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Your gas furnace pushes air at 120 to 140 degrees Fahrenheit. It burns four to six therms of natural gas between sunrise and sunset on a cold January day. That is5 to$7 in fuel every single day just to keep your house warm while
A DIY transpired solar collector built from $35 of perforated aluminum can heat a home with free solar energy, cutting gas furnace use on sunny winter days.
Key Takeaways
- A perforated, black-painted aluminum solar wall can provide substantial free space heating with minimal cost and no moving parts.
- Perforations improve heat transfer by eliminating the insulating boundary layer, outperforming flat plate collectors.
- The system works best in sunny, cold climates with long heating seasons and a suitable south-facing wall, and is less effective in overcast regions.
What the video covers
- A gas furnace delivers 120-140°F air and burns several therms of gas daily on cold days, costing roughly $5-7 in fuel.
- A transpired solar collector uses a black perforated aluminum sheet behind a clear cover to heat air that flows into the house without fans or electricity.
- Perforations eliminate the boundary layer of still air that limits heat transfer in flat plate collectors, raising thermal efficiency to around 70-80%.
- Materials include perforated aluminum soffit, high-temperature black paint, 2x4 framing, rigid foam insulation, twin-wall polycarbonate glazing, foil tape, and a Mylar check valve.
- The check valve prevents reverse thermosiphoning at night, which would otherwise pull heated air back out of the house.
- A single 4x8 ft panel can deliver over 1,000 watts of heat on a clear winter morning, with outlet air reaching 130-140°F.
- The narrator claims the technology has been used in over 30,000 buildings, including facilities such as a Quaker Oats plant in Iowa.
- The system is weather-dependent and performs poorly in heavily overcast climates, so payback depends on sun exposure and heating season length.
- Build steps include framing the box, insulating the wall, cutting inlet and outlet vents, sealing penetrations, painting and mounting the absorber, and installing glazing and the check valve.
- The video promotes the creator's book of 126 off-grid methods, which is a commercial self-promotion element.
Chapters
- 00:00The Cost of Gas Heating and the Solar Opportunity
- 01:04Introduction to Transpired Solar Collectors
- 02:04How Perforated Metal Heat Transfer Works
- 04:49Materials List and Costs
- 08:00Night Backflow Problem and the Check Valve
- 09:00Real Performance Numbers and Gas Savings
- 12:30Limitations, Climate Considerations, and Final Thoughts
Full Transcript — Download SRT & Markdown
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the sun is out. And the entire time that furnace is running, there is a giant nuclear reactor 93 million miles away blasting your south-facing wall with free heat that you are ignoring completely. A sheet of perforated aluminum painted black mounted on that
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wall produces a 140°ree air on an 18° morning. No gas, no electricity, no moving parts, $35 in materials, and a Saturday afternoon. That is not a typo.
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$35. I built one. I tested it. And the physics behind it has been proven for over 35 years in more than 30,000 buildings worldwide. From military bases in Edmonton to a Quaker Oats factory in Iowa. The technology works. The only
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reason you have never heard of it is because nobody makes money selling you a sheet of aluminum and telling you to paint it black. Here is the complete build, the science, the real numbers, and where it falls short. Because it
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does fall short. And I'm going to tell you exactly where. Before we get into it, everything I cover on this channel, every system, every build, I put in one book. 126 methods sourced and tested.
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Caleb Greeroffgrid.com. That's all I'll say about it for now. The concept is called a transpired solar collector. That sounds complicated, but it is the simplest solar heating system ever designed. A flat sheet of metal with thousands of tiny holes in it,
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painted black, mounted a few inches off your south-facing exterior wall with a clear cover over the front. Sunlight hits the black metal. The metal gets hot. Air gets pulled through the tiny holes by natural convection. Hot air rises. Cooler room air gets sucked in at
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the bottom to replace it. As the air passes through each perforation, it scrubs heat directly off the surrounding metal. The heated air collects in the space between the metal and your wall.
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So that gap is called the pleum and exits through a vent at the top directly into your house. No fan, no pump, no wiring. The temperature difference between the hot air rising and the cooler air below does all the work. It
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is called a thermosophon. Gravity and physics move the air for you. Now here is the part that makes this design dramatically better than a regular flat plate solar collector. And this is 200-year-old physics that most people have never heard of. When sunlight heats
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a solid, flat metal plate, a thin layer of still air clings to the surface.
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Physicists call it the boundary layer. That layer of dead air acts like insulation. It traps heat against the metal and prevents it from transferring efficiently into the moving airream. A flat plate collector loses 30 to 40% of its captured heat through that boundary
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layer. The heat gets absorbed by the metal, but it cannot get into the air fast enough. So, it radiates back through the glazing and escapes to the sky. A perforated sheet eliminates the boundary layer entirely. Each perforation, and they are tiny, between
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half a millimeter and a millimeter and a half wide, creates a miniature jet of air that scrubs heat directly off the surrounding metal edges. The boundary layer never forms because air is constantly moving through the surface itself. Every hole is a tiny doorway
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pulling heat off the metal. Every edge around every hole adds extra surface area in direct contact with moving air.
