Unmet Load Hours And Hours Above Cooling Throttling Range

Based on ASHRAE Standard 90.1 and LEED which compliance mandate specific limitations regarding unmet load hours, there is a concern about the number of hours buildings are not meeting load which generally, an hour in which one or more zones is outside of the thermostat setpoint range. So, Unmet Load Hours are defined in trace as the hours when the building load is in excess of the design coil capacity by 1% or greater and one or more rooms are 1.5 degrees Fahrenheit beyond the room thermostat setpoint range for that hours, and the coil does not have the capacity to bring it to setpoint.

The following are just a few strategies can be implemented to reduce unmet load hours:

• The first strategy is to review the Building Temperature Profiles report. This report can help to determine which rooms and zones are experiencing unmet load hours. If this is the case, the rooms could be rezoned. In addition, to verify that the rooms have a similar setpoint.
• Narrowing the gap between drift points (drift point and Setpoint) allows the equipment to achieve occupied setpoint faster, thus reducing unmet load hours. When the building is unoccupied, drift temperatures are used as building setpoints. Once the building becomes occupied, it may be the case that the HVAC equipment may not be able to handle the increased load and setpoint change if there is a wide variance between setpoint and drift point.
• Unmet load hours can also be reduced by applying an optimum start schedule. This control scheme allows the building automation system to condition the spaces before a building becomes occupied. Therefore, the building is conditioned to occupied setpoint when people enter the structure.
• A final strategy is to increase the plant and coil capacities in order to increase the building’s ability to respond to changing conditions.

Reducing/Eliminating Unmet Load Hours in TRACE™ 700