Build a Simple Solar Air Heater

This low-cost solar air heater plan lets you turn any south wall into a source of free heat.
By Gary Reysa
December 2006/January 2007
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The author and the south face of his Montana home equipped with a simple solar air heater.
Phot by Gary Reysa

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After walking into my workshop one December morning and feeling a bone-chilling 10 degrees, I decided to install a heating system. Given the rising costs of propane and my family’s environmental concerns about using nonrenewable fossil fuels, a solar solution seemed fitting.

I’m a retired aircraft engineer, but you don’t need a similar background to tackle this project. In fact, a solar air heater built into new construction or added to an existing building can be an easy and inexpensive heating solution. Following the simple principles and plan outlined here, you can heat your workshop, barn or even your home with free heat from the sun. If it works here in Bozeman, Mont., it’s bound to work wherever you are.

I reviewed many solar collector concepts and decided to install a thermosiphon air collector on the south wall of the workshop. The elegant, simple thermosiphon design uses only the buoyancy of heated air to create circulation through the collector, eliminating the cost, maintenance and power consumption of fans, sensors and controllers commonly used in other collector designs. On a sunny day, in a cold climate like ours, this simple system can produce heat equivalent to burning about $2 worth of propane (equivalent to about $3 for natural gas).

To minimize costs, I integrated the collector with the structure and used readily available materials. It cost me about $350 and took only one trip to the hardware store. I built and installed it in about three working days. Follow the suggestions below, and you may be able to do it faster!!

How It Works

The thermosiphon collector consists of clear, corrugated polycarbonate panels fastened to 2-by-6-inch studs. The clear panels admit sunlight, and an absorber suspended inside the collector captures the sun’s heat energy. The air around the absorber warms, expands and rises, creating a convection current. Vents at the top and bottom of the collector allow air to circulate through it. Cool air enters the lower vents from the interior, is heated by the absorber, rises to the upper vents and returns to the interior. Air circulation continues as long as the sun shines on the collector.

At night, airflow reverses as air in the collector cools to outside temperatures. Simple flapper valves on the top vents stop this reverse circulation and keep the heat inside.

Performance and Economics

On sunny winter days, the collector raises interior temperatures to between 60 and 75 degrees. In my neck of the woods, that’s 25 to 35 degrees above outside temperatures. Overnight, the building typically cools to 8 to 15 degrees above the outside temperature. On heavily overcast days, the collector does little heating, but on partly cloudy days or with a thin overcast it provides useful heat. It usually takes about three hours to warm the workshop from 35 to 65 degrees — a good excuse to sleep in! If you’re determined to start work early, more insulation, more thermal mass or an early morning blast from a backup heater would be in order.

Be sure to install adequate insulation and to control air infiltration. No solar collector — or any other kind of heating system — will heat a drafty, uninsulated area effectively. The walls and roof of my 700-square-foot workshop are insulated to R-19, as good as many houses.

According to my measurements, the heat output per square foot of panel compares favorably to that of most commercial panels—at less than one-tenth the cost! (See "Heat Gain, Running the Number" near the end of the article for more details.) Our only alternative would have been to heat the workshop with propane. Although a propane heater would have cost a bit less than building the solar collector, paying $2 per gallon for propane over our five-month heating season would have cost $300 to $400 per year. The simple payback period of the collector is a year or two on materials cost. I haven’t needed a backup heater, which is an additional savings.

Sizing the Solar Collector

Usually, the bigger the collector, the better. Most outbuildings suffer high heat losses because of high infiltration rates and inadequate insulation, so the heat a large collector generates can be put to good use. More collector area provides some allowance for partly cloudy and thinly overcast days. With this collector design, overheating usually isn’t a problem. You can easily close off upper vents or add thermal mass, such as water containers. Increased thermal mass also reduces nighttime interior temperature swings.

The added time and material cost is small to build a collector that uses the full south wall versus only part of it. Exceptions to using the full wall include locations with mild climates, well insulated and well sealed buildings, or buildings that are much longer along their east-west axis than their north-south axis. Even if the entire south wall is not available, using a portion still pays off.

