May I suggest Chris that this idea is too darn complicated, expensive, and less than fault tolerant for home use.
Ill suggest something cheep and complicated -- safer
Buy a larger electric water heater. Don't wire it up just place it in line before your current heater for a buffer tank. Now circulate both
tanks with a bronze pump off each output port - ( place a valve to slightly restrict flow to the pump from the primary) into your plate exchanger or DHW boiler leg. With a strap on Honeywell strap on stat on both outputs of each tank to start and stop the one circulator. Set the stat on the primary tank to switch off at 120 and the buffer tank to switch off the pump at 110. Simply accept the built in differential for close on temperature drop. Wire in parallel
to control the pump. The stat on the buffer will control the launch - the one on the primary will control the halt.
In this arrangement you get reservoir of hot water (the primary tank) to use before the boiler is called to service by cool output of the buffer tank - exciting the bronze pump and beginning the charging cycle -
This all happens in about the same time frame as your current system --- but you still have DHW service from the primary heater as the process begins postponing your need for heat DHW heat from a boiler that is ramping up to meet the demand.
Another feature and benefit is if you can circulate the DHW in your home system -- then you have almost instant hot at the lift of any hot water faucet - and its safe temp because circulation stops at 120 - stopping water temps of the DHW that can build above 120 from a boiler idling and a min of 140 --- water above 125 is not safe to use at a faucet.