I simulated a primary/secondary HW loop with an ideal, UnControlled Heat Exchanger in OS1.9, and check three outputs: "Plant Supply Side Heading Demand Rate" for both primary and secondary loop, "Fluid Heat Exchanger Heat Transfer Rate" and "Plant Supply Side Unmet Demand Rate".

The "Plant Supply Side Unmet Demand Rate" is negative, which is about 5% of loop demand rate. According to IO Reference, "The value is positive when the plant equipment cannot meet the setpoint, and the value is negative when the plant equipment provides more than enough heating or cooling to meet the setpoint. A negative value can happen when the amount of demand is smaller than the minimum capacity of the equipment using the minimum part load ratio." In my opinion, this means Supply Side Heating Demand Rate should be higher than demand side heating rate (which is "Fluid Heat Exchanger Heat Transfer Rate") in this case. However, results show that supply side heating demand rate is lower than heat exchanger heat transfer. Anyone knows the reason?