tank in a tank heat exchanger

[BruceM]

No copper inner tanks available here, though I did think of using aluminum beer kegs before deciding to just spend a bit more and go with copper tubing for it's low risk and longevity.

If you'll check the thread above, you can see my copper tubing exchanger, 180 feet of 1/2" soft copper (3 rolls), configured as 4 parallel loops, fed from bottom to top.  It has worked well for my domestic hot water.  It's been a nice luxury to use "endless" hot water for laundry and such since this spring, when the system became operational.  My washing machine now annoys me as it does not allow for a hot or warm rinse unless I fill the tank manually.  I have not turned on the propane backup, since I have excess capacity for just domestic HW.

The in-floor heat is run direct from the main storage tank, with no heat exchanger. The big question to be answered this winter is how often will I have to use the backup propane hot water heater.  At 5600 feet elevation in the White Mountains of AZ (central eastern) we do have sunny but real winters with sub zero temps and snow.

The copper pipe in the heat storage tank above the water line has a dark black oxide layer- seems 150F temp and the steam from when the system first starts is a fairly oxidizing situation.  No change to the copper in the water.  In the 6 months of operation, no smell of bacteria in the tank water- so far.  Perhaps just the copper will be sufficient.