1.2 System-level loads and sizing

System autosizing—the second sizing stage—runs the selected ApacheHVAC system to determine design sizing for system and plant equipment. Sizing at this level is thus the combined result of the zone loads for the design-day conditions, setpoints (including setback in the case of cooling), system configuration, and controller settings for the selected ApacheHVAC system.

This stage of the sizing process can be applied to system-level equipment for any (pre-defined or user-defined) functional ApacheHVAC system, and no Loads Data spreadsheets are required. Furthermore, except in the context of the Workflow Navigator for ASHRAE 90.1 PRM, system sizing does not require a specific ApacheHVAC target file name. Rather, the target HVAC system file is simply selected when the system level sizing is performed (when system sizing is launched form the ApacheHVAC toolbar, the system sizing target defaults to the currently open ApacheHVAC file).

 

System-level autosizing applies to the following components within ApacheHVAC:

·       heating coils

·       cooling coils

·       fans

·       boilers

·       chillers

·       hot water loops

·       chilled water loops

·       heat transfer loops

·       DX cooling

·       air-source heat pumps

·       water loop heat pumps

·       furnaces

·       water-source heat exchangers

·       other heating & cooling sources

A coil, for example, is sized according to the maximum load it “sees” during the design sizing run, given the settings within controls for leaving air temperature, dehumidification, air flow rates, and so forth.

System-level autosizing does not use or require the Loads Data spreadsheet for each system. Loads Data spreadsheets are required only for zone-level autosizing and for expediting setup of common component parameters, such leaving air temperature setting for coils, via the System Parameters dialog. Regardless of where they exist in the system (AHU or zone level) all coils and fans are autosized along with plant equipment during the system-level sizing run, which does not depend on the spreadsheets.

System load calculations and autosizing can be run from the System-level sizing button on the ApacheHVAC toolbar ( Figure 9-12 above), directly from within the ASHRAE Loads dialog ( Figure 9-9 below), or from the System load calculations action item in the System Prototypes & Sizing navigator. This third option is further described under in the System Prototypes & Sizing workflow navigator section below.

 

Figure 9 - 9 : ApacheHVAC system loads analysis type in ASHRAE Loads dialog

In addition to the System Loads Calculation reports, there are also a number of tools available for checking the number of “unmet load hours” according to various criteria. This is explained in the next subsection immediately below.