Converting electricity into heat is one of the most expensive uses of energy. If you’re not convinced, look at the power usage of electric appliances like microwaves, space heaters, water heaters, ovens, or coffee pots. The difference can be seen between your electric bill from a cold month, when you have to continually run your central heating, and during summer months, when the ambient temperature does not need to be heated.

If you live off-grid, you tend to be far more aware of what it takes to create electricity; you can literally watch your batteries’ voltage drop lower and lower when using an inefficient appliance. However, if you are tied into the grid, you should almost be extra vigilant as regards your power usage. Not only do you have to pay for it each month, but also the power plants that produce your electricity are rarely sustainable or environmentally conscious.

When considering the issue of heat, there are several green options, the best of which is the sun. Solar thermal energy can be harnessed in many ways to heat your water, food, or home. For the purpose of this article, we will be concentrating on an active thermal space heater to heat a room or house. The unit takes one person two days to construct, one day to install, and costs under $100. The temperature inside the unit easily reaches 160 degrees Fahrenheit on a 50-degree day, and can be blown into the house using a fan that pulls less electricity than a light bulb.

Please note that the unit should be installed on a south facing wall (if that is not possible, then a west facing wall will at least give you heat in the afternoon).

An additional advantage for rural Mexico is that the production of these units can create a local business. A couple of people could start making them with very few tools and initial outlay. Any new business in a rural area will increase the wealth of that community, as there will be more money being circulated locally. Furthermore, people can apply to their municipal’s Presidency for aid in putting them in schools and community buildings, to reduce the amount of firewood that parents have to provide. If you are interested in starting an environmentally conscious business in Mexico, please see our Spanish version of this Intructable.

For more information, view How To: Solar Heater

Please visit VelaCreations.com if you're interested in seeing more of our how-to guides. You can also follow our projects through the blog or our books.

Step 1: Materials & Tools


One 8’x4’ sheet of ½” foam insulation, with foil backing

One 8’x4’ sheet of transparent, corrugated plastic

Three 10’ pieces of 4” metal channel

Two pieces of 24” x 13.5” sheet metal (for shorter room-see step 1)

Four 8’ pieces of molding (1”x1/2 “)

16’x4’ of black screen (metal or plastic)

One 8’ length of 4” dryer vent tubing (for shorter room-see step 1)

Two 4” hose clamps 4” 100CFM fan (if fan is 12 VDC, you will also need a converter from an old computer or printer)

Screened vent Snap switch (45C action, 30C reset, Normal Open, Thermostat Temperature Control Switch, we use model KSD301)

10’ of 12 gauge stranded wire

Cheap extension cord (to reach your nearest plug)

½” metal screws

1” wood screws

12 x 2” screws and concrete anchors



Black paint

60” of 1” wide metal, 1/8” or ¼” thick

Insulation for the dryer vent (optional)


Tape measure


Tin snips

Chalk line

Wood saw




Staple gun

Caulk gun




Masonry drill bit (long as possible)