Nuts & Bolts

The collector frame is built of common dimension lumber and consists of vertical members, a bottom sill and a top sill. In mine, the six vertical 2-by-6-inch members divide the collector into five 4-foot-wide bays. The bottom sill is a 2-by-6, and a 2-by-8 is used for the top sill. The collector frame components are attached to the building with lag screws driven into them from the inside.

The collector is glazed with clear Suntuf corrugated polycarbonate panels. The manufacturer applies an ultraviolet-resistant coating to the panels’ sun-facing side to extend their life. I used 10 panels, each 26 inches wide by 8 feet high. Pairs of panels are overlapped and joined over a 1-by-1-inch vertical wood strip to make the 4-foot-wide panels for each bay. Two 1-by-1-inch horizontal members provide additional support for the glazing.

The absorber, a black metal window screen, is installed on battens fastened vertically to the uprights and across the top and bottom of the bays, about halfway between the glazing and siding. After measuring thermal performance with one, two and three layers of screen, I found that two layers work best. The top and bottom vents are simply holes cut through the building’s sheathing and siding. Simple light plastic flapper valves prevent back flow through the upper vents at night.

In summer, the vertical orientation of the panels and the high sun angle minimize overheating, but you should block the top vent openings to prevent the collector from delivering heat. I just staple a piece of cardboard over each top vent, but you could install hinged vent doors. Shading or covering the panels also would work, but is probably unnecessary. In the spring and fall, you can close some vents and leave others open to control the heat output.

Construction Tips

First, measure your building’s south wall to determine how you’ll need to adjust the collector design. Pay particular attention to available height and to stud spacing. Next, lay out the vents. Offset them from the wall studs enough to leave room for the vertical frames to be placed away from studs and lag-screwed from inside the building. Mark the vent locations on the inside and outside of the building to ensure you have no conflicts with electrical wiring. After you’re certain the layout is correct, take a deep breath and cut the vents.

Cut the top sill long enough to lap over the end verticals by at least 1 inch. Bevel the back edge of the top sill so that it slopes about 10 degrees when fitted against the siding, to shed rain. Next, cut the verticals, angling their tops to match the slope of the top sill. Line them up and cut notches for the two 1-by-1-inch horizontal glazing supports.

Next, prime and paint everything. You don’t have to repaint the siding that will be behind the collector, but painting it a dark color will improve the efficiency slightly. Keep in mind that a muted version of this color will show through the collector screen, so be sure it meets your aesthetic sensibilities.

After the paint has cured, mount the verticals to the siding. I fastened them from the inside using 5 inch lag screws, with washers, through the wall sheathing and siding. If your siding is not strong enough to support the weight, align the verticals with the wall studs and drive lag screws from the outside through them and into the studs. Take care to keep everything level, plumb and straight—this will save you a lot of four-letter words later.

Now, attach the top and bottom sills. If desired, install sheet-metal flashing above the top sill to repel rain. Then, seal the perimeter of the collector frame with silicone caulk. Mount the battens and staple the window screen absorber onto them. Fold over the edges of the screen to fit in the bays, which are slightly less than 48 inches wide.

Make your 4-by-8-foot glazing panels by joining pairs of the 26-inch-by-8-foot corrugated panels. Overlap the panels by one corrugation, applying a light bead of silicone caulk between the overlapped sheets. Reinforce the joint by fastening a 1-by-1-inch wood strip on the inside of the overlapped corrugations using screws with washers made of EPDM, a synthetic rubber that stands up to sunlight and high and low temperatures.

Install the horizontal 1-by-1-inch glazing support strips in the notches cut into the collector frame. The surface of the strips should sit flush with the surface of the frame. Do any cleanup, caulking or other work inside the collector frame now. You won’t be able to reach inside after the glazing is attached.

Next, mount the glazing panels. Install foam sealing strips, which are molded to fit the contours of the corrugated panels, on the top and bottom sills. Run caulk beads on the first set of verticals and mount the first glazing panel section. (You’ll quickly find out how square your frame is.) Fasten the panel sections to the frame using screws with EPDM washers. Install the rest of the sections in the same way. Overlap each new section over the previous one by one corrugation, using a bead of caulk in the overlap.

