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Using a temperature sensor to maintain a boil generally doesn't work well. If you set it to 250F it will likely be too vigorous and boil over. If you set it to 212F (assuming sea level), it will likely oscillate between not boiling and too vigorous. You will want a device that will let you manually set the power level. PWMs and PIDs with manual mode can do this, as well as software-based controllers if they are programmed to do so. These typically drive an SSR, and setting it to, say, 60% would have the SSR switch quickly so that the elements are firing 60% of the time. Another option is an SSVR, which allows you to regulate the voltage itself.
I would suggest a PID with manual mode with an SSR. You can have it maintain mash temperature automatically, then switch to manual mode for the boil.