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Building Manager Green Tip

October 2011

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Optimizing Fan Coil Unit Efficiency

Fan Coil Unit with Open Case

Fan coil units (FCUs), such as the one pictured above with its cover off, are nearly ubiquitous devices on campus that provide an effective means of heating and cooling buildings. This Green Building Tip examines three simple steps you can take to improve the efficiency of these units and save your department money through changes to operating modes and setpoints.

Linking Water Flow and Fan Operation

If water is flowing through the coil without the fan on, energy is wasted because conditioned air is distributed ineffectively and pumps moving the water must work harder to provide the same amount of heating or cooling compared to when the fan is on.  When the fan is on but the valve is closed, fan energy is wasted because the fan is only moving air around without heating or cooling it on the coil.  

FCUs Controlled by Siemens BAS - For FCUs that are operated by the building's direct digital control (DDC) system, setting the "CYCLE FAN" value to "YES" will instruct the fan to cycle on and off based on the valve postion.  The "STAGE FAN" value then determines how open the valve must be in order for the fan to turn on. Consider setting the "STAGE FAN" value to 20% to reduce fan run times even further.  

FCUs Controlled Locally - For FCUs that are controlled either on the device itself or via wall mounted thermostat, ensure that the thermostat selected is capable of controling both the fan and the valve regulating the flow of water through the coil.  Alternatively, a current sensor can be installed on the fan power line that is set to shut off the water valve when no current is detected.  Note that for some FCUs, the flow of water is controlled at the building level and not by a valve in the unit itself, so this may not be possible in all situations.

Seasonally Appropriate Setpoints

FAS Green Program FCU Setpoint Recommendation

As the image above shows, it is not necessary to heat and cool a building to the same temperature in the winter and summer. Changes in clothing and occupant adaptation to the surrounding climate can result in higher settings in summer and lower settings in the winter. Harvard University has enacted a temperature policy that defines seasonal ranges for departments to enact in their own facilities that is based on ASHRAE recommendations.  

Though the chart above from the FAS Green Program shows four-pipe FCU configurations, the same principal applies to other heating and cooling systems.

Programmable Thermostats and Occupancy Controls

Programmable Thermostats - While FCUs running on DDC systems should already have setbacks and schedules of operation based on expected occupancy, there are options for thermostats on locally controlled devices that will allow building managers to program nightime and weekend setbacks that can signficantly reduce consumption during these periods.

It's important to check how the existing units are powered, as many FCUs will require a line voltage thermostat that does not require a separate power system.  Consider using thermostats that are capable of 2 hour overides in case occupants are in the building after normal hours.

Occupancy Controls - If occupancy sensors are being considered for lighting, it's possible to order equipment with an extra set of dry contacts that would allow Siemens controlled units to be integrated with the sensors. This allows units to be programmed to switch to unoccupied setbacks whenever occupants are not present regardless of the time of day or week.  

Green Building Services provides consulting services to ensure that the design, construction and operation of Harvard's built environment has minimal environmental and human health impacts, maximizes occupant comfort and generates an awareness of sustainable design and building operations. To learn more about our work and services, visit http://green.harvard.edu/gbs.