There are several components, conditions, and methods that come together to provide full Central Air System sizing results. All calculations are handled by the powerful EnergyPlus engine and reported back into the variables seen on the platform.
System Design Calculations based on the sizing periods for each day will start after the Zone Design Calculations are completed. The system design sizing includes the weather data for outside conditions at each time step, the components of the system, and results from the Zone Design Calculations.
This article provides an overview of the core calculations behind all results for the Central Air Systems. The full detailed EnergyPlus documentation is also available and will cover each equation, step, and variable used within the engine.
Primary Cooling Coincident Load
The Sensible Cooling and Latent Cooling loads are calculated as the sum of the zone loads at the corresponding time and outside conditions. These values include the Cooling Factor which is assigned on the Simulation Settings. The Total Cooling is simply the sum of the sensible and latent components.
The Time of Peak is determined by looping over the sum of zone loads at each time step of the design day. The Outside Air conditions is reported from the Weather File for the exact Time of Peak.
Note: This does not yet include the outside air load that will be included in the Cooling Coil sizing.
Primary Heating Coincident Load
The Total Heating load is calculated as the sum of the zone loads at the corresponding time and outside conditions. The value includes the Heating Factor which is assigned on the Simulation Settings.
The Time of Peak is determined by looping over the sum of zone loads at each time step of the design day. The Outside Air conditions is reported from the Weather File for the exact Time of Peak.
Note: This does not yet include the outside air load that will be included in the Heating Coil sizing.
System Airflows
The flow rates for supply air, outside air, return air, and exhaust air are measured directly at the corresponding node of the EnergyPlus Air Loop during the Simulation Sizing Periods. This represents the actual flow rate that is passing through the system at peak time. The time step used will impact the exact values reported.
Primary Cooling Coil Sizing
Full coil sizing is handled as part of the System Design Calculations. Follows is a description of each reported value and the corresponding source in EnergyPlus. The coil sizing is done at the rating point which refers to the operating conditions specified, such as enter air and liquid temperatures. In this way the rating point is equivalent to the reference or design conditions for the coil. A full description of rating point and the coil sizing reporting can be reviewed here.
Total Capacity - Coil Total Capacity at Rating Conditions
This is the result for total capacity while at the operating conditions from the full coil model in EnergyPlus.
Sensible Capacity - Coil Sensible Capacity at Rating Conditions
This is the result for sensible capacity while at the operating conditions from the full coil model in EnergyPlus.
Airflow - Coil Final Reference Air Volume Flow Rate
This is the input or autosize result for the volume flow rate through the coil at the rating point.
Fluid Flow - Coil Final Reference Plant Fluid Volume Flow Rate
This is is the fluid flow rate at the rating point for the coil. Only reported for water source (not electric or DX type coils)
Entering Dry/Wet Bulb - Coil Entering Air Dry/Wet Bulb at Rating Conditions
This is the coil inlet air temperatures used when calculating the full coil model at the rating point specified. For hydronic coils these will match the inputs or the autosized mixed air temperature. For DX cooling coils the dry bulb value is 80F (26.67C) and wet bulb 67F (19.44C).
Leaving Dry/Wet Bulb - Coil Leaving Air Dry/Wet Bulb at Rating Conditions
This is the coil outlet air temperatures from calculating the full coil model at the rating point.
Primary Heating Coil Sizing
Total Capacity - Coil Total Capacity at Rating Conditions
This is the result for total capacity while at the operating conditions from the full coil model in EnergyPlus.
Airflow - Coil Final Reference Air Volume Flow Rate
This is the input or autosize result for the volume flow rate through the coil at the rating point.
Fluid Flow - Coil Final Reference Plant Fluid Volume Flow Rate
This is is the fluid flow rate at the rating point for the coil. Only reported for water source (not electric or gas type coils)
Entering Dry/Wet Bulb - Coil Entering Air Dry/Wet Bulb at Rating Conditions
This is the coil inlet air temperatures used when calculating the full coil model at the rating point specified. For hydronic coils these will match the inputs or the autosized mixed air temperature.
Leaving Dry/Wet Bulb - Coil Leaving Air Dry/Wet Bulb at Rating Conditions
This is the coil outlet air temperatures from calculating the full coil model at the rating point.
FAQ
Q: Why is there an option for mixed air temperature autosizing on the cooling side, but not the heating side?
A: Within EnergyPlus, the default behavior for heating coils ignores the heat recovery and reports a sizing based on the 100% outdoor air condition for the coil.