From the Engineering Reference documentation for the Ideal Loads Air System object:
The supply air flow rate is varied between zero and the maximum in order to satisfy the zone heating or cooling load, zone humidity controls, outdoor air requirements, and other constraints, if specified.
Besides maximum supply flow rate, you also input max heating supply air temperature. EnergyPlus then determines what the supply flow rate and air temperature need to be in order to meet the calculated heating load. Your error warning is stating that the REQUIRED supply air temperature in heating is equal to the MAXIMUM supply air temperature input. This happens when either the maximum flow rate or maximum heating supply air temperature input is too small for the load. Since 50C is a pretty high heating supply air temperature, I'm guessing that your "Maximum Heating Air Flow Rate" input field is too low (this input field can be set to "Autosize"), or that you are limiting the Heating Air Flow Rate.
It's interesting that your error message says this doesn't occur during Sizing, but does occur during Warmup and the annual simulation. Some things to check in your model:
- Are you defining a heating design day in your model?
- If you are, are you including 'WinterDesignDay' in your internal gains schedules (equipment, lights, people)?
- Is the Availability Schedule referenced by the Ideal Air Loads object set to 0 during 'WinterDesignDay'?
