Well it took a little over 40 years starting with a stoker, 30 years with a hand fire and back with a stoker for the past few seasons.
Heated my dhw on and off thru the years with countless combinations of coils and tanks and I think I finally got it tuned in right where I want it.
I only wish I ran across this fine site a few years ago.
Up until this last cold spell my Alaska 140 dual paddle feed was doing a great job maintaining my dhw between 140 and 160 degrees.
The cold spell pushed the temps up to 180 in the water heater. At night I would manually valve it over to the tempering tank bringing the uninsulated tempering tank to 120 on a few cold mornings.
I installed a upper element thermostat from an electric water heater in my existing water heater and set it at 150 . When it reaches 150 it controls 2 solenoids one N/O and the other N/C diverting the coil water to the tempering tank. My Aquastat is set at 170 with a 10 degree differential to control the circulator.
Working like a charm