For the flapper valves on the inside top vents, I went high-tech and used two thicknesses of plastic garbage bag. Before adding these, attach half-inch mesh hardware cloth over each vent to keep the flappers from being sucked into the vent. Then, staple the flappers along the top edge of the vent, just above the vent opening.

Collector Variations

A couple of variations could improve the system’s performance. Substituting dual-wall polycarbonate glazing for the single corrugated sheet would reduce thermal losses through the glazing (although some loss in energy absorption also will occur). This type of glazing, which consists of two sheets of polycarbonate separated by support webs, also simplifies installation, since it requires less support and doesn’t have corrugated edges that require sealing. Buildings in cold climates will benefit the most from this change, which would increase the cost by about $100.

Another option is alternating collector panels and windows on the south wall. This approach would admit more light and some direct heat gain through the windows, without the glare, high losses and overheating that accompany full window walls.

You can use the same concept to heat a house or cabin. With some refinement to integrate the vents with the interior wall, the same basic design can provide daytime heat to living spaces. One caveat—the National Mechanical Code prohibits circulation of conditioned air warmer than 120 degrees in wooden stud spaces. While this may not pose a problem for outbuildings, with in-buildings used for human habitation you might consider constructing the collector with metal studs, rather than wood. As an extra measure of safety, wood areas immediately surrounding exit vents also could be flashed with sheet metal.

In altering the collector design, make sure you don’t violate these guidelines:

  • The collector’s depth should be at least 1/15 of its height.
  • The absorber must offer low resistance to airflow.
  • The vent area should be at least 50 percent of the collector’s horizontal cross-sectional area.
  • The air path through the collector should be unobstructed and run the full length of the collector.

Other Ways to Build It

This collector could be built with less material and lower cost if it were integrated with the wall during initial construction. It also could be built on the outer surface of the wall, omitting the exterior siding behind it. This might save up to $2 per square foot. For a more finished appearance, you could install a higher grade of sheathing in the collector area.

For a more integrated approach, the wall stud cavities could be used for the collector itself. Using standard 2-by-6 studs at 24-inch spacing, inlet and exit vents could be incorporated in the inside finished wall, with the absorber screen mounted about halfway back in each stud cavity. The collector side of the inside finished wall could be lined with polyisocyanurate foam board insulation, which withstands high temperatures and provides high R-value at modest cost. The R-value of the insulated collector wall would be comparable to that of a standard wall. The glazing could be mounted on 2-by-2-inch members extending along the top and bottom of the collector, and over each of the vertical studs in the collector area.

The difference in materials cost between such a collector and a conventional wall would be close to zero! With a large collector, diagonal bracing or a shear-resistant inside wall may be required to maintain the wall’s shear strength. Be sure to work out an agreement with your building department on the details.

Other approaches surely are possible. If you work out a good one, I’d love to hear about it. Meantime, here’s to your warmth and comfort!

Collector Materials

This list shows what I used and my costs, and should help you figure totals for your installation:

2” x 6” lumber (verticals, bottom sill)

68 ft.


2” x 8” lumber (top sill)

22 ft.


1” x 1” lumber (glazing supports)

130 ft.


Suntuf 8’ x 26” corrugated polycarbonate panels*



Foam closure molding*

40 ft.


Screws w/EPDM washers*



Black metal window screen

4 x 70 ft.


Paint, caulk, lag screws, hardware cloth, etc.



Light plastic sheet (vent flappers)






*Your hardware store may need to special-order these items from the manufacturer, Palram Americas.

Solar Collector Pros & Cons


  • Simple (not much to go wrong or watch over)
  • Easy to build and does not require changes to the building structure
  • Long life and little maintenance (so far)
  • Very low initial cost (less than one-tenth the cost of most commercial panels)
  • Good economic return on the initial investment
  • Reduces our greenhouse gas production by about 1,800 pounds per year
  • Output can be adjusted by opening and closing vents—summer output can be reduced to zero
  • Does not affect use of building (I can still pile stuff against the wall, but now it’s not junk—it’s thermal mass)
  • It’s not ugly (or at least not too ugly!)


