Efficient DHW plan for a soaking tub? – GreenBuildingAdvisor

Hi all, 

My efficient DHW plan involves:

1) a sanco 2 heat-pump water heater

2) a compact, insulated, home-run distribution

3) low-flow fixtures…

…except for a soaking tub! I’ll be building it myself to fit the space, I’m estimating ~100-120 gallons, with a 1/2” tub filler fixture. 

I had initially planned to install the 119 gallon tank (first hour rating 135), but learning that this tank is not stainless steel, and we have hard water.  The 83 gallon (first hour rating 115) is stainless steel. As far as I can tell, the first hour rating accounts for the additional volume of water heated during use, but NOT the additional volume from a mixing valve. Since the sanco tank produces 150 deg water I’m guessing this will add a significant amount, but not sure if it will be enough to fill a 120 gallon tub. 

So, one option is to add a small electric tankless for just the tub (all electric house so no gas), but this will probably be inefficient, although I do have space for it.  

another option is to add a second 83g tank, but don’t have the space for that, and it seems overkill.  Are there other options I’m not aware of?  

thanks!!