However, I'd guess that the system won't be at a temperature outside the commercial or industrial temperature range all the time. Once the system is started, so long as power doesn't fail, the low-cost MCU can control the temperature. The crux, I suppose, is figuring out how to manage the situation in which the power does drop out at low temperature.

Restarting at low temperature means that the heater has to come on when the MCU or whatever controls it isn't operating, yet only when it's too cold, and that mechanism has to ensure that the entire system is within its safe operating temperature range before attempting to start it.

Further, there's the risk that the low-cost MCU fails. it's probably necessary to provide for that in the main MCU. I'm thinking that the main MCU has to ensure the system doesn't get too warm, and the auxilliary MCU has to prevent it getting too cold. Something has to ensure that the system is within temperature range before it's started. That means it has to be independent of the MCU's, oscillator, etc. so that no component is being operated outside its specified safe operating temperature range. There's probably a relatively simple way to do that, and the auxilliary MCU might not even be needed.

RE