Introduction: Cheap and Easy Solar Heater

Picture of Cheap and Easy Solar Heater

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

Picture of Materials & Tools

Materials

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

Staples

Silicon

Black paint

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

Insulation for the dryer vent (optional)

Tools

Tape measure

Marker

Tin snips

Chalk line

Wood saw

Clamps

Drill

Scissors

Staple gun

Caulk gun

Paintbrush

Chisel

Hammer

Masonry drill bit (long as possible)

Gorilla Glue

Electrical tape

Wire strippers

Step 2: Considerations

There are a few variations on this design, depending on the materials you have available and the type of room you will be heating.

FAN AND VOLTAGE

For the fan, you need to find a small, quiet unit that is capable of delivering at least 100 CFM. If you can find one of these that uses 110 VAC, then you don’t need anything else. Most small fans of this type are 12 VDC, which means that you need a converter from 110 VAC to 12 VDC. These converters are readily available from junk or surplus electronics, and are commonly referred to as “wall warts.” The converter should output at least 12 volts DC, and double the amps your fan is rated for. This gives enough room for the fan’s startup surge.

SIZE OF PLASTIC

The first problem you may encounter is the size of your transparent, corrugated plastic. If you can find a 4 feet sheet, so much the better, as that will match the size that the insulation comes in. However, if you can only find a 3 feet wide piece, don’t worry. You can still use this design, but you will have to cut your insulation down to fit it, and then adjust the metal channel walls, so that they too match the new size.

HEIGHT OF THE ROOM

The next issue is the inside height of your room. You will have two holes through your south facing wall, one at the top and one at the bottom. Air will be drawn from the bottom hole into the bottom of the heating unit outside. This is because it will take less energy to heat the air from inside the house than from outside, as it will already be warmer than the external temperature. The air that has been heated by the unit will then blow down into the room from the top hole. The problem with this is that, in order to make the most out of your heater, you will want the distance between the outside edges of the two holes to be about 8 feet (a little less is OK).

If your room is eight feet tall, construction of the unit will be easier, cheaper and more efficient. Easier, because you will be able to cut a hole through the bottom and top of the heater’s backing (insulation), instead of having to go out through the sides of the heater’s wall, which is more complicated. Cheaper, because you will need less 4” dryer vent, as it will be going straight through the house’s wall, instead of having to curve to the side before going inside. More efficient, because the temperature inside the unit will match the air coming into the room, whereas if you have to go to the side, you will lose a little heat through the extra dryer vent. You can compensate for this by painting the exposed vent black, or, even better, wrap it with insulation.

During the course of this article, we will be mostly describing how to attach this unit to a smaller room. For a start, it is probably more common to have rooms that are less than eight feet tall. Secondly, it is more complicated, and therefore requires a little more explanation. However, wherever the instructions differ for a tall room, we will add notes in bold.

Step 3: The Metal Sides

Picture of The Metal Sides
  1. Mark a piece of 10 ft. metal channel at 104” from one end (8’ 8”).
  2. Using the tin snips cut off the remaining 16” and put it aside for now.
  3. Measure and mark 4” in from each end of the 104” channel.
  4. Cut the sides of the channel off along these 4” parts, leaving a 4” tab on each end of an 8 feet length of channel.
  5. Mark one tab with a B for bottom, and the other tab with a T for top.
  6. Repeat 1-5 for another piece of 10ft. channel. These are the two sides of the heater box. Place them side-by-side, with both tabs marked B at the same end.
  7. Pre-drill holes, every 12” or so, along the inside edges of the 4” part of the channels. These holes need to be a little smaller than the 1” wood screws you will need to attach the metal to the insulation. The holes should be 1/4” from the side of the channel.
  8. Pre-drill holes, every 12” or so, along the 1” upright part of the channels that is on the outside (with the two channels side by side). These holes need to be slightly smaller than the ½” metal screws you will use to attach the plastic, and should be about 1/4” away from the edge of the metal.