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This is Newton's law of cooling applied to a 4x8 sheet of aluminum. Heat transfer rate depends on two things. The temperature difference between the metal and the air and the surface area in contact with moving fluid. A perforated
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sheet has thousands of extra edges and forced air movement through every single hole. More contact area plus more air movement equals dramatically faster heat transfer. The result, transpired solar collectors routinely hit 70 to 80% thermal efficiency. A flat plate
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collector struggles to reach 50. Same sun, same wall, same afternoon, and but the perforated sheet moves 30 to 40% more heat into your house. Here is what you need to build one. The absorber, a 4x8 ft sheet of perforated aluminum set
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vent material. You will find it in the siding aisle at Home Depot, Lowe's, or Menard's. The perforations are already factory punched. The exact size and spacing that makes this work. Cost 20 to $25. The paint. One can of flat black
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high temperature spray paint. Use barbecue or stove paint rated above 500° Fahrenheit. Rust oium and Kryon both make versions. Do not use regular latex house paint. When the panel hits 200° in full sun, and it will latex blisters,
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peels, and flakes within weeks. High-temp paint bonds permanently. Cost $6 to 8. The frame. four 8-ft studs of standard 2x4 lumber. This creates the pleum, the 3 and 1/2 in gap between the perforated sheet and your exterior wall
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where the heated air collects before entering the house. If you have scrap lumber on hand cost is zero new under $20. The insulation a sheet of halfin rigid poly isocyanerate foam board glued or friction fit against the exterior
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wall behind the pleum. This stops heat from bleeding backward through the wall into the siding instead of forward into your house cost about $10. The glazing.
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A 4x8 ft sheet of twin wall polycarbonate mounted over the front of the panel. This creates the greenhouse effect. Sunlight passes through, heats the metal, and the cover traps the rradiated heat against the absorber instead of letting wind strip it away.
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Do not use plain glass. Glass is heavy, fragile, and one hailstone or falling branch shatters it. Twin wall polycarbonate transmits about 90% of sunlight, weighs almost nothing, and survives impacts that would destroy glass. Cost $30 to $45. The ceiling
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aluminum foil tape for every seam. Do not use cloth duct tape. It dries out and peels within a single season of temperature cycling between 180 degrees during the day and 10 degrees at night.
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Foil tape handles it permanently. Cost $4 a roll. The check valve. This is the piece most people forget and the one that matters most after dark. At night, the metal cools down. The thermosifon reverses. Cold outdoor air sinks through
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the upper vent, fills the pleum, and pours into your house through the lower vent. Every BTU you gain during the day, walks right back outside while you sleep. The fix is a rectangle of Myar film, an emergency blanket from the
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dollar store works perfectly. Taped along its top edge over the upper outlet vent inside the house. During the day, warm air pushes the flap open. At night, gravity pulls it shut. Builders who skip the check valve report waking up to cold
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drafts pouring through the lower vent at 2 in the morning. Do not skip it. Total cost if you buy everything new at retail, about $75. If you have scrap lumber and foam on hand, which most people doing this kind of work do, $35.
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Tools needed, a drill, tin snips, and a caul gun. No soldering, no plumbing, no electrical work, no permits in most jurisdictions. Most documented builds are finished in a single weekend. Here's how you put it together. Build the frame
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from 2x4s to create a rectangular box that fits your south-facing wall. Glue or press fit the foam insulation board against the wall inside the frame. Cut two 6-in diameter holes through your exterior wall, one low for the cold air
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inlet, one high for the hot air outlet. Seal both penetrations with exterior grade silicone caul and flashing. Water damage from a poorly sealed penetration costs far more than any gas bill. Paint the perforated aluminum sheet flat black on both sides with the high temperature
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paint. Let it cure for 24 hours. Mount the painted sheet to the front of the frame. Install the twin wall polycarbonate glazing over the front of the panel sealed with silicone along every edge. Mount the Myar check valve
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over the upper interior vent. That is it. The panel is live the moment sunlight hits it. Now, here are the realworld numbers because the numbers are what make this either a project worth building or a waste of a weekend.
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A single 4x8 ft panel, about 3 square meters, delivers 350 to 450 watts of thermal energy per square meter. That is over 1,000 watts of free heat on a clear winter morning. The outlet air temperature hits 130 to 140° F with
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outdoor air below freezing. Rob Stevenson documented a build using the same aluminum set material. Total cost under $40 that measured 135° at the outlet with 22° outside. a 113°ree temperature rise through a sheet of perforated metal that costs less than
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dinner for two. Gary Rea at Build It Solar, who has been testing DIY solar air heaters longer than most people have been alive, measured his screen collectors consistently above 130° with outdoor air below freezing. He also ran a direct comparison between a pop can
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solar air heater and a screen collector. same size, same test conditions, same air flow. The screen collector produced 6% more heat output and required a fraction of the build labor. The Popkin collector cost $130 in materials. The screen collector cost $137
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for $7 more and a third of the work, the screen collector won. Over a six-month heating season with an average of four sunny days per week, a single panel offsets four to six hours of furnace runtime every sunny day. In gas savings,
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that is roughly $300 to $500 per season. The panel pays for itself before the first heating season is over. The second season is pure savings, and the third and the 10th. The polycarbonate glazing lasts 15 to 20 years before yellowing
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reduces transmission. The aluminum sheet and the frame last indefinitely. This is not fringe science. A company called Conserval Engineering, founded by the inventor John Hollik, has been selling commercial versions called Solar Wall Systems since the late 1980s. A
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professional solar wall installation costs $35,000. The Canadian military installed them on a base in Edmonton and documented a 50% reduction in heating fuel consumption.
