A load duration curve is a graphical representation that ranks all hourly or periodic building loads from highest to lowest, showing how long loads remain above specific threshold values throughout a year. This tool helps HVAC professionals understand load distribution patterns and identify peak versus average operating conditions across the 8,760 annual hours.
Technical Details
The horizontal axis represents hours per year (0-8,760), while the vertical axis shows load magnitude in kW or BTU/h. The curve typically displays a steep slope at the beginning (representing peak loads occurring for fewer hours) and gradually flattens (representing lower loads occurring for extended periods). Load duration curves separate into heating and cooling components, with summer curves often showing extended high-load periods during occupied hours.
Design and Operational Significance
Load duration curves enable right-sizing of HVAC systems by distinguishing between design-day peaks (rarely occurring) and typical operating conditions (frequent occurrence). This distinction allows engineers to optimize equipment selection, consider load-shifting strategies, and evaluate part-load efficiency. Utility companies use load duration curves to match generation capacity with demand patterns and plan infrastructure investments.