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[EnergyPlus_Support] Mixed air setpoint manager -- bad mixed air setpoint calculation

My mixed air setpoint manager is calculating a very low setpoint, i.e.
less than freezing, subsequently causing the economizer to suck in
lots of cold air in the winter.

For the first ten minute time step on Dec 15, relevant system node
temperatures are as follows:

Outdoor Dry Bulb [C]: 1.7
Return air temp[C]: 13.2 (after return fan)
Mixed air setpoint temp [C]: -5.3   <---------- !!!
Mixed air temp [C]: 1.7
Supply fan inlet temperature [C]: 1.7
Supply fan outlet temperature [C]: 1.9
Temperature at reference setpoint node[C]: 12.7

Question 1: What could possibly cause this?
Question 2: I am usig 6 time steps per hour and noticed big
fluctuations  from one time step to another for a number of output
variables. What could be causing this instability?

Many thanks,

Brian



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