Passive Solar Heating

When my wife and I purchased our home last year we ran into a neat feature I had never seen before: passive solar heating. Most of the articles you’ll find online use the same term for the planning, design and building of homes and buildings with an eye towards using thermal mass, windows, and sun orientation/angles to try and make use of the sun’s heat to ┬ápassively heat during the colder months and block the sun off during summer months. This is a nice concept, but doesn’t really help people who already have homes built or aren’t planning to do a huge makeover on the south-facing side of their homes. Ours is an actual system added to the home. From what we can tell it appears that the system was installed in the 1980’s when the original owner added an expansion to the second floor (more than a decade after the house was built).

Solar is all the rage nowadays as companies are coming up with innovative financial solutions to get people to install them. Seems like the most important innovation now is financial – to let people lease the system for cheaper than their old electric bills, thereby defraying the large upfront costs for a longer term savings. Pretty much exactly the issue electric cars are running into these days. But I digress…

The solar installations that are making these headlines are expensive solar panels that generate electricity, and while that’s awesome I’m not convinced they’d be all that feasible in a climate like we have here in western New York. We’ve got an interesting other type of solar installation, and I’m willing to bet a very handy person could even make a DIY project out of this – and see a much quicker return on investment.

On the south side of our house is an array of sealed glass panels with a black interior. They look sort of like a wall of storm windows looking in on a black wall. Inside each of those “windows” is some black material and netting. Technically this would be the “thermal mass”. The sunlight hits the black material and wall and heats it up. The enclosed glass panels keep the hot air inside (obviously some leaks out).

panels

The concept is simple: when the sunlight hits them, they heat up and keep the heat in the boxes. Think of a car with a black interior parked out in the sun.

At the top and bottom of these panels are insulated flexible ducting. The ducting at the bottom goes into my basement and has a fan connected inside. They continue on to three floor vents in various rooms of the house. The top ducting is fed in from the attic, and returns air from two ceiling air returns – one at each end of the house on the second floor.

Then there is a simple “brain” behind all of this that controls a handful of dampers to open or shut off the ducting, and turn the fan on or off. This controller takes in readings from a few sensors: outside temperature, inside temperature, and temperature in the panels – plus from a thermostat in my living room. When the temperature in the panels is higher than the inside temperature, the fan is turned on.

My thermostat controls where that heat is blown. If the termostat is set to a temperature higher than our current indoor temp, the heat is directed to the heating vents in my family and dining room. If it’s set lower, the heat is “dumped” to the basement. The heat dump serves as a more indirect heating, which will penetrate the rest of the house slower. For some solar heating systems, it may be necessary as an outlet when the panels get too hot and may degrade the materials inside the panels (the thermal mass).

If it’s summertime and my house is plenty hot enough I simply go down to my basement and shut the system off entirely at the panel for the season.

Now, I recall seeing a post or two on reddit’s frugal sub-reddit asking about how one might build a system such as this, so I figured I would include more detailed photos of the controller (“brains”) that takes in the temperature sensor feeds and does the work of turning the fan on or off and moving the dampers. I’m guessing someone with some electrical engineering background or an Arduino/Raspberry Pi hacker might be able to make this themselves in a much smarter, cheaper and smaller package nowadays.

2 thoughts on “Passive Solar Heating

  1. Wow, this is pretty awesome. I can see where it would be much cheaper than PVs and as you said, a do-it-yourselfer could come up with a similar system themselves. My dad was actually one of them; making one to heat pool water.

    I am also thinking of doing something similar, but probably just to save money with the hot water heater.

    Do you have any idea how much money this thing saves? A ballpark estimate would be fine. I would think your furnace would hardly have to run once the sun starts baking this thing. Perhaps there is latent heat in the tubes after the sun goes down too?

    Finally, don’t count PVs out, even in your neck of the woods. Germany leads the worlds in PVs and is at a higher latitude than you.

    I am going to install PVs at some point, but will do it myself to save money.

    Thanks for the write-up, most appreciated!

  2. Thanks for the comment, Mr. 1500, and for spurring me to write about this!

    I’ll have to do some research to get any sort of estimated savings – it’s hard to come up with a figure without getting some typical sunlight data for my region and figuring that out against the natural gas rates.

    Anecdotally, I can say that from September through May it runs and replaces natural-gas heating during a good portion of the day. This past week it typically ran 9am – 4pm, and the heat inside the house kept the furnace from running for a few hours after. The collector itself cools off pretty quickly once the sun disappears. Now, the weather isn’t very conducive to it all the time – I’d say it ran full time Friday and Saturday and probably ran part of another couple days this past week.

    Using those numbers as a base, we can assume it cuts the heat bill roughly in half when run on a full day, and runs about 3 of 7 days, which means roughly 20% reduction in heating bills? Don’t quote me on that though, I’m pulling numbers out of thin air here ­čśë

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