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-   -   Designing & building a solar hot water tank (https://ecorenovator.org/forum/showthread.php?t=1608)

menaus2 05-18-20 08:29 AM

Never too late to start again! I like how this project is half Marie Kondo half Engineer. :thumbup:

Daox 05-25-20 08:28 AM

I've been working on this project a little bit here and there. I ordered the EPDM tank liner. 90 lbs of rubber! It'll be fun wrangling that thing around haha.

I'm also starting to think about how I want to do the heat exchangers on this thing. I'm going to need 3 different loops. The first loop will be to put heat into the tank from the solar panels. The second will be for domestic hot water. The third will be for heating purposes. I like the idea of a big PEX heat exchanger like I've seen on build it solar for DHW setups. However, I'm not sure that'll work for the other two loops. Do you guys have any suggestions?

menaus2 05-25-20 09:44 AM

I've been impressed with how copper heat exchangers have worked on my tank. You can always buy copper wort chillers on Amazon if you want something premade that will look nice. They can be run in parallel if necessary. Running your space heating on an open loop using tank water directly would be 100% efficient & save exchanger space, provided you aren't worried about it freezing.

jeff5may 05-25-20 06:38 PM

The material you use for heat transfer makes a massive difference. Copper pipe is the king when you consider raw heat transfer and time lag versus dollars . Pex is about 6 times worse per surface area than copper at heat transfer, and also introduces lag. If you have the volume, pex will work (best for slab heating and ground loops). For domestic hot water, and other high drain, not so steady-state processes (quick, high dT/dt pulses), copper pipe is the cool kid.

Rule of thumb for copper is 100 feet of 1 inch will reliably move 100k btu per hour with a dT above 25 degF, steadily, forever. Two parallel runs of 3/4 inch, four runs of half inch, move equivalent amounts of heat. With PEX or poly pipe, the bigger the diameter, the worse the heat transfer. So if you push your luck, half inch might have a factor of 4 or 5 times less effective.

The 1 inch and larger diameter poly pipe has a much, much thicker wall. This thermal resistance and slow flow isn't insignificant. A 1000 foot spool of 1 inch poly pipe will not let you take a 15 minute hot shower with a 150 degF thermal store. Once the stagnant water in the pipe runs out, the water comes out lukewarm. I'm not talking about an eco low flow, locker room showerhead, and a crispy cool shower here.

If you need fast, high density, compact heat transfer, it's hard to beat a brazed plate heat exchanger. If you're thinking about a beer chilling thingy, they work great for sea water. Both are pricey compared to a roll of copper tubing for the raw BTU capacity.

WillyP 07-20-20 03:13 PM

I've been reading this thread and want to toss in my two cents. I build a lot of wooden stuff. If you are looking for the best structural bang for your buck, invest in high quality wood glue. "Woodtight3 is awesome stuff that wont change your fitting dimensions (unlike liquid nails). The level 3 adhesive is water proof, but shouldn't be submerged in water (not that you would do that). But it is what you want to use anywhere near water.

The metal joiners are almost a waste of money. You are much better off just double lapping the wood and using good glue.

On a 4X8 plywood side, you should really put a support at least every two feet. Two by fours are really cheap. Adding a couple of extra 2X4s, can prevent some serious problems latter on. Glue them to the plywood, then add a couple of screws, and you will have a wall that will hold a lot of water.

It is a really good idea to rip about a quarter inch of wood off the 2X4 (if you have a table saw) This will give you a much tighter edge. Even though the wood is thinner, it will be stronger because the (glued) fitting will be tighter.

Always drill a pilot hole when using wood screws. The pilot hole should be the same diameter as the solid bolt under the threads. A good test is to hold the bit on top of the screw. You should only see the threads of the screw. The switch it so the bolt is over the screw, the bit should disappear (from view, not reality).


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