Step 4: Insulation Box

Picture of Insulation Box

  1. Using the wood saw cut two 8 ft. pieces of molding.
  2. You want to clamp these pieces of molding to the 4” part of the pieces of channel, but at a diagonal. The ends near the B tabs want to be ½” in from the inside edge, so that when the two channels are side by side, the molding from one channel will be close to the other at the B end. The insulation (which is ½” thick) will eventually slot into this ½” space.
  3. The other end of the molding, near the T end of the channels, wants to be 1” from the outside edges. This is where the corrugated plastic will eventually slip in.
  4. Once you have the molding tightly clamped to the metal, turn the metal over and screw it into the wood (through the metal). You may need to pre-drill through the metal. Your screws should be every 12” or so.
  5. Turn the two channels up on their sides, facing each other, so that the molding that is ½” in from the side is on the bottom.
  6. Place the insulation in between the channels, slipping it inside the metal and the bottom molding.
  7. With the 1” wood screws, screw it in place using the holes you pre-drilled in the previous section, going through the metal and into the side of the insulation.
  8. Cut a piece of molding, 47” long (this is the width of the insulation, 48”, minus the thickness of the two side pieces of molding, ½” each).
  9. Place this 47” piece inside the two side pieces of molding, in roughly the center of their 8 ft. length, and screw it in place.
  10. Paint the whole inside black.

[If you have an 8 ft. tall room, as described in Step 1:Considerations, you will need to do the following steps at this stage:

11. Cut two 4” holes through the insulation (you can put your 4” dryer vent in place against the insulation and draw around it to get the right size). One hole needs to be in the center at the top, with its top 2” down from the edge, and the other in the center at the bottom, with its bottom 2” up from the edge. The distance between the top of the first hole and the bottom of the second hole should be 92”.

12. If the unit can fit inside the house, place the unit up against the wall where you want it to go, and mark the two holes, through the insulation, onto your wall.

13. If the unit doesn’t quite fit inside the room, but almost, drill a hole through the wall from the inside, a little up from the floor. Make sure you drill straight, not sloping up or down.

14. Then drill another hole through the wall, exactly 92” straight up from the first, but near the ceiling.

15. Hold your unit up on the outside of the wall, so that the very bottom of the bottom circle is just below the drilled hole, and the very top of the top circle lines up with the top drilled hole. Make sure the unit is level up and down, and then mark the wall through the 4” circles.]

Step 5: Screen and Plastic

Picture of Screen and Plastic

  1. Lay the 16 ft. x 4 ft. screen out inside the box (you will have 8ft. of it that is on the ground outside of the box.)
  2. Staple one edge of the screen to the underside of one piece of molding.
  3. Pulling the screen taut, staple the other edge of the screen to the underside of the opposite molding.
  4. Fold the excess screen into the box and staple one edge of it to the top of one piece of molding.
  5. Pulling the screen taut, staple the other edge of the screen to the top of the opposite molding.
  6. Place the corrugated plastic on top of the screen. Clamp one 8 ft. edge under the lip of one piece of channel, and screw them together through the predrilled holes, using ½” metal screws.
  7. Clamp the other 8 ft. edge under the lip of the opposite channel and screw them together through the pre-drilled holes.
  8. Bend the 4” tabs of the channel at right angles, in toward the center of the heating unit.

