18 August 1998
DIRECT DIGITAL CONTROL IMPLEMENTATION
1. GENERAL. The programs described in Chapter 7 can be applied to existing or new systems. Most
of these programs may be applied to several types of systems, but others may only be applicable to
special types of systems. For example, the boiler monitoring and control program is only applicable to
facilities with boiler plants. Due to the interactive nature of the programs, the inputs and/or outputs of one
implemented program may provide inputs to other programs.
2. INSTRUMENTS AND INPUTS. Certain instruments and inputs can be common to an entire
building or, in some case, the entire UMCS. Electrical consumption and demand instrumentation do not
need to be duplicated except in special cases, such as when a UMCS serves an extremely large
geographical area, or multiple utility substations. When applications require OA measurement, the
minimum requirement is one OA temperature and one RH instrument (when used) for each building.
However, the designer may need to increase the minimum requirements to satisfy site specific building
and system conditions. For example, separate OA instruments will be specified where intake
temperatures of the OA measured on a roof mounted unit may vary significantly from other air intake
locations, causing erroneous economizer calculations.
3. DIAGRAMS. Graphic diagrams of typical systems showing UMCS devices and functions for direct
designer for each system's controlled devices in the event of a field equipment panel malfunction. Failure
modes will be based on climate, type of system, and user requirements. The failure modes shown are for
example purposes only.
4. TABLES. Database tables listing UMCS software and settings applied to typical systems are
system to be controlled or monitored by the UMCS. Two or more identical systems within the same
building, having the same occupancy schedule, may be listed on the same database table. The table's
contents and setpoint values will be tailored to the system being controlled for each specific application.
5. SYMBOLS AND ABBREVIATIONS. A listing of symbols and abbreviations used in the system
schematics is provided in Appendix B.
6. SEQUENCES OF OPERATION. The design will include a sequence of operations for each system
under direct digital control of the UMCS. The sequence of operations will be tailored for the specific site,
system and application. Sequences of operation will identify required control loop accuracy when
different from default requirements identified in the guide specification CEGS-16935. Sequences of
operation for typical HVAC systems are provided in the following paragraphs. These sequences are
keyed to the corresponding figures and tables.
a. Steam/Hot Water Converter and Primary/Secondary Heating System Sequence of Operation
(1) All Modes. The UMCS will start and stop the primary and secondary pumps in sequence
based on the signal from an outside-air temperature sensor as shown. The UMCS will enable control of
the steam/hot water converter control valve when the primary pump is operating. The UMCS will control
the primary loop hot water supply temperature by modulating the steam control valve in response to a
temperature sensor element and transmitter located in the hot water supply line to maintain its setpoint.
The UMCS will reset the hot water supply temperature setpoint with respect to the outside air
temperature in a linear schedule as shown.