Gorilla Glue

Electrical tape

Wire strippers

<p>Do you have any temperature in vs temperature out data? I'd be interested to see how well it's working out. I've seen a ton of projects like this from vacuum tubes to soda can heaters, and I always find them interesting. Someday maybe I'll finally have space to build one.</p>
<p>Yes, we measure the temperature in at 60 degrees F, temperature out at 135+ degrees F. The soda can ones are a good idea, but the screen versions like this one have a much higher efficiency, and are easier to build.</p>
<p>I can see how this would be much easier to build. I'm impressed you get such a large temperature gradient. How would you describe the flow rate your fans achieve? Is it more of a gentle breeze?</p>
<p>I measured it at 170F at the top in the collector, but that's as high as my thermometer goes. You could improve a bit by insulating the top pipe, and the top of the unit to extract more heat. </p><p>The fan is decent, but could be better. It's more than a gentle breeze, but not a strong wind. 2 fans or one that is a bit more powerful would extract more heat, but would increase the noise. Also, shorter runs (straight through the wall) would increase the flow. <br><br>Once it turns on in the morning (8:30 am), it runs all day, so the fan rarely cools the air down enough to trigger the snap switch. Ideally, you want a fan that can move enough air to trigger the snap switch regularly during the day, so that you are sure to get all of the heat out of the unit and into your house.</p>
<p>Can't help but wonder if you couldn't use the internal convection to replace the fan? I know that a similar device has been used to replace electricity on a ground-air cooling system. I that case, there was a rooftop turbine extracting mechanical energy from the heated air, which turbine then turned the fan by means of a u-joint. Didn't seem very efficient, so I wondered if you couldn't use just a portion of the heat energy to drive ventilation?</p>
<p>yes, you can, but it is not as efficient. If you turn off the fan, heat will still enter the room, but at a much lower air flow. The fan helps you get all the heat out of the unit, and at a very small price of electricity (a few watts).</p>
<p>Hey Vela C,</p><p>Good stuff! But don't forget your friends 'down under' . . . New Zealand, Australia, South Africa, the Pacific Islands and South America all have to turn their screens to the North!</p>
<p>Yes, that's very true! I guess we should say &quot;point your solar heater towards the sun!&quot; for the global version. Thanks for reading!</p>
<p>&quot;Toward the equator&quot; is unambiguous too.</p>
<p>This is so thorough! I love the detail that you put into it. I love all your instructables! Keep up the good work. </p>
another awesome instructable!!<br>I've been pondering on an idea something along this line. there should be no need to heat a camper during the day under the Texas sun
<p>Nicely done.</p>
<p>Great Instructable! A thought for a starting point: picking up an old 'tin can' (the enclosure for a commercial sign) from a sign maker would probably save a lot of work, and maybe a lot of money.</p>
<p>Made something similar to this using roof flashing ( scrap ) we sent the heated air to big insulated container of river rock. We were able to then extract the heat when needed by siphoning it into the house with another fan.. Great inle </p>
<p>I heard a case study of an early solar thermal installation which used river rock as the storage. The owner/builder/designer neglected to factor in the energy needed to drive the water out of the rock, and they had to use supplemental heat for the first 3 or 4 heating seasons until the rock dried out sufficiently to warm up</p>
<p>Neat idea. I just could't figure out what is the function of the screen.</p>
the screen is the heat collector. Air passes through it, heats up, then goes into the room.
<p>Interesting. As an Instructable, it would be at least as helpful (if <br>not moreso) to include the &quot;why&quot; in your considerations. A detailed <br>plan is great ... if I have a very similar set of circumstances and <br>materials as you do. Telling why (for instance, why slant the backing? Why paint things black? Why use screening?) in each step creates an informative and adaptive Instructable. Good inspiration, interesting build to study.</p>
nice project. The old mother earth news one was called the heatgrabber, the nice feature of its design (when circumstances allowed) was no holes in the wall.<br><br>I once did service on a big res. air heating system that included 20 tons of rock as thermal storage, owner loved it.<br><br>oh, and don't say heat rises, that's a misnomer. Only hot air rises.<br><br>
<p>velacreations, always, great stuff.</p>
<p>Great instructable! Have you made any designs for heating domestic water? Since we all use hot water year-round and the warmer months tend to have more sunlight (duh!), this might be an even more efficient use of whatever exterior wall-space someone has available. Maybe fill the collector with a loop of black-painted PEX tubing with the bottom end connected to the house's cold water supply line, the top end feeding to the cold water inlet on the house's water heater. Water would only flow when there was demand from the water heater so the efficiency (and temperature) might be erratic. Also, the system would have to be valved-off from the house and drained when it's freezing outside.</p>
<p>Actually, we do use a similar design for a water heater. We started off snaking PEX through the box, but it gets too hot and the pipe split on several occasions. We ended up biting the bullet and using the more expensive alternative: copper. Since installing copper, we have not had an issue.</p><p>The hot water is circulated through a well insulated tank, and that tank is connected to our hot water taps. We have a pump that begins to circulate when the temperature in the panel is higher than the temperature in the tank. Our water temperature is generally about 125 degrees (except on cloudy days!).</p><p>Also, because the water heating unit does not need to rely on the principals of air rising, we have the unit titled back further than the almost vertical design of the space heater.</p>
