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Or maybe a better idea. Put a temp sensor at the max exceptable frost height close to the coil and other in free air. if that readings are close, no ice. If they are different that means the one close to the coils is insulated by ice and start defrost cycle.
Hi, Love the project. I think there is a easier and more reliable way to check for the outdoor coil icing up. Taking temperature measurements at the start and end of the outdoor heat exchanger. As the temperature difference gets smaller it means the heat exchange rates are getting lower (icing, dirty coil, outdoor temperature to low, bad condenser fan...) I don't know how reliable that ice sensor is going to be if it gets dirty or icy.