cavie

Good to hear the less expensive #6 copper will work :-) Thank you very much !

Why don't you just turn on the sink, let it run and see which tank starts to fill up first ?

I just checked this AM. outside temp is 44. Inside set point 74. T-stat saying 74 and holding. Unit is on. 8 kw strip. Drawing 2.5 amps so I guess I'm good to go.

You can always just disconnect the back up wire and connect it again if you should need to. I suspect the Ecobee connects to your wifi and so gets the outdoor temp from that unless they wired in a remote sensor. I doubt that because you said you had the stat before this unit was installed.