<p>I would have thought that it would be better to draw air from the top of the room, where it will already be warmer than at the bottom, and blow it in at floor level, which is where you want the heat to be. I want warm feet, not a hot head!</p>
we take the coldest air in the room and heat it.
Yes, but you are trying to force warm air to flow downwards, which is the opposite of what it wants to do, and it will never reach floor level, so will never have maximum benefit.
<p>No, we use the natural path of warm air in the panel, from bottom to top, the warm air is not flowing downwards.</p>
<p>Most excellent!</p><p><br>The following is a <em>Nerd Alert</em>!</p><p>just sayin'...<br>After reading through several solar-heating projects, I wonder why no comments are made of the correlation between latent heat and sensible heat, since this is really what it is all about. <br>Latent heat is the energy one puts into a material in BTU's (English measurement) or calories (Metric measurement) to raise its temperature 1 degree fahrenheit or celsius, at STP (Standard Temperature and Pressure, i.e., in a room, at sea-level). Different materials, (air, water, sand) require different latent-heat inputs to raise their respective temps up a degree. I.e, one might put in 5500 BTU's raising water one degree over a time to go up a degree, but 11,000 BTU's over the same period to raise the temp of sand the same degree. This doesn't change with the starting temp of the material--it is specific to the material; hence the assigned term, &quot;specific-heat carrying-capacity&quot; of each and every material. For example, the specific-heat carrying-capacity of water is 25 times that of air--so water is a more efficient purveyor of heat, because it holds more heat. Conversely, then, it is &quot;harder&quot; to heat because more heat has to be put into it to increase its temp.<br>So then--why all this wind about latent heat, you may ask? Well, it is because it is not about the temp of the room, but how well the room holds the heat after it reaches the desired temp in the first place, right? As we all know, heat flows only from hot to cold. The variables of conduction and convection and radiation within the room holding the heat put into it, should govern more of the project characteristics than the actual heat source--but having a &quot;matched&quot; heat source--neither too large or too small, is very important. Ideally, in other words, if no heat left the room, at a certain temperature no more heat would be required to be put into the room, since no heat is escaping from the room, and the room would remain at the desired temp indefinitely. So, one could heat a room with a big candle, if it burned long enough in time and fuel to generate the necessary amount of BTU's or calories to heat the room to a certain temperature--with little or no heat loss. Are you getting my drift here? It is all about heat Transfer. Once you get the heat (latent heat--BTU's/Cal's) into the room--keep it there! :) You still have to ventilate the room for human occupation, however...<br>If you had a room full of high specific-heat carrying-capacity materials like aluminum or copper, the heat would flow from the heated air to the cool aluminum and copper, heating them up (in latent heat terms) until the air and the aluminum and copper were the same temperature (but each material would be holding different amounts of latent heat, right? And in so-doing will cool the air. The heat is still there, but it is in the aluminum and copper! Aluminum and copper each have a specific-heat carrying-capacity higher than that of air, so they will &quot;sponge&quot; up more heat than air); the temp of the materials would not rise or fall until enough BTU's or cal's had gone into them to raise or lower their temps a degree. This is <strong>conduction</strong>. If the hot air flowed through a crack or hole in the walls or ceiling, this would be heat loss through <strong>convection</strong>. If the hot air rose to a cold ceiling, the air's heat would be transferred to the ceiling--this is <strong>convection/conduction</strong>. Blowing the hot air around the room, allowing it to contact other materials, would be<strong> forced convection/conduction</strong>. An example of <strong>radiation</strong> would be visible infrared heat coming from a red-hot-cored electric heater &quot;shining&quot; on a material.<br>OK--getting back to the most excellent design of the heater in this Instructable--using the black screen is brilliant, because one major, major, factor in heat transfer is any project materials' surface area. As you said, the 3-D measurement of the screen is crucial. Calculating all unwanted openings to be less than 3-sq.in. for minimal convection-loss, and adequately insulating the room, this solar heater is a winner! Thank you for a great Instructable.</p>
<p>Thank you for the very thorough and enlightening comment!</p>
<p>Great instructions &amp; I love the concept as soon as I get done with another project I am going to make this- it would be nice if we had a way to store excess heat for the night time...</p>
<p>I agree.... I have a sun room attached to the house and by day (when the sun is out) the temperature in the winter can (and has) hit 100 degrees F, but once the sun goes down the room cools off quick! Have been kicking solar air heating around for awhile but am thinking a &quot;glycol&quot; (SP) radiant floor heating scenario with the sun as my heat source might be a better route for the nite time...</p>
<p>Mother Earth News back in the 70's and 80's did something similar with a twist, they developed theirs to mount to a double hung window so it could be used by just about anyone. Nice job !</p>
