pex dhw coil

I had a similar thought a couple years back. After running a few numbers regarding the amount of pex (or copper for that matter) required and the potential payback period involved it made very little sense. I think you'll find it's not worth your time or money to proceed with this kind of rig....

Pex will work for heat exchanger tube but not nearly as good as copper or other metal(obviously). For what it's worth my coil-in-tank ex. is pex(700-800 feet I think) and handles
60 kbtu fine.

However I think it will be a waste of time wrapped around the outside of your tank, to little contact area for pex...

You might get tap water for hand washing based on the volume within the pipes, but the heat exchange is going to be anemic
at best. It will not keep up with a flow, such as a 5 minute shower.
Spraying foam over the tubing will diminish the heat exchange since there will be less area of the tubing that is heated.
Copper would be better, but it will still not be very good.

If you are looking for some decent heat exchange and don't want to get into the tank, use a plate hx to a small tank. A couple
small pumps will move a lot more heat out of the tank.
It is more complicated, but you will have a decent shower (if that is a goal).

There are a number of heat recovery devices that wrap copper around a drainage line that do some fairly good exchange. For the cost it is actually a pretty good return on investment. The link below shows some of the performance on one of the brands.

Also the Rheem SolarAide solar tank, been around for decades, is a 5/8" copper coil wrapped around a steel tank. With an external coil the get around the double wall HX requirement. Not as powerful as an internal coil but a bit less to manufacture. Check the second link below.

If you have the copper, why not. Be tough to retro fit after you foam


www.renewability.com/uploads/documents/en/gen_res_bro.pdf

https://shop.solardirect.com/pdf/water-heaters/tanks/solaraide-he.pdf

I'm reluctant to contradict Tom on this one, he seems to be the expert in these areas, but if you want to try it, make sure you use something to get better contact between the steel and the pex besides the "tangential" contact from wrapping it. Foam is an excellent insulator and the heat will flow mostly through the one point in contact with the steel. doitsolar.com recomends silicone for a bedding compound for homemade solar collectors. Sprayed on mortar would also work, mortar is a poor conductor, but far better than foam.