Step 6: Top and Bottom Manifold

Picture of Top and Bottom Manifold

  1. Out of the last piece of 10 ft. channel, cut out two 4 ft. lengths. These are the top and bottom of the box.
  2. Cut four pieces of molding, about 12” long each.
  3. Screw two into the inside of one 4 ft. piece of channel, about 6” from either end, and 1” down from the top (for the plastic to rest on).
  4. Do the same with the remaining pieces of molding and 4 ft. piece of channel. [If you have an 8 ft. tall room, as described in Step 1:Considerations, you can skip to the next section]
  5. Draw two rectangles on the inside of each piece of channel. The rectangles should be about 2” x 4”, centered both horizontally and vertically, and about 1.5” apart.
  6. Take one of the 24” x 13.5” pieces of sheet metal and lay it of a flat surface.
  7. Along one 24” length make marks at 2”, 6”, 7.5” and 11.5” from the right-hand side.
  8. Do the same along the other 24” length, also from the right-hand side.
  9. Make a chalk line along one 24” length (parallel with the marks you just made), 2” in from the edge.
  10. Do the same along the other 24” length, but only 1” in from the edge.
  11. Draw a line from each of the marks you made to the chalk line closest to it.
  12. Cut out the two rectangles that have been formed on each edge, the first being 4” wide and 2” from the end, the other being 4” wide and 1.5” from the first.
  13. Place the side of the sheet metal that has deeper rectangles on the back of the channel, lining up the rectangles of the two materials.
  14. Clamp and put some ½” metal screws in along the 4” length of the rectangles.
  15. Now bend the sheet metal so that it forms a tube, and line up its shallow rectangles over the holes of the channel.
  16. Clamp and put some screws in all around the rectangles, including the overlap.
  17. Now you want to make two caps, one for each of the manifolds you just made. You will be using left over pieces of channel to make a roughly circular shape (it’ll actually be more pill-shaped). It wants to include pieces of the upright parts of the channel, as these will be uses for tabs. You also want to make two extra tabs on the flat part, which you can bend up. Look at the photo above for clarification.
  18. Place these caps on the end of the manifolds that you want sealed (the opposite end of where the dryer vents will be entering the house). Make sure all four tabs are on the outside of the manifold.
  19. Screw the cap in place with ½” metal screws.

Step 7: Completing the Box

Picture of Completing the Box

  1. Slot the two 4 ft. pieces of channel into the heater box, making sure that the insulation and plastic are inside. You want the tabs from the 8 ft. lengths to be on the outside.
  2. Put a few screws from the channel into the plastic, and from the channel into the insulation.
  3. Screw the tabs on the box’s sides into the top and bottom channel.
  4. Run a bead of silicon around every seam, to make sure that it is all airtight. Make sure you get the cap and sides of the manifold, as well as all around the plastic and insulation.

Step 8: Holes in the Walls

Picture of Holes in the Walls

This step is largely dependent on the type of walls you have and how thick they are. The principals are the same, but solid concrete walls, for example, will be a lot harder than adobe, and will take you longer. If you have loose fill inside your walls, you will have to be prepared to support the fill as you chip out the wall, as they do in mines, so that the fill doesn’t all fall into the room.

  1. This step is likely to make a mess, so it’s a good idea to put sheets over your furniture against dust, and tarps on the floor for the chunks.
  2. If you have an 8 ft. tall room, you will already have marked your holes on the wall (in the section headed “Insulation Box”). If you have a short room, you will need to mark your walls now. Know roughly where your panel will be hung, as these holes will be to one side of it.
  3. On the inside of your room, hold a piece of dryer vent up to the wall close to the floor (about 1” or 2” up from the bottom). Draw around the tube onto the wall.
  4. Do the same up near the ceiling. The two holes want to be in line with each other.
  5. Using a long masonry bit, drill some holes on the inside of the circles you have marked. Make sure you always drill straight, not angling up, down or to the sides. You can also drill some in the center.
  6. With a chisel and hammer, start tapping out the material of the wall. You want to go all around the circles you marked, and then the center.
  7. Once you have chipped out a circular tunnel as far as you can, you’ll need to go to the other side of the wall and begin there. If your drill holes were straight, you should have a pretty good idea of where that circle is. If not, drill a hole through the center and look through it to make sure it is straight. Then draw a circle around this hole with a 2” radius.
  8. When the two holes are complete, try putting your dryer hose into them. You will probably need to chip little bits out.
  9. Take your fan and center it over the top hole. If it is square or larger than the hole in any way, draw its shape onto the wall. Chip out this shape a little more than the thickness of the fan, so that it can sit inside the hole without any part of it sticking out.
  10. Your major mess making is now over. You can clean up the worst of it, just to get it out of the way, but don’t worry about dusting the house just yet!