<p>very very interested in this sort of thing and grateful for your instructable. </p><p> In PA it gets pretty cold , now if it is 5 or 10 degrees outside do you think it will still warm up enough to perhaps keep a crawlspace above freezing? If we could have air that is 40 degrees F under the house, or warm up my unheated garage workshop would be great. </p><p> Off the grid is a dream of mine. </p>
<p>How warm is the air under the house now? Do you get sunlight? The air coming out of the unit will be 100F or more. You may need more than one unit, depending on the size of the space. So, build one, see how it works, then build more as you need.</p>
<p>An excellent detailed and informative instructable. An improvement may have been to have picture or schematic of the completed project, so that as we went through all the details we would have an overall concept of how each step was developing. As it stands it is a bit like working through a maze with no concept of where it is going. Great project</p>
<p>Did I miss it somewhere? What is the &quot;16&rsquo;x4&rsquo; of black screen (metal or plastic)&quot; for? Seems like it would &quot;shade&quot; the inside of your collector.</p><p>BTW, the way the Instructable materials list is formatted, your black screen item is not on a separate line. Not as easy to see as the other items.</p>
<p>Thanks for the typo, we've now adjusted it.</p><p>As for the screen, it is the heat collector. The sun heats it up because it's black, and the air then passes through it and transfers the heat into the air very efficiently. We mention its installation in Step 5: Screen and Plastic. </p>
&quot;The sun heats it up because it's black, and the air then passes through it and transfers the heat into the air very efficiently.&quot;<br><br>Really? I would think that a plastic screen wouldn't absorb much heat and a metal one wouldn't do much better. Doesn't seem to be much mass there.<br><br>Perhaps it slows the air down enough for the air to heat up more than without it? Have you tried you heater without the screen?
<p>They do absorb a lot of heat, and the air passing through them transfers the heat effectively, because of the greater surface area. We selected this design based on tests of side by side collector designs:</p><p>http://www.builditsolar.com/Experimental/AirColTesting/Index.htm</p>
<p>I'm with ClodHopper, I didn't understand what the screen was for until your above reply. You explain installing it, but not what it is for in step 5. I would not think that it has enough thermal mass to make a difference, but I have not made one of these. I'll check out the link, but have you tried a side by side comparison?</p>
<p>You don't want thermal mass, though, you want efficient solar to air heating. The screen heats up in the sun, and because of the huge surface area, and the air passing through it, it transfers that heat to the air.</p><p>That link above has several side by side comparisons, but think of it this way. With a flat surfaces, the air to surface contact is just the surface area of the flat surface. Air that is even an inch away from the flat surface is not making good contact. <br><br>With a screen, it's is the 3D surface area of the screen that is in contact with the air, and has considerably more surface area across the whole flow of air.</p><p>When constructing the unit, you can slot the plastic in (don't screw in place) before adding the screen, and stand upright to feel the heat coming off. It is some. Now, install the screen and do the same test. You will notice a significant difference.</p>
<p>What do the inside walls look like or suggestions so that it blends seamlessly with your interior room?</p>
<p>Look at Step 11. Choosing your vent covers will make it blend seamlessly on the interior.</p>
<p>Nice work! To save some electricity, why not make the fan solar powered? It would also negate the need for a thermostat in only running the fan when there is sun. I don't think it would need too large a solar array to power a medium sized 12 pc fan.</p>
<p>You can use a solar panel to power the fan, for this fan, you'd need a 15-20 watt panel. You still need the thermostat, though, so that the fan only turns on when the panel is up to temperature, so you get the most heat out of the unit. The thermostat is only $5, and it will perform considerably better than a solar panel alone.</p>
<p>Thank you for the article, it should be helpful to many. </p><p>I would suggest that if it is possible that the collector be mounted horizontally which would negate the restriction of room height. Also the uninsulated reflective dryer venting is a source of heat loss and directly venting the collector directly through the back of the collector should be comparatively easy. This will also greatly improve the look of the installation, The horizontal positioning will also have the added value of being able to place the heat exaust lower in the room which will help to keep the heat from staying at the top of the room.</p>
<p>The problem with horizontal is that it is less efficient, because you lose more heat though the long side of the panel (heat rises). Also, the taller the unit, the greater the air flow due to convection.</p><p>It is definitely better to insulate the air ducts.</p>
<p>This is an excellent instructable. It is well thought out, and well explained. I don't know much about solar heaters, but it really looks like something I could build. </p><p>As an electrical engineer I have to mention that conversion of electricity to heat is 100% efficient. All electric energy used by a resistance heater is converted to heat energy, but it is usually more costly than other heat sources. </p>
<p>I'm just curious, how efficient would this be for heating a one car garage?</p>
<p>That depends a lot on the size of the garage and the amount of sun you get in a day. This collector should output about 7,600 btus</p>

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