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September 19, 2017

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Building a Solar Thermal Tank

January 10, 2018

This is part one of a series of post about my solar hot water system. Make sure to follow along!


I built a large water tank in my crawlspace under my home.  This is a wood-framed and insulated tank connected to a solar system for heating water.


The crawlspace under my house is fully inside the thermal envelope.  There is an R-26 of foam board insulation at the ground level, and no insulation in the joists.  So, the crawlspace is always within a few degrees of the living spaces year-round.  Being inside the thermal envelope meant I didn’t have to worry about any freeze issues.


Dimensions of the tank were dictated by the space I had to work with.  The height is 29 inches to the underside of the floor joists, and about 8.5 feet from wall to center girder.  I still needed enough space to crawl around the tank.  In the end I framed the walls 23 inches tall, and made the outside framing dimensions 66 inches. 


The structure of the tank is held together with five “ribs” making a square around the tank.  There is a layer of plywood inside, that spans from rib to rib, and holds the ribs in place.  I’m not a structural engineer and never performed calculations on how much wood was needed for strength and stiffness.  I just made what felt good to me.


The ribs were made by from some old 2x6’s I had salvaged from a building demo.  I ripped a straight edge using a jig and my circular saw.  Then I ran the boards through my table saw to get 2x3’s.  I used a dado stack in my radial arm saw to cut half/lap joints in the ends.  I have a video on this here:




I brought down the individual pieces of wood through a trapdoor and assembled everything in the crawlspace.  The ribs are glued together at the half-lap joints.  I stapled the ends together with pneumatic 1.5-inch staples.  Then I screwed the plywood to the inside face of the ribs.  I spaced the ribs tightly near the bottom of the tank, and farther apart near the top.  I figured there would be more water pressure at the bottom.  To see all the details of framing the tank watch this video:




Tank Construction in Crawl space.






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