After a recent conversation with @joe90 about ASHPs frosting up, I have had a few minutes to think about this problem.  As usual, data is needed. Does anyone monitor the input and output air temperature and relative humidity of their ASHP?  This would be a couple of sensors either side of the radiator and in the airflow.  Ideally knowing when the ASHP is on would be useful as well.   My thinking of the frosting problem is to monitor the temperature and RH on either side and calcul