ASHRAE 90.1 Reference

Unmet hours

"Unmet hours" of a building are the summation of the number of hours when the heating or the cooling set point temperature of a zone is not met either by the HVAC system or by the plant. The data is provided in the Summary report as:

• Time Setpoint Not Met During Occupied Heating
• Time Setpoint Not Met During Occupied Cooling

Understanding/Interpreting/Calculating the number of unmet hours:

• There will be an "unmet hour" for "Time Setpoint Not Met During Occupied Cooling" in a particular zone when the zone indoor temperature is higher than the cooling set point specified for that hour. Likewise for heating when the zone heating setpoint temperature is not met.
• The number or the percentage of unmet hours in a building is usually given as one of the outputs of the simulation.
• Zone wise, unmet hours can be read from the EnergyPlus: Output Variable, Time Cooling Set point Not met. When two zones are unmet at the same hour, this will count to one unmet hour for the building.
• When two zones have unmet hours during different non overlapping times of a day, the total number of unmet hours in that day is the summation of these unmet hours of each zone. This total for the year should be considered as the total unmet hours of the building.

Example

When each zone is unmet in the specified hours as beside

Zone 1 unmet during hours: 6 8 14 16

Zone 2 unmet during hours: 6 8 12 16

Zone 3 unmet during hours: 7 8 12 13

Total number of unmet hours of the building: 7 hrs and not 12 hrs.

6 7 8 12 13 14 16

• When percentage of unmet is specified, then this is the percentage of total number of hours (1 year- 8760 hours) for which the simulation is performed (not just the occupied hours).
• As per ASHRAE 90.1-2004, the unmet load hours of the total building should be less than or equal to 300 hours and unmet load hours for the proposed design shall not exceed the number of load hours for the baseline building design by more than 50.
• If unmet load hours in the proposed building exceed the unmet load hours in the baseline building by more than 50, then the size of equipment in the baseline building shall be reduced incrementally, until the condition is satisfied.
• ASHRAE and USGBC say that it is OK if the baseline unmet load hours exceed the proposed unmet load hours by more than 50, provided they are both below 300.

The above information on Unmet hours is sourced from a post by Ashu Gupta (and subsequent posts) on the bldg-sim mailing list