LaSalle Plaza
Class A commercial office building, comprising 803,529 square feet total, 25 story office floors each covering 20,000 square feet. LaSalle occupancy is near 100%. Energy costs had risen from $1.44 per sq. ft. in 1994, ($1,156,336.00 total building energy costs) to $1.85 ($1,489,229.00 total building energy costs) in 1995 primarily as a result of an increase in tenant occupancy to the present levels. BTUs per square foot had similarly risen from 89,198 to 118,305, an increase of 32.63%, while heating degree days increased 3.16% and cooling degree days increased 43.76%.
Client Goal:
Reduce overall energy costs, primarily steam consumption, without negatively affecting tenant comfort or adversely affecting IAQ. Steam consumption accounts for 16% of the total utility costs while chilled water & electricity each accounts for 42% of the remaining energy costs in 1995.
Scope of Work
(Base Project):
Install one CO2 duct mounted sensor per AHU, (one AHU per floor), revise EMS control program to accomplish CO2 Demand Controlled Ventilation and revise control of minimum outside air dampers to incorporate full modulation vs. Fixed minimum setting.
Scope of Work
(Extended Project):
Install VFDs on all vane axial fan motors to take advantage of the reduced air flow requirements using CO2 Demand Controlled Ventilation strategy. Install VFDs on the remaining tower floor AHUs (approx. 12) to coincide with the reduced volume of air supplied and exhausted to each floor.
Install VFDs on 2-30HP & 2-50HP chilled water pumps with pressure transmitters for accurate control of chilled water to all tower AHUs to take advantage of the reduced chilled water demand occurring from reduced air volumes requiring conditioning.

Revise control sequence for all of the systems above to accommodate VFD control.

Each floor AHU is limited to providing approximately 6000 CFM outside air, sufficient to meet ASHRAE design conditions for the expected occupancy levels. Each floor AHU is powered by one 20 HP motor.

Outside air is delivered to the floor AHUs by means of a central vertical duct system capable of providing approximately 175,000 CFM. The OA fan system is comprised of twin fans located in the sub basement and penthouse. The sub basement fan system is made up of 1-20HP & 1-60HP vane axial fans. The penthouse fan system is made up of twin 40HP vane axial fans. Each system has steam coils to precondition the outside air to near discharge conditions of 55F. The sub basement fan has a heat recovery coil to capture wasted heat from the building exhaust fan system.

High pressure steam and chilled water is provided by the Minneapolis Energy Center: Steam costs are $.00001096 per purchased BTU. Chilled water costs are $.00002278 per purchased BTU.
Heating savings
Estimated at approximately $500 per AHU annually were on the basis of 50 MMBTU per 1000 CFM of air saved*.
Cooling savings
Estimated at approximately $3000 per AHU annually on the basis of 150 MMBTU per 1000 CFM of air saved*.
Energy savings
Total annual of $87,500 were projected netting an expected simple Return On Investment of less than six (6) months for the CO2 control portion of the project.
Estimated savings from the proposed installation of VFDs and essential EMS changes on all systems is expected to yield an ROI of approximately two (2) years. The original estimated energy savings represents an approximate reduction of 6% from the 1995 total energy costs.
Project Status
as of
March 10, 1997:
Both chilled water and steam consumption have been reduced substantially. When compared to February of the previous year (1996) steam consumption has been reduced by approximately 35%, chilled water has been reduced by approximately 55% as a result of changes to the chilled water pump control sequence and early results of the CO2 Demand Controlled Ventilation strategy.

CO2 levels on the controlled and monitored tower floors have typically not exceeded approximately 700PPM, allowing the significant reduction of outside air required to meet conditions for acceptable IAQ. Outside air levels are currently calculated to meet the general and bathroom exhaust fan CFMs to maintain a slight positive pressure relative to the outside.

Comparison of the actual CO2 results against the Telaire model indicate that the modeled profile is slightly more conservative in predicting expected maximum CO2 levels, thus it is expected that incrementally greater CFM reductions are likely.
Project Status
as of
August 31, 1997:
Total Chilled Ton Hours have been reduced by 181,900, or $51,136.
Total Steam in 1,000 pounds have been reduced by 5,809 or $69,710.
Total Electric KWH have been reduced by 549,200 or $29,657.