The Passive Solar Heater Project

Why the passive solar heater project?  Well, not long after summer, when our electric bill becomes reasonable again, our natural gas bill begins its rapid ascent.  I wanted to find a cheap way to lower the the heating bill, so I started poking around online at solar heater plans.  Online, you’ll find may DIY solar heater plans that leverage recycled materials like pop cans, scrap wood, and old windows to make a cheap, supplemental home heater.  I figured I’d give it a go myself.

This year in particular has been very cold, most of the USA experienced extreme, arctic cold early on (even before the official start of winter).  Prior to Christmas, we experienced a solid week of subzero temperatures and even had one day with the -6 F for the high.    It doesn’t matter how well insulated the home is when it’s that cold.  On days like that, the furnace runs nearly nonstop, burning a constant stream of natural gas.  Next month’s gas bill is going to be a beast.

The Passive Solar Heater Plan

After culling through all of the online solar heater plans, many of which are brilliant, I decided I’d go a different, potentially less-brilliant route.  When I looked at the resources that I had on-hand,  I found two lousy pop cans – not gonna work.   A dual-soda-can solar heater, though far easier to make, would not provide sufficient home heating.  Now, had I used some forethought and planning, I would have been saving beer and soda cans for some time now.  My wife can assure you that, in fact, an ample supply of beer cans did happen through our household in the previous few months; they were all recycled.  If you want to make the solar heater on the cheap, plan ahead.

There were two requirements for my solar heater plan at this point.  They are:

1.  Keep it cheap.  Since I had to acquire the resources to build the heater, I defined cheap as under $50 for the entire build.
2. Keep it easy.  The solar heater plans I reviewed online weren’t necessarily difficult, but they did look to be labor-intensive – boo!   I don’t like my piddling-around at home to be too laborious, so the idea of drilling out the bottoms of 100 aluminum cans and connecting them together didn’t appeal to me.

Solar Heater Materials

Also, many of the plans online seemed to instruct folks to build the frame of the heater with plywood or foam insulation.  It seemed to me that using cement-board would be the better choice.  Being the armchair scientist that I am, it was readily known to me that rock retains heat for longer periods of time than other less hard/dense surfaces like wood.  Cement board seemed like the most efficient substitute to somehow affixing stones or rocks to the interior of the heater.  So, I bought a sheet of cement board and some other materials from the local home improvement store.  Here’s a rundown of my material purchases and their cost:

• Cement board – $9.50 (You’ll find this in the drywall section of your local home improvement store.)
• 2 standard 2×4’s – $4.25
• 15′ flexible dryer vent – $12.60
• High-temperature black spray paint – $4.45

My total materials purchase for the solar heater project came to $30.80.  I did not purchase any glass or plexiglass to cover the heater as I was certain that I had an old glass storm window saved in the shed for this very purpose.

Assembling the Passive Solar Heater

The first step in this process is to assemble the frame of the heater.  Using a miter saw, I simply cut

the four 36″-long sections of 2×4’s and screwed them together into a square.  To reduce the chances of splitting, you will want to pre-drill the holes where your screws will be installed in the frame.  With the basic heater frame assembled, it was time to attach the cement board.

A box cutter is the only tool you will need to cut the cement board.  It helps to use a portion of 2×4 as a guide to score your cut line along the surface of the cement board.  Unless you are something of a brute, you will likely have to re-score the same line a few times to cleanly cut the cement board.  I found that scoring one side of the cement board then carefully cracking the board along the scored line was most-effective.  You may need to follow-up with another cut along the other side of the cement board to cut the board’s plastic mesh.

I tried to maximize the use of the cement board and cut one large section to fully cover the back of the frame.  I used the remaining section of cement board to cut four additional side pieces.  All of the cement board pieces were attached to the 2×4 frame using a combination of screws (specifically designed for concrete board installation) and liquid nails.  I did have both of these material on-hand when I started the solar heater project.  Using either screws or liquid nails alone would have likely been fine, but since I had both available, I figured why not.

The completed frame with attached cement board looked like this:

Next, I needed to add the flexible venting to the solar heater box.  To do this, I threaded large zip ties through holes that I had drilled through the cement board.  The zip ties were then wrapped around the vent and secured at each bend.

Be sure that you are fully expanding the flexible vent as you go, this will ensure that you have enough vent to serpentine throughout your entire solar heater.  Once all of the zip tiles were secured, I drilled and cut a vent inlet hole at the bottom of the cement board backing.  I then did the same for the heater outlet hole, but this was done at the top of the solar heater cement back.  The idea is that the cooler air will be drawn in at the lower inlet, and since heat rises, the heated air will be expelled from the outlet at the top of the heater.

Here is what the solar heater looks like at this point (prior to spray painting):

You can see the black spray painted solar heater at the top of this post, but that is not the completed box yet.  It turns out that the glass storm window, that I had thought I had saved in the shed, was not saved.  Agh.

More information and results of this project coming soon.  Check out more about passive solar heaters and heating, here.