CEMP-E
TI 810-11
30 November 1998
to maintain expertise in each manufacturer's
equipment. Contract maintenance might be
the only feasible way to maintain such
systems, which could require the
management and quality assurance of
numerous contracts by the installation. The
installation might also be subject to the high
costs often associated with sole source
maintenance contracts.
Since each DDC vendor has a different
Interfacing an SLDC-based control system
communication protocol and the
to a higher level computer could be
development and acceptance of an industry
achieved via a 4-20 mA signal which
standard communications protocol is not
eliminates problems with proprietary
complete, the systems of different vendors
manufacturer protocols.
cannot be easily interconnected and the
application of any global control strategies
would be difficult.
DDC, used in a networked architecture, can
SLDC-based control systems have been
provide a significant degree of functionality
interfaced to a higher level computer in a
in that a multitude of operating parameters
networked architecture, but the functionality
can be accessed, viewed, and modified
is more constrained than that of DDC and
from a remote location. This functionality is
the methodology and technology for
a built-in, well established, and time tested
accomplishing this is less well established
feature of most vendor's individual DDC
and more expensive than that of DDC.
systems.
Competitively procured commercial DDC
In the event of a failure, only one unit or one
systems may have a lower first cost in
control loop of an SLDC-based system
larger buildings where one panel serves
would be affected until a nonproprietary
several air handling units. However, if the
controller or other component could be
panel fails, several HVAC systems may be
replaced. Since the single-loop controller is
out of service until the appropriate
firmware-based and not software-based, no
maintenance personnel can locate and
programming is required.
diagnose the problem and make the
necessary repairs. Reprogramming of the
panel, if necessary, would require the
services of an individual trained in that
system's software.
Software-based DDC systems can provide
Many local energy saving features as well
some energy saving features that local
as traditional cost effective global features,
SLDC-based panels cannot provide. The
such as demand limiting, can be provided
actual incremental savings of these features
by SLDC-based panels in conjunction with
is very site-specific.
the standard EMCS, a modified EMCS or a
central FM system.
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