The Performance Rating Method and this manual use the term “unmet load hours” 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 blocks but is summed for the building as a whole. For a thermal block, it represents the number of hours during a year when the HVAC system serving the thermal block 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. An unmet load hour occurs only during periods when the HVAC system is scheduled to operate. One hour with un-met loads in one or more thermal block counts as a single un-met load hour for the building. If unmet load hours for more than one thermal block coincide (occur at the same hour), they count as only one unmet load hour for the building. Un-met load hours include periods when the space is either under cooled or under heated.
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 undersizing. It is the responsibility of the user to address other causes of unmet load hours in the proposed design. There can be many reasons, but the following checklist is offered as a starting point:
- Make sure that thermostat schedules agree with schedules of HVAC system operation; occupant schedules; miscellaneous equipment schedules; outside air ventilation schedules and other schedules of operation that could affect the ability of the HVAC system to meet loads in the thermal block.
- Check to make sure that inputs for internal gains, occupants, outside air ventilation are reasonable and are consistent with the intended operation of the building.
- Examine the simulated operation of controls to determine if primary or secondary heating or cooling equipment (pumps, coils, boilers, etc.) is activated. Verify that the controls are not resetting in a way that reduces modeled capacity.