Step 9: Mounting the Unit

Picture of Mounting the Unit

The unit will be almost upright, although the bottom wants to be held a couple of inches out from the wall and the top touching the wall. This slight angle means that the bottom wall braces will be slightly less than right angles.

The heater is not that heavy, so the weight to be supported is not much. The braces are more to protect it from wind damage.

  1. Cut the 1” metal strip into two lengths of 12”, two lengths of 10”, and two lengths of 8”.
  2. Drill two holes, evenly spaced, into the last 4” of both ends of each piece of metal. The holes should be large enough for your masonry screws to pass through.
  3. Bend the 12” strips at less than right angles. The upright part (which will attach to the wall) wants to be about 4” tall, and the horizontal part (the last 4” of which will attach to the heater) will be about 8”. These are your bottom braces.
  4. Bend the 10” strips of 1” metal until they are at right angles. The upright part (which will attach to the wall) wants to be about 4” tall, and the horizontal part (the last 4” of which will attach to the heater) will be about 6”. These are your side braces.
  5. Bend the 8” strips of 1” metal in the middle until they are a little greater than right angles. These are your top braces.
  6. Place these two bottom braces up against the wall where you want to mount the heater, about 36” apart. Make sure the two braces are level with each other.
  7. Through the holes in the brace, mark the wall.
  8. At these marks in the wall, drill holes, 2”deep and just large enough to squeeze your concrete anchors in.
  9. Put a dab of gorilla glue on each concrete anchor and push them into their holes.
  10. Place the braces back against the wall, lining them up with the holes in the wall and screw them into place with the 2” masonry screws.
    • [If you are making a unit for an 8 ft. tall room, you will now need to attach your dryer vent to the holes in the back of your insulation.
    • The top one wants to be the thickness of the wall and the bottom one wants to be the thickness of the wall plus 2” or 3”. Dryer vents are like accordions – once you have cut your pieces, stretch them out a little so that they are a little longer)
    • Put the shorter vent into the top hole of your heater, and push it in about 1”. It’s a good idea to attach it by cutting a couple of 4” x 1” strips out of some left over channel, bending them into right angles, and then screwing one part of the “L” into the vent and the other part into the insulation.
    • Put a bead of silicon all the way around the dryer vent.
    • Do the same for the bottom hole, using the longer dryer vent.
    • Now, as you put your heater onto the bottom wall braces, you will want to feed the dryer vents gently into the holes in the wall. This will require two people.]
  11. Lift your heater onto the bottom wall braces and center it over them.
  12. Using self-tapping screws, screw the horizontal parts of the braces up into the bottom of the heater.
  13. Position the side braces about half way up the heater, one on each side.
  14. Mark the wall through the holes in the braces.
  15. At these marks, drill holes, 2”deep and just large enough to squeeze your concrete anchors in.
  16. Put a dab of gorilla glue on each concrete anchor and push them into their holes.
  17. Place the braces back against the wall, lining them up with the holes in the wall and screw them into place with the 2” screws.
  18. Using self-tapping screws attach the side braces to the heater.
  19. Position the top braces, about 36” apart, on top of the heater.
  20. Mark the wall through the holes in the braces.
  21. At these marks, drill holes, 2”deep and just large enough to squeeze your concrete anchors in.
  22. Put a dab of gorilla glue on each concrete anchor and push them into their holes.
  23. Place the braces back against the wall, lining them up with the holes in the wall and screw them into place with the 2” screws.
  24. Using self-tapping screws attach the top braces to the heater.

