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D.7.6 Building Control Systems
Building energy management control systems (EMCS) control operation of a building’s mechanical and electrical systems. A good EMCS design takes advantage of advanced control strategies to meet the building’s original sustainable design intent. After commissioning (see Chapter 9), the building operator guarantees the building continues to perform sustainably by adjusting the EMCS to accommodate changes in occupant requests and building functions. Poor building operation will reduce the energy saving benefits of an otherwise good design.
An EMCS is an integrated network of sensors, controllers, actuators, and software. When programming an EMCS, designers typically specify use of reset schedules for supply air discharge temperatures, hot-deck and cold-deck temperatures, mixed air temperatures, variable volume duct pressure and flow, heating water temperature, condenser water temperature, secondary chilled water loop pressure, and chiller and boiler staging. A good EMCS sequence avoids conflicts between these schedules so that savings achieved by one component are not offset by losses at another component. Evaluate each component control strategy on an individual basis and then determine the cumulative effects of various configurations. Consult with a controls representative to assist in identifying where interferences may occur.
An EMCS saves energy and money by:
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Optimizing the equipment start and stop times (e.g., turning fans off during unoccupied hours).
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Operating the equipment at the minimum capacity necessary (e.g., running the fans in a VAV system at the minimum speed needed).
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Limiting peak electric demand.
Direct digital control (DDC) systems use electronic signals to actuate, control, and send/receive input and feedback to/from equipment. Centralized DDC systems monitor the building systems as a whole instead of controlling individual equipment separately, reflecting the basis for designing and operating energy-efficient buildings. Central systems also provide opportunities for remote access through modems or networks and can record historical data about equipment operation that can later be used for troubleshooting and diagnostics.
D.7.6.1 Guidelines for Designing an EMCS
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Specify and install EMCSs in all new Fort Carson buildings. If a building is very small, such as a transportable office, install programmable thermostats.
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Require a detailed sequence of operation for all systems controlled by an EMCS. The sequence must describe all modes of operation of the system and how they are accomplished.
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Commission EMCSs and periodically check calibration of critical sensors.
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Allow as wide a “dead band” as possible for temperature and humidity set points, and increase the dead band during unoccupied hours. Thermostatic controls for office areas can be programmed for a cooling set point of 75°F and heating set point of 72°F. Plan for an adjustable dead band of at least 6°F (± 3°F) to reduce the cycling of the heating and cooling equipment and to prevent switching back and forth between the two systems. Having different set point temperatures for heating and cooling seasons increases personnel comfort because people tend to dress according to the season (e.g., warmer in the winter). Also, the temperature and humidity of unoccupied spaces can float (drift beyond the levels required when the space is unoccupied) until the space becomes occupied again or when the space gets too cold (below 55°F).
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Integrate economizer controls with the mechanical cooling (leaving air temperature controls) so that mechanical cooling is only operated when necessary and to avoid overcooling the supply air.
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Design the systems and controls so that operating the economizer does not increase heating energy use.
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Control VAV systems to have a reset temperature such that one box is always fully open. This strategy reduces the supply duct static pressure.
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Use a sensor for multiple purposes, if possible. For example, a current transducer (CT) may be used to verify that a pump is operating properly and to calculate and record pump energy use. Also, tie occupancy sensors that control the lighting to the VAV boxes serving the same space to control temperature and the amount of outside air (e.g., only condition and supply outside air to the space when it is occupied).
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Provide controls that automatically reset supply water temperatures (heated and/or chilled water) by representative temperature changes responding to changes in building loads or by outside air temperatures.
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Use lockouts based on time of year or outside temperatures to prevent simultaneous operation of the heating and cooling systems.
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