Hydronic System Efficiency

Achieving energy targets with improved HVAC performance

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HVAC upgrades often are overlooked when owners seek to improve efficiency of building systems due to perceived high upfront costs. Furthermore, the impact of energy-efficiency improvements is often difficult to extract at the individual building system level since energy use is typically billed at the building level.

However, a research project designed to obtain a granular understanding of exactly how much energy would be saved through HVAC system enhancement in a commercial building proves the value of updating mechanical systems. 

The project was a collaboration among water technology company Xylem Inc., building manager Related Management Co., and the Sustainable Engineering Lab at Columbia University.

The Test Subject
Astor Place was built in 2005 and includes 39 residential units with commercial space on the building’s first and second floors. The HVAC system uses hydronic space heating through boilers and cooling with central absorption chillers. Fan-coil units provide HVAC to residential floors; HVAC in the common and commercial spaces employs air-handling units.

The intent of the project was to improve the energy efficiency of the system without modifying the heating and cooling loads.

On the cooling side, Xylem Bell & Gossett e-1510 20-horsepower (hp) pumps replaced the original 30-hp condenser water pumps. The lower horsepower but more technologically advanced pumps immediately delivered savings in electricity use—from 25.3 kilowatts (kW) to 18.0 kW.

Other modifications included new piping to create primary/secondary chilled water loops out of the original primary-only system. This was completed with the addition of Bell & Gossett Series 80 7.5-hp primary pumps. Decoupling the loops in the original one-loop system allowed for the installation of variable flow technologies on the secondary loop. Bell & Gossett e-1510 15-hp pumps outfitted with variable frequency drives (VFDs) replaced the original 20-hp distribution pumps.

Sensors were installed at the end of each branch to provide vital performance data. All data was logged at one-minute intervals and accessed through a newly installed building management system (BMS).

VFD Impact
Centrifugal pumps installed in HVAC systems typically operate in variable load applications that see a fluctuation of flow requirements based on the heating or cooling load of a building at any given time. The original pumps specified for Astor Place were running at constant speed along with being oversized for the true operational demands of the building.

VFDs proved the best solution at reducing energy.
Even at peak cooling load, electricity reduction was more than 50 percent compared to constant-speed operation, according to the test data. VFDs bring the most benefit in terms of energy consumption. The pumps consumed just enough energy to provide proper service needed for the building’s cooling load.

The key finding from the testing was that right-sized pumps paired with VFDs, as well as properly balanced system hydraulics, can deliver a 95 percent reduction in pumping energy. 1

Research further revealed that pumping electricity reduction with VFDs was far more significant than the effects of reduced pump size and pressure-independent control valves. Annual replacement of the chilled water (ChW) distribution pump electricity was 92 percent lower with VFDs than without.

Coupled with the findings in an American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Journal article that concluded energy improvements in the HVAC system could substantially influence 43 percent of a building’s total energy, this has a dramatic impact on the overall building energy footprint.2

Crunching the Numbers
Monitoring system behavior at Astor Place yielded three primary benefits: determine if equipment was operating as designed; assess the amount of energy being used and the dynamic adjustments to the load’s demands; and gain a broader understanding of system behavior.

Prior to the retrofit, nearly 30 percent of the building’s common system utility costs were for pumping electricity, primarily because of the pumps being oversized for the demands of the system and the constant speed operation at partial loads. Oversized pumps were found to cause unnecessarily high pressure differentials and flow rates within the system.

When considering both heating and cooling, the retrofit resulted in a computed annual pumping electricity usage of 316 megawatt-hours (Mwh), a 41 percent improvement in pumping energy requirements and an estimated 12 percent reduction in the building’s central operations’ energy bills. With all retrofits considered, the total annual cooling pumping energy reduction was computed to be 36.1 percent compared to the original system.

The study illustrates that replacing constant speed pumps with more appropriately sized equipment can significantly reduce energy usage. The results from the Astor Place Energy Improvement Project can help inform decisions about new equipment and the potential for energy savings.

References
1. Copeland, C. “Improving Energy Performance of NYC’s Existing Office Buildings,” ASHRAE Journal, 2012. 

2. Waite, M.B. “Analyses of energy infrastructure serving a dense urban area: opportunities and challenges for wind power building  systems and distributed generation,” Doctoral dissertation,  Columbia University, 2016.

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