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Originally Posted by Acuario
It's been a while since I updated this post so I thought I would share my experience this winter...
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Great to hear from you again Nigel, and thanks for your good analysis.
I don't know if it will help your defrost project here, but I have noticed that in the cheap de-humidifiers I have disemboweled, there is a thermistor pounted to a small flat plastic mount that is inserted so that the plastic goes between the fins of the evaporator, leaving the thermistor just above the fins. The thermistor is positioned on the air-in side of the evaporator, in the fin area, close to the area that frosts up. It is not at the area where frost would first occur, but a bit further out. The de-humidifiers I have looked at rely on the evaporator coils frosting up, and the subsequent defrost cycle to melt, capture and contain the humidity that is being removed from the air.
I have wondered what temperature the thermistor sees... must be the freezing point of water. This would work in an interior where there was certainty that the machine wouldn't be used in freezing weather. You likely couldn't count on this for your heat pump.
Perhaps you could use this idea to sense the presence of actual frost to trigger the defrost cycle.
Optical maybe?
IR maybe?
Physical probe maybe?
Maybe something that could detect the cessation of airflow through the most likely part of the evaporator to frost up?
Best,
-AC