This 'manual uses the term “Unmet Load Hours” (UMLH) as a criterion for sizing equipment, for qualifying natural ventilation systems, and for other purposes. The concept of unmet load hours applies to individual thermal zones but is summed for hours whenever any thermal zone in the building has unmet loads. For a thermal zone, it represents the number of hours during a year when the HVAC system serving the thermal zone is unable to maintain the set point temperatures for heating and/or cooling. During periods of unmet loads, the space temperature drifts above the cooling setpoint or below the heating setpoint. A thermal zone is considered to have an unmet load hour if the space is outside the throttling range for heating or cooling. The throttling range is defined in Chapter 5|topic=5. Building Descriptors Reference as the space temperature difference between no cooling and full cooling or between no heating and full heating. It is assumed that the cooling and heating setpoints are “centered” on the throttling range, so that a cooling setpoint of 75°F results in an acceptable temperature band of 74°F to 76°F. The throttling range is fixed at 2°F for simulating both the standard design and proposed design.
An unmet load hour can occur only during periods when the HVAC system is scheduled to operate. Unmet load hours are accounted for in each zone of the building. No zone in the building can exceed the maximum allowed unmet load hours.
Unmet load hours can occur because fans, air flows, coils, furnaces, air conditioners or other equipment is undersized. Unmet load hours can also occur due to user errors including mismatches between the thermostat setpoint schedules and HVAC operating schedules or from other input errors, for instance, high internal gains or occupant loads. The term, as used in this 'manual, only addresses equipment that is undersized. It is the responsibility of the user to address other causes of unmet load hours in the proposed design.