The Optimal Operation with building mechanical equipment has been taken notice due to energy saving, global warming, running cost reduction and commissioning concept dissemination. On the other hand there occur a number of operational faults to thwart the optimal operation after delivery of new building because appropriate testing, adjusting and balancing are carried out under restricted conditions. The series of paper illustrates how the optimal operation has come into effect in a high-rise office building for two-year operation after delivery. The fourth paper elucidates how the fault reform activities were carried out on the ice-storage system and secondary chill water supply.