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menaus2 04-18-12 11:42 AM

Pex Solar Water Preheat Project
I'm starting a drainback type solar preheat system on a rental property. I have the tank mostly built and that will be fed by a pex collector I have yet to build. Off to move the tank into the property's basement, I'll have pics up soon.

GaryGary 04-18-12 09:55 PM

You are probably aware of this already, but you want a steep tilt and only single glazing on the PEX collector to avoid stagnation temperatures getting high enough to damage the PEX.

This one is tilted at 70 degrees and still shows occasional stagnation temps as high as 230F, which is about a high as you ever want to go with PEX even in a non-pressurized system:
$1000 Solar Water Heating System -- Performance


menaus2 04-19-12 04:19 PM

Thanks for the tip Gary, I plan on mounting it vertically to get around the stagnation issue and btw I must I'm a huge fan of everything you do on

Daox 05-01-12 01:31 PM

Any updates on this?

menaus2 05-06-12 12:32 AM

Yes! I've been busy recently but I got my pictures finally downloaded onto my computer. Right now I'm probably 75% done on the Storage tank side and I'm generally following the $1000 design on I plan on having 166 sqft of collector area that drains back into a tank roughly 3.5ft x3.5ft x3.5ft of interior storage space that yields 322 gallons of storage capacity.

I started with 3 4'x8' sheets of 3/4" treated OSB as I wanted something strong, moisture resistant and economical. I cut them into 4'x4' pieces. The sidewalls rest flush on top of the edge of the bottom sheet. The wall widths are 3'11&1/4". This allows them to but up against the next one in a "circular" fashion (you'll see in later pics).

I screwed on the top and bottom supports (from the tank inside of course ;) ) using treated 2"x4"s I have them overlap each other for easier final reassembly. One thing in retrospect I wish I had done is to directly attach the bottom support to the base sheet as the sheet would take the outward pressure at the bottom walls better. To address this issue I simply added another layer of 2x4s to reinforce it more. Also, Torx bits are your friend :thumbup:

Here I glued and screwed the middle supports onto the walls. A good New Glarus Fat Squirrel beer makes things go much faster (or maybe it only seems lol ) I also used Gorrilla Glue brand woodglue, idk how much of a difference it really would make over regular woodglue, but I figured it couldn't hurt.

menaus2 05-06-12 09:03 PM

Disassembly to put 2 coats of sealing primer on each piece.

Detail of how the base rests on 4" of XPS foamboard.

Closeup of how the corners interlock. One issue I had was that after priming, it was harder to reassemble everthing cause the pieces liked to stick together.

Final check before moving the unit. The inside looks so neat and clean after the paint job!

menaus2 05-06-12 09:22 PM

Moving the pieces and reassembling in the apartment basement. Space was tight and had to fit in a space between 2 walls, a dryer and a basement jack. Definitely had some tetris moments.

Finally assembled!

I put nail plates to reinforce the outside supports as an extra precaution. You can also see how the interior polyiso insulation fits on the walls. I put 2" of polyiso on the floor of the tank first.

How the polyiso wall insulation is layered.

menaus2 05-06-12 09:39 PM

Tank with insulation.

Cut the insulation flush with the top. I also made up and printed a label and a some safety warnings, laminated them in 2 layers of contact paper and stapled them up.

Fitting the liner. I used a 15'x15', 45 mil EPDM sheet. Took some finagling to get it mostly fitted.

So that's where I currently am in the project. I have to tackle the lid next and then probably the pex heat exchanger and all the necessary plumbing.

Daox 05-07-12 09:08 AM

Wow, 166 sqft. That sounds like it should be quite capable of providing nearly all the DHW unless there are many people in the building. What is this setup going into?

Nice work with the tank too. It looks like it JUST fits! Are you planning on adding any additional insulation on the outside?

menaus2 05-08-12 10:26 AM

Currently there are only 3 people, but with changing renters that number could always easily change. The tank feeds into 2 hot water heaters (one gas one electric) that each have their own water supply. I debated putting extra insulation, and maybe I should have, but that's something I could do later with some spray foam and EPS. It might be tricky though with the tight fit haha.

I figure in 5+ years when the water heaters need to be replaced, I could use cheaper smaller electric models and super insulate those :).

I'm looking at a Swiftech MCP355 pump to lift the water around 16 ft up for both panels. It's a cheaper option around $70 USD but what do you think of the sizing and do you think it'll have enough head pressure to do the job? Looking at the PQ Curve I'm a little confused... MCP355 - Rouchon Industries Inc., dba Swiftech - PC Liquid Cooling Systems CPU Cooler VGA Water Block Heatsink Pump Radiator Heat Exchanger Kit

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