  • It hurts a bit to cut holes in the wall (but you’ll get over it)
  • The building might require additional thermal mass and insulation to keep inside temperatures from dropping too much at night

Heat Gain: Running the Numbers

In a recent test I did at midday, under typical sunny winter conditions, the collector produced a 50- to 60-degree temperature rise between the lower vent and the upper vent, and an average air velocity at the upper vent of 110 to 120 feet/minute. Peak output was 25,000 Btu/hour, and the total daily heat gain was about 130,000 Btu (38 kWh). This is equivalent to burning about 2 gallons of propane (about $4 worth) at 70 percent efficiency.

Heat gain estimates are based on measurements of the temperature rise and the vent exit velocity. Combining these with the specific heat of air and the density of air gives the collector heat output. I estimated the rate of heat gain using the following equation:

G = A x V x D x (Tu - Tl) x H

G is heat gain rate; A is vent area; V is velocity of air through the vent; D is air density at prevailing temperature and altitude (you’ll have to look this up); Tu is air temperature at the upper vent; Tl is air temperature at the lower vent; and H is the specific heat of air (a measure of the heat required to raise the temperature of a pound of air one degree Fahrenheit; it varies with temperature and pressure but is about 0.24 Btu/lb-F° under everyday conditions).

I measured the temperatures with several $2 Taylor thermometers from the hardware store. The vent exit velocity was taken using a Kestrel wind meter. Although this instrumentation might not meet Sandia National Laboratories’ standards, I believe it does provide a solid estimate of the collector’s performance.

Gary Reysa worked for Boeing Aircraft. Since 2000, he has lived in Montana, where cold winters, high heating bills and ample sunlight sparked his interest in solar heating. He has posted many other DIY solar projects at his Web site, Build It Solar.

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Post a comment below.


6/26/2013 8:28:06 AM

If the solar system can't furnish sufficient space heating, a back-up or an auxiliary framework gives the supplemental high temperature. Despite the fact that some early system passed sun based heated air through a bed of rocks as energy space, this methodology is not proposed on account of the inefficiencies included, the potential issues with buildup and form in the rock bed and the impacts of that dampness and form on indoor air quality


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Tracy Armbruster
6/9/2012 6:46:33 PM
Do you think this would work on a flattish surface? I have a flat roof over the garage I can use... it is the best space for a sun trap and there are vents that side of the house (just above the garage roof) that could take the warm air into the house (which is actually what I want to heat)?

1/6/2012 11:18:43 PM
Click on the "Photo Gallery" in the blue box at the upper right of the article above to get pictures.....I built a small (5'x5') version of this a few yrs ago and stuck it on the south side of my barn. I was amazed to feel a swift breeze coming out the vent at 105degF on brite, sunny days from 10 AM to 2 PM with outside temp at -5degF! The problem is, with even only ~30& cloud cover, no heat to speak of was generated at all. Here in NE IL, we get about one sunny day out of five over the winter and even then it was only effective for about 4 hrs in the middle of that day. Don't cancel your gas service.

Bambi Freeman
1/6/2012 6:34:24 PM
It sounds like a great idea but it sure would be nice to have some photos to go along with the article. Thanks!

7/23/2011 9:58:20 AM
Thank you for subscription

B Knight
10/29/2010 4:22:06 PM
We've done something similar, but on a smaller scale - an air pre-heater for the air exchanger on the R2000 home. Here's a picture of the unit: One advantage of this setup is that the blower already existed. On low fan, there is a 20 degree Celsius gain between the intake and exit ports. When running on high, the gain is still 15 degrees Celsius. We picked up the sealed double pane window from a "reuse" store... total cost to make was just $50. We may need to replace the glass with "thermal" glass, as it cracked in the first couple of days due to heat/expansion. Bruce.

Robin Wilson
12/14/2009 9:56:33 AM
I loved this idea. I modified the system to let the sun shine through the glass under the house and used the screen on the wall. I used two layers of the black screen with a 3/8 inch separation between them and the back wall which is aluminum flashing spray painted with high heat flat black paint. Our siding is board and baton so there is no problem like with vinyl siding. The vent into our house feels like a forced air heater on sunny days. We use a "Bakers Choice" wood stove for heat on the other days which works very well for cooking, water heating and home heating.

