I can't believe that one can do better than this heat exchanger here:

$1000 solar water heating system -- storage tank

Like others, I would be concerned about cross contamination of anti-freeze with the drinking water at the solder points whereas, if one uses intact coil of pex tubing, it has plenty(!) of surface area for heat transfer, it holds a good bit of water in the tubing as a reserve, and I wouldn't have reservations about its integrity of the tubing and cross contamination (although they don't use anti-freeze, the water in the storage tank is not exactly potable having sat in the tank for a year or more).

I would have put a section of finned baseboard on it, thereby getting some free radiant heat on the vertical run. ( I have some lengths that I scrounged somewhere) FYI

I can't believe I missed this thread! Sorry to revive something long dead, but I didn't do it first...

Ben, how did this experiment turn out? Did it work? If so, did it do well? Did you retire the rig? I'm interested if you can't tell.

I know from experience that this type of supplemental heating can work really well. Not just for solar, but for other types of heat sources as well. From a design standpoint, maximum efficiency of the heat exchanger is not paramount. With a low-grade heat source, there are only a certain number of raw btu available to transfer each pass through, so even with a superconducting exchanger, the gains are not enormous like with steam or a flame. As long as the heat doesn't leak out of the loop, it will come back through on the next pass through.

With this "shotgun" heat exchanger, the best way to improve the efficiency is easy. The unit can be built just like Ben made his, then a length of refrigeration-grade tubing (or a quadruple helix or what have you) can be coiled around the straight length of hard pipe, in between the tees. The heat transfer fluid line can be teed off at the supply and return ends, so that the fluid flows both through the center of the water pipe as well as around the outside. This greatly increases the surface area of both loops, and allows for a higher flow rate through the heating loop. Both factors increase hx effectiveness. If you feel the need to maximize contact area on the outside wall, and don't want to solder the tubing to the pipe, toothpaste or zinc oxide ointment are nearly as good at conducting heat.

I also want to do like this something new for solar hot water system. But let me know that how it's working exactly and the materiel what should we collect how to start this? How many problems you face during this period?

"If you feel the need to maximize contact area on the outside wall, and don't want to solder the tubing to the pipe, toothpaste or zinc oxide ointment are nearly as good at conducting heat."

Jeff, with heat do these two materials get runny? I guess I could do the experiment and put some toothpaste in the microwave . . . .

This is a great suggestion as the standard heat conducting gells are expensive.

Thank you for this quick, easy and inexpensive hack.

Steve

The zinc oxide stuff is normally oil-based and it depends on what product you use. It is much more like heat sink grease than toothpaste. Once the joints heat up, surface tension sucks the compound into all the nooks and crannies. I usually use the chapstick stuff on close-fitting joints due to its tenacity. The fish oil and diaper rash stuff works well on joints that move against each other due to dissimilar expansion. Neither of these two really harden completely.

OTOH, toothpaste is a hardening product. It doesn't flow out when it heats up. Over time, the water evaporates out, and it ends up like stale polishing compound, cured drywall mud or hazy car wax. Being water-based, it is much easier to rework or clean up later.If the dried assembly is subject to rattling or dissimilar expansion, the toothpaste will eventually lose contact with at least one surface and crumble off where it can.