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Dear all, Thank you very much for all the inputs. It seems that the major reason for all the unmet hours was that the VRF system has fixed fresh air supply and because its availability schedule has been set to "always on", it kept dumping fresh air into the spaces even at night. This caused the indoor temperatures to drop to near outdoor temperature giving the system a hard time during morning start-ups for a given heating capacity. I created a reasonable on/off schedule based on cooling and heating schedules for the VRF system and the unmet hours decreased significantly in most thermal zones. There are still high unmet hours in a few individual zones which I am looking into at the moment but that could be caused by some of the reasons mentioned in other comments in this thread. Regards, Daniel