How are the Zone Component Loads Summary fields calculated?

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When there are multiple pieces of equipment serving the same zone, EnergyPlus will not automatically split the load. The user must control this. There are a variety of places to specify load fractions depending on the specific equipment type: DesignSpecification:ZoneHVAC:Sizing, DesignSpecification:AirTerminal:Sizing, Sizing:System. and in some cases directly in the zone HVAC equipment object (e.g. ZoneHVAC:Baseboard:RadiantConvective:Water Heating Design Capacity Method).

The zone equipment "priority" (which is old terminology, current field names are Zone Equipment 1 Cooling Sequence, or Heating) is really just the order in which the equipment is simulated. It has no impact on sizing calculations.

Without knowing the exact types of objects used here, my best guess is that you have an AirloopHVAC providing the ventilation air. The following change will tell the air loop to size to the ventilation flow rather than sizing to meet the heating and cooling loads.

Sizing:System,
Airloop1, !- Name
VentilationRequirement, !- Type of Load to Size On

However, this may not be enough since the heating supply air temperature and the zone heating setpoint are both 70F. Even with VentilationRequirement, the zone sizing first attempts to calculate a heating design airflow rate, which will be near-infinite with no deltaT. I would suggest setting the Sizing:Zone Zone Heating Design Supply Air Temperature to something greater than 70F to get a reasonable airflow rate which will later be proportioned by the system sizing to match the OA requirement. The system design supply temp and control setpoint can still be 70F.

Also, if you want to allow for the 55F ventilation air when sizing the radiant panels, in Sizing:Zone, see the Dedicated Outdoor Air fields.