The issue is that the miner has to turn off if it's too hot. Already heated water is effectively heating the miner. Heat shortens the lifespan of the device.
It is more complicated, but two systems, one that directs the flow through to the normal heating vessel, and have it's heater dial down so it let's the miner provide a baseline and a secondary flow path to a radiator with fan once the water temp rises above the optimal max for the miner.
Or to integrate the thermostat controls so it turns on the miner at low temp ranges and flips to the resistive element is the tank when it passes a threshold, then the miner is baseline and the element is backup and tops off to reach max.