Step 10: Dryer Vents

Picture of Dryer Vents

[For the 8 ft. tall room model, your dryer vents should already be through the wall. Gently stretch or pull the accordion to bring them flush with the wall on the bottom or flush with the beveled out part for the fan at the top. If you find that you have to push hard, get someone to hold the vent on the outside of the wall, and push against them rather than against the insulation of the unit.]

  1. Take a section of dryer vent and gently put it around the open end of the bottom manifold on the heater.
  2. Once you’ve got it on properly, about 1” overlapping, put a 4” hose clamp over the join and tighten it.
  3. On the top manifold, take your snap switch and fix it against the side of the solar heater. The switch should be firmly pressed against the metal, you can use some wire or a screw to hold it in place. Attach 2 wires to the snap switch, with the length being long enough to go from the switch to the fan on the inside wall.
  4. Feed the wires through a dryer vent, and attach the vent like you did on the bottom manifold.
  5. Feed the two dryer vents into the holes in the wall. Dryer vents are like accordions, so you can pull them apart or push to change the length. You want the bottom vent to be flush with the inside of the wall, and the top vent wants to come to the beveled out part for the fan.
  6. With a little mortar, stucco around the vents against the wall, both inside and outside.
  7. Paint the part of the vents that are showing on the outside black. Even better would be to wrap them with insulation, especially at the top, to prevent wasted heat.

Step 11: Fan

Picture of Fan
  1. Pull the wires from the snap switch through the dryer vent from the top manifold. These will attach to your fan on the positive line.
  2. Fit your fan in place, and attach one of the wires from the snap switch to the positive wire of the fan.
  3. If your fan is AC rated, then cut the extension cord, leaving enough wire to go to a wall outlet and then connect to the fan. Connect one wire of the extension to free wire of the snap switch, and the other wire to the fan. Skip to step 5.
  4. If you are using a converter, then cut the appliance-end adapter from the converter cord. Plug the converter into a wall outlet, and with a multimeter, test the voltage coming from the converter. This will also show which line is positive and which is negative, as your multimeter will read a positive voltage if the leads are on the correct wires (red to positive, black to negative). Attach the positive to the free wire of the snap switch, and the negative to the negative of the fan.
  5. Plug the converter into the wall to test. It should come on, if the solar heater is in direct sunlight. If it does not come on, then unplug the converter, and wire the positive line from the converter directly to the fan’s positive line. Plug it in again to test. If it works this time, then you may have an issue with the snap switch (either it is too cold, or it is not making good contact with the solar heater).
  6. Once the fan is tested and working correctly, unplug the converter, and insulate all connections well with electrical tape. Push the wires into the dryer vent, and then put the fan in its place. Secure the fan against the wall with screws. The fan needs to blow air into the room, so make sure you put it in facing the right direction (blowing, not pulling, air).
  7. Attach the vent cover over the fan and screw firmly into the wall.
  8. Attach the bottom vent cover over the bottom hole and screw into place.

So now you have a brand new solar heater installed. As long as there is sun, the fan will run and heat up your room. The air coming from the fan will be nice and warm, but it won’t get hot enough to burn or damage anything.

During the warmer months, you can unplug it and remove the top dryer vent from the manifold to prevent hot air entering the home. Alternatively, just cover the solar heater with a thick blanket to prevent any solar gain.

Comments

kakashibatosi (author)2015-02-28

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.

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.

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?

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.

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.

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.

eruger (author)velacreations2015-03-08

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?

velacreations (author)eruger2015-03-08

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).

ClaudioP34 (author)velacreations2017-05-17

Since the Heater will be mounted facing south (or toward the equator), have you thought about replacing the plugged fan with a solar powered fan? There are some cheap fans for camping, or cars, that are already solar powered and that can be tried. If it works, you do not need to use the power from the grid or from the batteries. Just Google "solar powered fan", smaller models can be very cheap (need some hacking).

corieltauvus (author)2015-03-01

Hey Vela C,

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!