9/9/2009 9:22:32 PM
(Continued) This building has the CUBIC VOLUME of 12-2000 sq. foot homes and literally cost less then $2.00 of electrical power, per month, to operate the fans motors, which "Sucks" the cooling or heating into the project in Vancouver Wash on Mill Plain Blvd. CURIOUS? and I will gladly show HOW to do the above. Anachronism 9-9-09

9/9/2009 9:19:17 PM
9/9/9 Mother Earth: Passive Solar Heating/25 years of use and design. ANYBODY DESIRE TO HEAT HIS OR HER HOME/BARN/SHOP? The following is where it started with my first effort in 1983. The original cost of the Burlington project, beyond the original cost of $100 for a used "Hot water flash Boiler" and $100 for 500’ of used sheet metal ductwork from a "Sea First" bank being remodeled in Bellingham was the purchase of a "Adjustable Remote" temperature sensor that indicated the "AIR" temperature in the attic and a distance of 24" from the inside pitch of the roof covered with black composition roofing. The one question for you is the following: What were the conditions "outside" and on the roof on December 22,1983 in Burlington, Washington? When the Passive Solar system turned "ON" for the first time, the building was 65 degrees inside and I was sweeping the floors. I had turned the hot water furnace off and the remote attic control was set for 65 Degree. I quickly went outside to look at the weather and found "3 inches" of snow on the roof and a temperature of 22 degrees. SO I guess "Solar Heating" and "Passive Solar Cooling" are impossible. The building size is 95' long by 55' wide and 27' tall with 3 floors with 150 rooms inside. MY monthly cost to heat the "COMPLETE BUILDING" averages $1.00 (per day) in the winter. Look on and go to "photos (10) "and see 5 projects in operation for 25 years. I cool the project in Vancouver Wash for "NO COST" beyond the operational cost of two fans in the back-up gas-heating furnace. LOOK AT THE TEN PICTURES of OUR PASSIVE COOLING & HEATING SYSTEM AND IS TOTALLY PASSIVE SOLAR AND USES NO MACHINERY/COMPRESSORS/ or any power to create the COOLING for the complete project. This building has the CUBIC VOLUME of 12-2000 sq. foot ho

8/26/2009 3:20:07 PM
I don't see a single relevant picture with this article. Am I missing something, or am I just supposed to somehow visualize the final product in my head?

Gary Reysa
2/5/2009 8:31:22 PM
Hi Kim, I don't think that moist air will be a problem. The collector pulls air from inside your garage (not outside), and it will heat it up by about 50F to 60F degrees as it passes through the collector. The temperature increase lowers the relative humidity, and should to to evaporate any moisture that might be in the collector at startup -- it should dry out very quickly once the sun is on it and air is circulating. Gary Reysa

Kim _4
1/17/2009 7:31:21 PM
I really want to build one of these to heat my garage that's attached to my house. I live on the Oregon coast where the air has a lot of moisture. Will the collectors condensate and push moist air into the interior? Or (hopefully) will the air flow in the collector dry out the air as it moves through?

Doug Smith
1/13/2009 1:00:31 PM
You could use a 2X4 placed flat against the outside wall and secured the adjoining studs to make a more rigid connection point to lag screw the verticals too. Also to create a more rigid wall to support a large collector one could install a long narrow piece of plywood across the effected studs, counter sunk to allow even finishing of the interior wall.

1/6/2009 10:09:23 PM
THINKING INSIDE THE BOX. Most people already have the makings of a thermo-siphon heater. People intrigued by the concept, but not willing to spend hundreds of dollars and hours, may find that they are already halfway there. Foam insulation, metal collector, glazed enclosure, are all parts of home components otherwise called an insulated steel entry door and full-view storm door. The only things missing are the supply and exhaust vents, and flat black paint.

