Understanding and Minimizing Unmet Load Hours – A User Guide - Part 3: How Unmet Load Hours are determined in the IES Virtual Environment

Part 3: How Unmet Load Hours are determined in the IES Virtual Environment  

As described in Part 1 of this guide, an unmet load hour is any hour of any day of the year when one or more zones in the building are outside of their temperature control range. This control range includes both the zone temperature setpoints and associated throttling ranges. The UMLHs test uses the cooling and heating setpoints plus tolerances to account for the extension of throttling ranges above and below the cooling and heating setpoints, respectively. Part 2 described the determination of which spaces, dates, and hours are subject to UMLH tests.

Part 3 of the IES Unmet Load Hours guide delves into the specifics of how UMLHs are determined for those spaces within the IES Virtual Environment software. The focus is on relevant user inputs in the setup of the model and HVAC systems, and how these inputs are used in automated UMLH analysis for ASHRAE 90.1 Appendix G, CA Title-24, and other performance rating and energy code compliance tools. This will help you understand the mechanics of UMLH tests within the IES Virtual Environment.

Related topics previously covered in Part 1: Understanding Unmet Load Hours

• Temperature Control and Throttling Range
• Tolerance
• Allowed UMLHs
• Spaces for which UMLHs are counted
• When UMLHs are counted
• Specific UMLH Requirements of Codes and Standards

Related topics previously covered in Part 2: Spaces, Dates, and Hours subject to Unmet Load Hours tests

• Space types excluded from UMLH checks
• Settings in Thermal Templates and Space Data
• Relevance of the ASHRAE-55 Comfort Analysis
• HVAC Zones and Master Rooms
• Hours and seasonal date ranges included in automated UMLH testing

Topics covered in Part 3

• Setpoints for space conditioning and UMLH tests
  • Space conditioning setpoints used in Unmet Load Hour tests 
  • Schedules and related functions that influence the setpoints used in Unmet Load Hour tests
    • Morning Start-up
    • Optimal Start and Morning extension
  •  Special-use zones
  • Setpoint results variables used in UMLH test, and how these reflect space setpoints and schedules
• Throttling Range and Tolerance for UMLH tests
• Unmet Load Hour Results Reporting

Part 4 will describe the use and application of manual UMLH checks for performance assessment, tracking down sources of UMLHs, and identifying problematic HVAC zoning. Subsequent parts of the guide will present various additional strategies for minimizing UMLHs and addressing particular situations.

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You can view our other Understanding and Minimizing Unmet Load Hours articles by clicking HERE.