Yes, that's very true! I guess we should say "point your solar heater towards the sun!" for the global version. Thanks for reading!

farticus (author)velacreations2015-03-01

"Toward the equator" is unambiguous too.

tomatoskins (author)2015-02-27

This is so thorough! I love the detail that you put into it. I love all your instructables! Keep up the good work.

WilliamL225 (author)2017-05-17

Where did you find the transparent corrugated plastic?

StandsWithABeer (author)2015-03-02

Most excellent!


The following is a Nerd Alert!

just sayin'...
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.
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, "specific-heat carrying-capacity" 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 "harder" to heat because more heat has to be put into it to increase its temp.
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 "matched" 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...
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 "sponge" 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 conduction. If the hot air flowed through a crack or hole in the walls or ceiling, this would be heat loss through convection. If the hot air rose to a cold ceiling, the air's heat would be transferred to the ceiling--this is convection/conduction. Blowing the hot air around the room, allowing it to contact other materials, would be forced convection/conduction. An example of radiation would be visible infrared heat coming from a red-hot-cored electric heater "shining" on a material.
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.

Interesting Instructable -Thanks very much. Also this comment, but a small nit-picking follow-up: the use of "Latent Heat" here is not really correct, as that term applies to the heat taken up by a substance in changing it's phase state from a solid to a liquid, or from a liquid to a gas, at the constant temperature of that phase change. The appropriate description is "Specific Heat" (mentioned lower down in the comment), which refers to the heat energy entering or leaving a substance which causes a change in its temperature. As mentioned, the Specific Heat Capacity of a substance is a definite number of Joules of energy per unit mass per degree centigrade change. Water has a far higher Specific Heat Capacity than most materials, including metals. Air is quite low. It might be a matter for investigation, whether air or water would be the best medium for transporting the Heat energy captured from outside to inside.

On the point about heating the room air from upper or lower levels: the point has been made that extracting cold air from the bottom level gives the greatest possible heating range but the hot air blowing in at the top level means it will never sink to floor level. (As with all convector heaters. a cool region remains unheated close to the floor). Blowing heated air in at or near floor level would be better, as it will inevitably rise and heat all levels. The problem with that though is that extracting and inputting air at the same (lower) level would lead to a warm air mix being re-heated. Perhaps a water heat-transfer medium would be better, in that it could be piped to radiators, or transferred to blown air by a heat- exchanger. Either method can be at the low level.

Thank you for the very thorough and enlightening comment!

northcalgreens (author)2015-03-01

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

joelparks (author)northcalgreens2015-03-07

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

Mass is mass. Whether it's rock or water, it will heat up equally. Do you "dry out" your water when you heat it up to make coffee ? No ! Maybe the rock was drawing a lot of humidity in the house for the first week or so. Most rocks don't absorb water anyway.

Rock doesn't phase change in the range of temps produced here. So rather than store heat in the rock, the heat was evaporating the water in the rocks.

Why "phase change" ? Rocks can absorb heat without changing phase. Leave a rock in the sun and it will get hotter.

And it doesn't take a few seasons to dry out rocks.

Another way to think about it is by considering what happens when you sweat

Yes, yes and yes. But it will never take 3-4 heating seasons to dry out rocks. Most rocks absorb less than 0,4% of their weight in water. It will dry out in a few days and maybe 1% of the energy will be "lost" to evaporation. And this water vapor will be released in the house and will contribute to warm up the air.

A wet towel absorb 90% of it weight in water and it dries out in a few hours.

When I sweat, I drink water. Rocks that sweat don't.

Thermodynamics is not that complicated. Mass is mass even if it's water. It absorbs heat and it gives back heat.

The water will evaporate, changing phase from liquid to gas, which requires an input of heat energy.
So if you try to heat up rocks that contain water, some of the heat being put into the system will be spent evaporating the water, rather than to contribute directly to increasing the temperature of the rocks.

