idea though.

David Profitt_1
10/12/2009 4:14:43 PM
Excellent update on the solar window heater! The design has evolved nicely since I sketched out the idea for a material handling salesman from PA in late 1976. My original version used half-ht aluminum beverage cans soldered to an aluminum isolator plate, encased in a plywood box enclosure with single strength glass. I didn't pursue it commercially but saw an identical heater offered for sale in a newspaper ad the next fall. This new MEN version, while perhaps not delivering as much heat output as the original design and not as durable, is far easier to construct. Well done! To address a couple of questions: Linda - Plexiglas, though lighter and safer, will gradually cloud with exposure to sun, reducing the effectiveness of the heater. Clear Polycarbonate would be a better alternative. You will need legs to set your exposure angle properly and for durability. My original design had legs going back against and attached to the house. With the MEN design being constructed out of foam as opposed to wood, I would suggest you attach the bottom of the grabber solidly to the house or the ground Also, put an ice dam on the roof above it. A couple of carefully placed 1/8" holes with 8d nails inserted will lock most windows at the desired height. Allen - Having the chambers as close to the same size as possible is ideal, though 1/8" difference (I'm guessing that's the result of not allowing for the isolator panel thickness) between the two will not have a significant effect.

Henrietta Hughes
7/3/2009 7:28:37 PM
I have a question: Has anyone design any type of security feature that can lock your window down when you have this in place? Maybe it's included in the Mother Earth deisng, but I missed it? If I do this, I am going to find some way to secure my window with this in place, or will be easy target for break ins. Regarding questions on using glass, I have seen an alternate design like this that uses corrugated polycarbonate like that from SunTuff. This alternate design though was vertical an required you to cut 8" holes in your wall at top and bottom. I am going to do a "hybrid" of these two, using the materials recommended in the vertical design with the window box design of Mother's. Here is the alternate design:

Henrietta Hughes
7/3/2009 7:26:15 PM
I have a question: Has anyone design any type of security feature that can lock your window down when you have this in place? Maybe it's included in the Mother Earth deisng, but I missed it? If I do this, I am going to find some way to secure my window with this in place, or will be easy target for break ins. Regarding questions on using glass, I have seen an alternate design like this that uses corrugated polycarbonate like that from SunTuff. This alternate design though was vertical an required you to cut 8" holes in your wall at top and bottom. I am going to do a "hybrid" of these two, using the materials recommended in the vertical design with the window box design of Mother's. Here is the alternate design:

Henrietta Hughes
7/3/2009 11:49:22 AM
I have a question: Has anyone design any type of security feature that can lock your window down when you have this in place? Maybe it's included in the Mother Earth deisng, but I missed it? If I do this, I am going to find some way to secure my window with this in place, or will be easy target for break ins. Regarding questions on using glass, I have seen an alternate design like this that uses corrugated polycarbonate like that from SunTuff. This alternate design though was vertical an required you to cut 8" holes in your wall at top and bottom. I am going to do a "hybrid" of these two, using the materials recommended in the vertical design with the window box design of Mother's. Here is the alternate design:

Mark Dionne
2/12/2009 10:00:35 PM
Apparently the Thermax insulation is getting hard to find. This article explains how to build a similar unit without using Thermax:

2/12/2009 8:45:24 AM
Thermax brand insulation is made by Dow. To find a distributor in your area call Dow Customer Information Group at 866-583-2583. - Mother

Jerald Holcomb
2/9/2009 1:36:46 PM
Can I use plexiglass instead for plan glass and still do a good job of heating

2/8/2009 12:24:44 AM
So, where does one buy thermax? I looked around online and am having a hard time finding a store that sells it or how much it costs. Also, has someone on this board actually built this thing THEMSELVES and found it to work? Thanks! - Chris

2/8/2009 12:24:10 AM
So, where does one buy thermax? I looked around online and am having a hard time finding a store that sells it or how much it costs. Also, has someone on this board actually built this thing THEMSELVES and found it to work? Thanks! - Chris

1/16/2009 3:31:47 PM
Wow, this gives me an idea about heating my basement some.

12/25/2008 5:46:52 PM
I think I will try this one out for myself,I used to read this the mid 70's, any chance of getting reprints of back issues?

10/28/2008 7:32:23 PM
Image Troubles To those of you having trouble seeing the images - there's a better picture of the window mounted collector here Plans are available to buy here if you don't mind spending $10 Also if you click on the image gallery and go to the plan image and click on it again you can print out a fairly decent printout of the image without spending money on it Hope this helps BG in NC

William Jurgens
10/18/2008 8:46:41 AM
I am gong to build this window heater asap

10/8/2008 8:42:31 AM
Hey All, you may want to check out some new technology from I just ordered 8 Model 1536's that are a Module System that should give me about 3,800 BTU's. I purchased those plans from three years ago and built six units and joined them together and they worked great! Time to upgrade to a fixed system. After my research and a review of their "Patented" technology, has really peak my interest.

10/6/2008 9:08:41 PM
had anyone tried this in a cloudy climate? i live in portland, OR, where there isn't all that much sun in the winter. i need to see if its worth convincing my friend to help me build one of these :)

Lulla Sierra Johns
10/4/2008 1:59:50 PM
Wondering if we can see step by step pictures of this collector. Its very hard to image the collector just with the instructions described on Mother Earth News site. Thanks.

10/1/2008 3:38:43 AM
My house has no south-facing windows and the western side of the house is shaded by a covered porch. Will a heat grabber work well in a window that faces east?

9/28/2008 9:39:44 AM
Has anyone tried using plexiglass instead of glass? I've seen a kit you can buy at that uses plexiglass which would seem much safer. Anyone out there who has made one, do you have it resting on the ground or did you build legs under it - am wondering if this material is waterproof enough if it's sitting in snow. Also, can anyone tell me what the current cost is to build the Heat Grabber?

9/16/2008 1:01:07 AM
Is the Thermax referred to in this article the same as this Thermax ?

allan smith
9/3/2008 6:28:38 PM
I can't wait to build the heat grabber. But the plan are not clear. Are the cold air and hot air chambers the same size in height? I appears from the plans, that the cold air chamber is 2 1/2" tall and the hot air chamber is 2 3/8" tall. Is this close enought? Thanks. Allan Smith

7/22/2008 7:00:32 PM

7/21/2008 2:06:44 PM
Bruce, once you open the gallery and you see the blurry photos, click on the page and they will clear up. hope this helps.

7/21/2008 1:20:50 AM
Did anyone else but me notice that the image gallery is so severely blurred for this online article that it is completely useless?

Lloyd Gieg
7/17/2008 4:57:16 PM
Any reason why you couldn't build something like this as a free-standing unit and deliver the output through insulated tubing?

7/15/2008 11:08:40 PM
My dad's side were poor Germans living in Johnstown, PA at the turn of the century and the black boxes they built and put into the windows kept the house so warm the upstair windows had to be raised. Just 15 years ago my dad built one box for their dining roon window to show my mother free heat could be obtained. After a month my mother had him take out the box for it made the whole floor too hot. Solar heating does work.

11/24/2007 9:53:26 PM
What a blast from the past. I purchased the plans and built one of these back then when the article came out. It really worked! I wish I still had the plans for now I am in a new hokme with much better south-facing windows and a very large heating bill. Jim

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