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How are the Zone Component Loads Summary fields calculated?

asked 2018-08-29 10:40:42 -0500

updated 2018-08-29 10:46:34 -0500

I have a radiant panel system with a dedicated 100% OA unit. The OA unit supplies air at 55/70 for cooling and heating. The radiant panels do the actual zone conditioning and they are specified with priority 1 over the zone equipment for the OA unit. The space setpoints are 76/70 for cooling and heating.

We are doing sizing of the equipment, and to verify that the loads are correct we are using the Zone Component Loads Summary.

The field Main Fan Air Flow shows an astronomically high airflow during the heating operation. How is this air flow calculated? Is it taking into account the contribution of the zone radiant panels? It looks like it is not and it is assuming that the DOAS air stream is the one contributing to the heating of the space. We would be expecting the Main Fan Air Flow to match the Outside Air Flow.

I have checked the Engineering reference, the Input Output Reference, and the Output Details and Examples document.

I have attached a picture of the report section.

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answered 2018-09-05 20:08:36 -0500

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.

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Asked: 2018-08-29 10:40:42 -0500

Seen: 139 times

Last updated: Sep 05 '18