This manual uses the term “unmet load hours” (UMLH) as a criterion for sizing standard design equipment and for other purposes. The concept of unmet load hours applies to thermal zones. 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 or cooling or both. During periods of unmet loads, the zone temperature drifts above the cooling set point or below the heating set point. A thermal zone is considered to have one UMLH if the zone temperature is outside a specified tolerance below the heating or above the cooling set point for the entire hour. The set point tolerance is defined in Chapter 5.6.1: Space Temperature Control and is the same for both the standard design and proposed design.
An UMLH can occur only during periods when the zone is occupied. UMLH are accounted for in each zone of the building. No zone in the building should exceed the maximum allowed UMLH.
UMLH can occur because fans, air flows, coils, furnaces, air conditioners, or other equipment are undersized. UMLH can also occur because of user errors such as inappropriate supply air control set points. It is the responsibility of the user to address other causes of UMLH in the proposed design.
UMLH apply to thermal zones that contain any space type that is normally occupied. Thermal zones that contain only the space types listed below will not have UMLH criteria applied to them:
•Commercial and industrial storage areas
•Corridors, restrooms, stairs, and support areas
•Electrical, mechanical, telephone rooms
•Laundry rooms
•Locker/dressing rooms
•Unoccupied gross floor areas
•Zones that are not subject to any UMLH checks or restrictions are listed in Appendix 5.4A.