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The National Renewable Energy Laboratory published a technical report TP550 CO2 Tunai 913 studying a solar wall installation at a Quaker Oats facility in Cedar Rapids, Iowa, where the system offset over 300,000 cubic feet of natural gas per year. By 20120, more
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than 30,000 buildings worldwide had commercial transpired solar collector installations. The heat transfer through perforated metal works the same way at $35 as it does at $35,000. The physics does not check your budget. Now, here is where I have to be straight with you
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about the limits because this system has real ones and pretending otherwise would be dishonest. This is a daytime supplement. It is not a furnace replacement. When the sun drops behind the roof line or a thick cloud bank rolls in, the heat output drops to zero
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within minutes. There is no built-in thermal storage. At night, you still need your furnace or wood stove or whatever you use for heat. On overcast days, expect roughly 30% of rated output at best. In a week of solid gray skies,
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the panel does almost nothing. This is a solar system. I It needs the sun. If you live somewhere that gets 200 overcast days a year, your math changes significantly. The panel needs a south-facing wall with direct winter sun
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from roughly 9 in the morning to 3 in the afternoon. Trees, fences, neighboring structures, anything that shades that wall kills the performance.
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If your south wall gets 3 hours of direct sun instead of six, you get half the output. No amount of engineering overcomes a shadow. This works best above 35 degrees north latitude where the low winter sun angle hits a vertical
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south wall almost headon. In South Texas or central Florida, the sun angle is too high in winter for a vertical panel to catch efficiently and the heating demand is so low that the payback takes longer.
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If you are heating a house in Vermont, Michigan, Montana, or anywhere the furnace runs 5 months a year, this panel earns its cost back in weeks. The minimum wall space you need is about 32 square ft of unshaded south-facing
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exterior wall. If your house faces east west with no south exposure, this is not your project. But if you have a garage, a workshop, a barn, or an outbuilding with a south wall, that works perfectly.
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For scaling up, build multiple panels. Two panels on a south wall double the heat input. Three panels on a 40ft barn wall can keep a workshop comfortable on a sunny 20°ree day without turning on a heater. The commercial solar wall
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installations cover entire building facades, hundreds of square meters because the economic scale linearly. Twice the area equals twice the heat. A few more things from running this system that are worth knowing. The average American household spends over $900 a
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year on natural gas, and that number climbs 6 to 8% annually. A cold climate heat pump retrofit costs $15,000 and still charges monthly to operate. A new gas furnace costs $5,000 installed and burns fuel every hour it runs. A 4x8
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panel of perforated aluminum costs $35, and the fuel arrives every morning without a bill. For an off-grid cabin or homestead, this is the most cost-effective space heating supplement you can build. It pairs perfectly with a wood stove. The panel handles the sunny
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daytime hours. The wood stove handles the nights and overcast stretches. Between the two, your propane bill goes to zero. For a greenhouse, mount the panel on the south wall with the outlet venting directly into the growing space.
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On a clear February morning, that 140°ree air keeps the interior above freezing without any electricity. Pair it with the sand battery we covered in another video, and you have solar heat during the day from the panel and stored
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heat at night from the sand. Two systems, both under $300 total, and your greenhouse grows through winter in most of the country. For a garage or workshop, one panel on the south wall takes the edge off a cold morning in 20
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minutes. You walk in, the sun is hitting the panel, and by the time your coffee is cool enough to drink, the shop is 15 to 20° warmer than it was. No space heater humming in the corner, pulling 2500 watts from your breaker panel. The
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simplest version of this, the version you can build this weekend, is a 4x8 sheet of perforated aluminum set from the hardware store, a can of flat black barbecue paint, a frame made from scrap 2x4s, a sheet of polycarbonate, and a
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myar check valve from a dollar store emergency blanket. Mounted on your south wall, cut two holes, seal them, the sun does the rest. The same technology heating 30,000 commercial buildings worldwide. The same physics proven by the National Renewable Energy
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Laboratory. The same principle the Canadian military uses to cut fuel bills in half. Except yours costs $35 and takes a weekend. If you want every one of these systems in one place, stepbystep, it's at caleb greeroffgrid.com.
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Links below. Have you built a solar air heater? What design did you use? Screen, pop cans, flat plate, and what temperatures are you hitting? Drop the numbers in the comments. That real world data is what drives this channel.
Topics:transpired solar collectorsolar air heaterDIY solar heatingperforated aluminumoff-grid heatingsolar wallpassive solargas furnace alternativehome energy savingsthermosiphon
