Significant Electric Demand Peaks

Irvine Office Building has a variable-air-volume (VAV) system equipped with variable speed drives (VSDs) and electric duct reheat supplies conditioned air to the building. Two 375-ton centrifugal chillers (one equipped with a VSD) provide chilled water to the air-handling units (AHUs), one pre floor, and heat rejection is provided by an induced-draft vertical discharge cooling tower, also equipped with a VSD. Outside air is provided to these AHUs in fixed proportions by a make-up air unit.

It was suspected that the electric duct heaters (which are not under the control of the building EMS) were firing during the morning start-up in the cooler perimeter zones, contributing to the observed demand spikes. A selection of circuits serving the electric duct heaters on three typical floors of the building were monitored for a two week period in May and June to investigate this possibility. The resulting data for almost all of these monitored duct heaters indicate that this is indeed the case.

Although short in duration (lasting only a few minutes in some cases), these episodes lasted long enough to set the peak demand and resulting demand charges for the entire month.