10/13/2008 2:18:22 PM
FROM GARY Hi Colette, I have heard of a small number of cases of this happening, but its very unusual in thermosyphon collectors because the temperatures are not that high. From what I have heard from others, the smell will likely go away with time -- you may want to use more ventilation until it does. One thing you might try if the glazing can be removed easily is to wait for some warm, sunny weather, and remove the glazing for a couple days and let the sun and air at the internals of the collector. This may also allow you to figure out which material the smell is coming from. I would also wipe off the inside of the glazing before you re-install it as some of the volatiles may have deposited on the inside of the glazing -- this will reduce the collectors performance. Another thing you might try is to pull the glazing away a bit at the top and the bottom of the collector, and then shut off the vents that go to the shop. This way the collector will ventilate itself with air coming in at the bottom and out at the top. Let it run for a few days like this, and see if that gets rid of the odor. As to whats causing the odor, I'm a bit suspicious of the sealer, as I've used a several different paints with no problems. Another potential source of odor in collectors is the caulking -- silicone caulk is the only one I would inside of inside of collectors. If there is any insulation inside the collector, it should be a high temperature insulation. The polyisocyanurate rigid foam board that many lumber yards carry is good. If you used polystyrene foam board (the blue, pink, or white stuff), it will not hold up to collector temperatures, but I'm not sure if it smells as it degrades. As another way to try to identify which material is causing the smell, you can bake it in a kitchen oven at about 250F. For example, if you suspect the paint, then paint a scrap of wood with it, let it dry, then expose it to the oven overnight and

Colette _1
10/11/2008 11:15:46 AM

8/25/2008 11:36:28 AM
First of let me thank you for this. I have out building all over my farm that heat in the winter would be nice to have but never had electricty. This is a great way for me to do it. But as a way to control the heat you could add simple plywood doors painted a very light color that could be closed on warm days to block heat build up on the outside. then as winter months come you simply go outside and open the doors to allow for heat. on larger setups you could even use a double door and on warmer days when not as much heat is needed simply open only one door. Just a suggestion.

1/21/2008 4:40:08 PM
Teresa from S. Georgia, I live in N. Carolina and have the same problem with variable temp. in winter. If you use this method and design it to heat the house well on the coldest days (teens and twenties), then it will probably overheat the house on a 70 degree day. Without getting into design specifics, you could vent the air from the solar room into a water heat storage system. Dave

1/19/2008 8:00:51 AM
i make self made soler heater for every one in pakistan due reasone shot of energy and poor pople this best method but more better home made method pl send email other .This chraty FOR POOR POPLE THANKS. P.O.BOX# 4024 MULTAN ROAD LAHORE54500 PAKISTAN.

1/11/2008 12:35:19 PM
I live in So. Georgia and would like to convert (at least partially) to solar heat. Is this the best way to do this? There is a lot of info. for the colder regions, but none for the south. Today is 1-11-08 and it is between 75 & 80 degrees. Last week it was 21. Any suggestions? Any help would be appreciated.

1/8/2008 10:40:58 PM
To Dan and Toeknee, That's the beauty of putting it on the south facing wall: The sun is 46 degrees higher in the sky in summer than in the dead of winter. In Bozeman, at a latitude of 46 degrees, the sun will be at a high of 23 degrees from vertical at high noon on June 21 (compared to 21 degrees from the horizon at high noon on Dec 21). Because of the high angle, the collector is effectively in the shade and no direct sunlight enters (you could be a modest sized overhang to make sure). Thus there is no problem with overheating in summer due to the collector. Dave

1/4/2008 11:46:04 PM
Could this be used to pre-heat recirculated swimming pool water?

1/2/2008 1:07:37 PM
I agree with Dan... I'm looking at building a Garage/Workshop this spring and would consider adding this to my south wall during construction. But the summer months is a concern....

12/21/2007 1:30:33 PM
Very informative article, and I think this will come in very handy in my garage. However, I'm curious what you do in the summer to keep the heat out? Do you plug the vents, or do you remove the whole thing? Just wondering. I don't want my garage to be 300 degrees in the summer. Thanks, Dan

Gary Reysa
11/8/2007 9:44:40 AM
Hi, I did a pretty careful measurement of collector efficiency on the collector last week using my new pyranometer from Apogee to measure the solar radiation. The efficiency came up 70.2% for the conditions I tested under, which were full sun and outdoor temperature around 45F. I was very very happy with this -- it shows you can get efficiencies as good as commercial collectors costing $30 a sqft with this simple $2 per sqft collector. Gary

3/4/2007 2:12:25 PM
the artical on simple solar heat from south wall was very informative on how to build. thats one reason i subscribe to mother earth .

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