I'm coming a little late to this discussion but I am agreeing with joelparks. If anyone has ever dropped a wet rock out of the river and seen it explode will know that "wet" rocks and rocks that have had time to dry out do behave differently.

NYT1 (author)2017-05-17

I know there are other Instructables on similar Solar ideas (and I'm slowly reading through many of them), but how would something like this fare in the N.E. (US) during Winter? The output measured maxed out your thermometer, so I wasn't sure if that number may be higher... or if similar results would be achievable.

IngenuityAtWork (author)2017-05-16

I made a solar concentrator using a fresnel lens... could be a good way to heat water on demand. This is an excellent idea and write-up, and I like that you go the further step of promoting its use and assembly for and in places that need it more than others.

IngenuityAtWork,

Great idea! I've played with the idea too. What stops me is the helio tracker. If the lens does not track the sun it will be far less effective. Just sayn'...

For sure. Although I have seen at least a few great Instructibles on trackers.
Also had the idea of using fiber optic cable, although I don't know how thick it can be found, to collect the focused light, removing the requirement of precise alignment/positioning for the heat exchanger that the water flows through.

Scotticus (author)2015-03-03

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.

I once did service on a big res. air heating system that included 20 tons of rock as thermal storage, owner loved it.

oh, and don't say heat rises, that's a misnomer. Only hot air rises.

kwhitacre (author)Scotticus 2017-05-17

I don't understand your statement. Isn't heat in the air and thus rises? (This is all new to me.)

barnabas09 (author)kwhitacre2017-05-17

You are correct...read my previous reply

barnabas09 (author)Scotticus 2017-05-17

Heat indicates temperature...air and water absorb heat energy and expand thereby become thinner in density...as a result, colder water and air become the platform for the warmer air and water and the rise is simply due to density differences.

kwhitacre (author)2017-05-17

I have no exposure to the south so I was thinking the west would work. I have a railing along the west side of our home. It has a roof over the railing (think Japanese fence) in an Asian style. I was thinking this roof might make a good place to put the unit. Would a long narrow configuration work well? Could I take the top vent off one end and the bottom vent off the other? I would insulate them and run them under the deck for which the railing it used. The railing is about 50 feet long and runs the length of my basement, which is what I want to heat. In the summer it is even too cool. (Unless we get into the 100°F, which is rare) I use space heaters year around and would love to create something to heat this space without costing more money. Thank you for this timely post.

ApprenticeWizard (author)2015-03-04

Interesting. As an Instructable, it would be at least as helpful (if
not moreso) to include the "why" in your considerations. A detailed
plan is great ... if I have a very similar set of circumstances and
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.

I think the slant is to encourage the air to travel through the heated screen. Had wondered the same thing.

Popeye1959 (author)2017-05-16

I wonder how much heat this would provide a 1 car garage sized shed in areas with air temp near or below 0F.

moorwoble4444 (author)2017-05-16

Great project. Thanks for the a,aging instructions. This s there anyway you share some pictures of the completed unit?

ddavis662 (author)2016-10-16

another awesome instructable!!
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

FlyPot (author)2016-10-09

Nicely done.

eruger (author)2015-03-08

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.

Azze01 (author)2015-03-04

Neat idea. I just could't figure out what is the function of the screen.

velacreations (author)Azze012015-03-04

the screen is the heat collector. Air passes through it, heats up, then goes into the room.

sherab (author)2015-03-03

velacreations, always, great stuff.

joshem (author)2015-03-02

Beautiful!!!!Thanks

wbogen (author)2015-03-02

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.

velacreations (author)wbogen2015-03-02

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.

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!).

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.

beesontoast (author)2015-03-01

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!

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.

No, we use the natural path of warm air in the panel, from bottom to top, the warm air is not flowing downwards.

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