Building a Solar Thermal Tank

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: https://youtu.be/jKr4An8m_tM

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: https://youtu.be/jeXR7o68c0Q