Planning Our Passive Solar Heating Design

by Angela aka Farmer Jane on October 24, 2012 · 3 comments

This is the south facing side of our home. It gets full sun all day and only has one window located at the east side of the wall.

We’ve thought about what we wanted to do with this area so many times it could make your head spin.

The south side of our property is the area we’ll be putting a very large aquaponic greenhouse, as well as having the majority of our future large animals in this section.

We went back and forth wondering whether to espalier tropical fruit trees on this wall.

Dom wanted to use this space to install a solar hot water heating system. We’ve decided that we can do that on the north east side of the house, and concentrate on making this area the passive solar heat collector to warm the house.

As you can see, there’s a brick planter box that travels down the whole wall to the gate which leads to our courtyard. We’ll be digging down into the planter box about three feet.

After measuring the area, we need to figure out the BTU’s of a 2 liter bottle filled with water.

We’ll be using 550- 2 liter bottles to trap and hold heat throughout the day.

So far the plan is this (it could change!) :

  • Dig down three feet
  • Dry fit 2 liter empty GLASS wine bottles
  • Using slip straw and cob, create a tight glass insulating barrier on the ground
  • after cob sets, add water filled, capped, 2 liter plastic soda bottles standing straight up
  • String together the necks of plastic bottles to form a web of string between the bottles
  • Add 4-6 inches of cob to the top of bottles to seal bottles in, sequestering any plastic particles from becoming airborne and entering our home
  • Paint brick wall and cob black
  • Frame out the 3 1/2′ x 22′ area with wood up to the window and roof
  • Install glass on framed area
  • Of course vents would be added…just didn’t want to get to technical right now. ;)

The photo to the left is similar to what we’d like to do. Once we figure out the technical part, like how many BTU’s does a 2 liter bottle of water hold, and how many would be needed to heat roughly 3,000 sq ft of home. Our home is larger than that, but we won’t need to heat the basement which is about 400 sq ft.

We thought we’d go the full length of the wall and include the window, but in the end, that becomes a dangerous situation if we don’t have a way to escape in case of a fire.

If the 550- 2 liter bottles filled with water will not be enough to heat our home, we can always add courses on top of the cob, and then seal the next level of bottles in with cob also.

We ultimately decided we wanted to go passive solar because it’s a low cost way to heat our home year after year, without the need to purchase wood, gas, or oil. We do have fireplaces and we’ll be adding more, but they will be used for area warmth as well as ambiance. I couldn’t see us needing to buy lots of cords of wood to heat our home, nor do we want to continue to purchase gas from the gas company in order to keep our family warm.

It just doesn’t make sense to us any more. Why would we pay for heat if we can make it with the sun? Wouldn’t a passive solar design help to make conventional modern ways to heat the home obsolete?

We still have a lot of planning to do, but we have some of the basics down as well as most of the materials needed to work on this project in the next month.

{ 3 comments… read them below or add one }

Christine Baker October 25, 2012 at 10:42 am

I can’t wait to see how that works out for you. We have the same layout and I added an adobe addition to the south side for seedlings and to overwinter all the plants that would otherwise freeze. We never heat this addition and it’s never been below 40 F, even when it got down to 4 F at night and we had several CLOUDY days. The adobe is great.

The house also stays cooler in summer and warmer in winter.

There’s a window to the kitchen from the addition and I had planned to leave it open to let heat into the house, but that’s not working as planned because there are always gnats since we also have the worm bins in the addition. And we don’t want to put up the window screen because the worm food, containers etc. are handed through the window. Now that I think about it, I’ll try putting the screen back on and just taking it off at night when I usually work out there and we need the window open. It’s not that big a deal to take the screen off. It’s cold in the house right now, so I’ll go looking for the screen.

In spring and fall it gets hottest in the addition since the sun is low and in summer it isn’t as hot as I had expected.

We’ve been using the 1 gallon plastic bottles in the greenhouse and sprayed them black. But they only last about a year before they break. I had never considered using glass bottles, was worried they’d freeze and break. Wish I’d had a test bottle out there when during the cold winter of 2010.

Looking forward to updates on this project.


Angela aka Farmer Jane October 25, 2012 at 11:10 am

The reason we considering sequestering the plastic bottles, essentially encapsulating them is because of the heat. I didn’t want them directly in the sun where they could break down easier. I’ve see milk jugs and other gallon containers exposed to heat only make it maybe a full year, but then they need to get rid of it.

The glass bottles we’ll be using are mainly as an insulation. They are often used as insulation on the bottom layer of cob ovens. We thought the same could be used for the bottom of our passive solar area.

We’d also like to use the reflective insulation during the spring and summer months to cover the passive solar glass. With the area protected from the summer sun, a thermosiphon loop can still be used throughout the summer, providing a draw of cool air from the same passive solar area, cooling down the interior of the house, since the bottles will remain cool without any contact with the sun. I’m not sure how much it would contribute to natural air conditioning, but its worth a shot. ;)


Christine Baker October 25, 2012 at 3:18 pm

I can’t imagine how you could cool a house from the SOUTH side. You have a basement and that should have cool air, but not enough to cool a huge house like yours.

As soon as I looked at our kitchen window I realized that we hadn’t been using the window screen because there are all these plants in the addition on a shelf right in front of it and it would be very hard to get it in and out. So I got some mosquito screen and cut it to size. That will work perfectly, I’ll just tape it to the top of the window and rig it up so the gnats can’t get in but we can lift it easily. Since there were no gnats today, I opened the window and it keeps the addition much cooler, way better for the plants. Can’t believe we didn’t do that in previous winters!


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