hi Acuario I think that you should check this page for defrost algoritm if you have not solved it yet
harizanov.com/2012/02/defrost/
basically he incorporates an outside humidity sensor and changes the time cycle for defrost based on relative humidity.
so his plan is to:
No defrost above +5 degrees C
Default time between defrosts (Td) is 45 minutes of accumulated heat pump work time.
If air humidity is <65% then increase time between defrosts by 135 min
If air humidity is between 65% and 75% then increase time between defrosts by 90 min
If air humidity is between 75% and 87% then increase time between defrosts by 45 min
If air humidity is between 87% and 99% then increase time between defrosts by 0 min and BREAK further processing (humidity too high to risk defrost delays)
If outside temperature is between 6 and -4 then increase time between defrosts by 0 min
If outside temperature is between -4 and -7 then increase time between defrosts by 45 min
If outside temperature is between -7 and -99 ,increase time between defrosts by 90 min
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