Water node mass flow rate in EMS

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There is a typo in the third input field for Output:Variable or Output:Meter Name. The WaterHeater:Mixed object type represents a water heater that provides hot water. From its list of output variables, it looks like you want to use Water Heater Use Side Mass Flow Rate. The severe error you are seeing is because the Water Use Equipment Hot Water Mass Flow Rate output variable that you tried to assign is actually for a different object type -- WaterUse:Equipment, which is used to define individual components that use hot water (showers, sinks, etc.).

As mentioned in the two posts I linked to in my comment above, the third input field of the EMS sensor should match with a viable output variable or meter that you can find in the report data dictionary (RDD) output file generated by EnergyPlus after each simulation. One downside of the RDD file, however, is that it only gives you variable names without the object type that is generating each output variable. If you want to confirm that information, then you should read through the Input Output Reference documentation. Each object type has it's Input fields described, as well as each Output variable the object type can generate -- hence the name Input Output Reference.