How does this work with interrupts, say in an embedded context, that execute on the current stack, and that may in some cases, be interrupted themselves. Do you add the maximum stack depth of all interrupt routines that could go off at the same time?
I read the firmware, it's build with the Arduino IDE and PWM for the heater element is bitbanged and updated from the main loop. Meaning it can easily get stuck sending a MQTT message over wifi when it loses connection, causing the heater to possibly be left on indefinitely.
A neat project for sure but as it stands this has major safety issues. The control logic is able to lock up and keep the boilers heating element in an forever "on" state till something snaps. I couldn't find a mention of safety in the manual nor see any mechanical safety failsafes in case of overheating.