I use a dual tank set-up- and it works great! The first tank is preheated by the superheater in my geothermal heat pump. In the summer, this provides about 80-100% of our hot water needs. I used to have my drain back solar system plumbed into it, but I get a lot more efficiency from the GT heat pump by cooling off the desuperheater.

This tank is just a 50 gallon standard dual element hot water tank. It was a "dent" special from Lowes for $50.

The output of that first preconditioning tank then goes to a tankless water heater for a small boost (if needed).

My ground water temp is about 60F and I need 110F water - delta t of ~ 50 degrees F. In the summer, the temp is often 100-105F (or higher) in this preconditioned tank, so I save some 80-90% on water heating costs.


I do have the electric resistance elements hooked up in the preconditioning tank for back-up, but I have literally never turned them on. This tank is wrapped in fiberglass, then scrap bubble wrap and finally some re purposed laminate fashioned into a cylinder (additional R40) surrounding the extra insulation. Put it on a safety pan with a drain hose and it just sits there and has for years.

At installation, I did put in a bypass (with unions) for the tankless so that if I need to remove the tankless for service/replacement, then I still have hot water.

Very basic, inexpensive and it works well. I can't understand why a air source geothermal hot water tank then feeding into a tankless would not offer similar attributes.

Steve

Here's what we can tell about the site and OP:

Two sources of energy exist, not including solar: electricity, propane. This suggests a location outside a municipality, due to no natural gas availability. Usually, this is a favorable condition for solar, no HOA problems, less stringent building codes, etc.

OP is using all available sources of energy. He is trying to improve the overall efficiency of the system. Serious consideration must be made before chopping up a working rig. Otherwise, the expenses add up in a hurry. Cobbling things together doesn't usually turn out well. Resistive electric tank is burning a hole in the bank account.

Future solar plans: wish-I-coulda-hadda. Not going to happen this cycle.

For today, a possible upgrade could be a new split heat pump (minisplit or whole home) with a desuperheater plumbed into the existing water heater tank. Another could be a retrofit HPWH such as the Airtap unit Xringer has chronicled. In tandem with a gas-powered tankless WH (as steve just mentioned), lots of electricity would be saved over resistive heating normal DHW during modest usage events. When a high-usage event comes along, the tankless unit could serve as a backup source.

A PV module could be added now or later to directly improve energy savings. This would not affect operation of the rest of the system whether the sun was shining or not.