Passive Solar Air Heater
Using the naturally occurring laws of physics, a passive solar hot air heater uses the heat from the winter sun to heat indoor living spaces.
Because the sun follows a similar, predictable pattern throughout the seasons, if the solar heater is positioned in a certain place in a certain way, it will not receive any summer sun.
This type of heater is also referred to as a thermo-siphon solar air heater.
I’ve seen these types of solar heaters on barns and houses, but this particular heater was totally passive solar. (Cool!)
This means that it works without electricity, simply by being in its location (on the south side) and by how it is built.
While this article is being posted in the summer, keep in mind that this project would take only a day or two to build, and summertime – when it’s warm, is the ideal time to build this type of project. (This particular one involves knocking holes into your walls.)
I participated in a passive solar heater workshop on a on a cool, spring afternoon in Silver City, New Mexico.
The solar heater was built from easily obtained materials and was mounted on the side of a Habitat for Humanity ReStore. (The heater can be seen just under and to the left of the ReStore sign.) Since the office was located on the south side of the building, it was a perfect position to build a passive solar air heater.
How a Solar Air Heater Works
Upon entering the office to view the solar air heater, I noticed that the room was noticeably warmer. This was a good thing, as I visited on a cooler spring day. While the outside temperature was about 68 degrees Fahrenheit, the air blowing into the space was 103 degrees Fahrenheit.
There were two holes located at the top of the heater, and two at the bottom.
The passive solar design creates a natural vacuum that allows cool air to be sucked into the lower section of the solar heater where it immediately starts to warm.
Within the heater, there are two pieces of darker aluminum screens sandwiched in the middle that help to warm the passing air.
Since hot air rises, it continues to be heated until the top is reached, where it travels into the room.
This natural cycle allows the air to circulate and transfers the solar heated air into the conditioned space.
When sun goes down and the outside temperature starts to drop, there is a built-in function that stops the airflow into the room.
When the temperature outside the building drops below the building’s inner temperature, the parchment paper located at the top of the screen is sucked against the screen and stops the cycle of airflow, keeping the cooler night air from cycling into the building. (Although this heater was not insulated, so cooler air would inevitably enter the space during the night.)
Basic Materials Needed
The passive solar heater was built out of glass panels that were formerly used as shower walls.
The other materials were 2×4 and 2×6 pieces of wood, aluminum screen, nails, caulk and parchment paper.
This is a good project to try on a barn or garage, but could also be integrated into a main living area.
It can be built in a day and serves as another way to warm a space naturally with the sun.
If is it mounted on a south facing wall, it will warm the space in the winter, yet with the sun ‘traveling’ a higher and wider path in the summer, with a proper overhang, the heater will not receive any sun, hence heat, in the summer.
Here’s another way of building a solar air heater.
I found this diagram after doing a basic search. I’d recommend researching this method before building it. According to the diagram, it looks like cooler air is (in theory) enters from the top. It would make more sense to have cooler air enter from the bottom, however, if that’s the case, according to this design, then the cooler air would be coming from outside. Winter air would be much cooler, thus having a great temperature differential that would cut down on its overall efficiency.
Also keep in mind that this will make whatever materials are utilized in its construction, very hot. When building and updating building, remember the mantra: Do no harm.
I recommend using tempered glass for building passive solar air heaters, since polycarbonate and other oil based transparent materials will off-gas as they get hotter. One doesn’t want to poison oneself while trying to warm the house.
Here’s a somewhat long-winded, yet informative video that looks at a large passive solar air heater installation. The info starts at 0:54.
I’ve also seen these types of solar air heaters built with aluminum cans.
Thank you to the ReStore and to Asher Gelbart for giving the workshop.
A solar air heater would be a great summer project. Have fun and remember that it’s better to build these when the weather is warmer.