Pipe the cold water to the keystoker and back again to the cold water inlet of the water tank. Put a valve between the two tees and in each branch line.
but but but -- opps there goes that pesky motor boat again
What if the tank sits all day when folks are away - there is no chance for thermal siphon on the tank or replenishment of lost energy in the standing vessel ? Why is there even any hot water in the morning after this design standing stagnant all night - unless the DHW tank heat train is still active and this arrangement is only to augment energy used during peak use and washing?
But then -- whats the use ?
I value your advice steamup
You are correct in noting that there is repleshment of loss from the tank in non-use hours such as daytime work and nighttime sleep. Standby losses can be significant is some cases. However, I have found that my tank hardly ever, if ever, ran - mainly due to the much hotter water being dumped into the tank from the tankless coil. It takes a long time to cool down to the point the water heater kicks on.
My tank is over 15 years old and is PVC vented power draft type. No stack losses in the off mode. Good quality unit with fair tank insluation.
Newer, better quality electric tanks are insulated with non-cfc foam and really hold their heat.
I figure the use of a little standby energy was much cheaper than pumps, wiring, controls.
With all that said, my new coal arrangement does'nt permit me the luxury of a simple setup. I am going with a heat exchanger off of the boiler return water, bronze pump and a preheat tank. Much too complicated but I got a good deal on stuff off of Flea-Bay at the right time. A indirect water heater would be my best option but they are too pricey for my wallet at this time. Could'nt resist another science experiment.