User Guide

Control Option Comparisons—Start/Stop Time Optimization (SSTO)
Siemens Building Technologies, Inc. 2-11
Defining SSTO
Three PPCL commands are defined in the field panel for SSTO. They
must be specified in the following order:
1. TODMOD – Defines the specific mode for each day of the week.
This command is shared with the Time-Of-Day group of
statements and is necessary for SSTO to work.
2. SSTOCO – Establishes the thermal characteristics of the
building based on how the building reacts to changes in
temperature.
3. SSTO – Calculates optimal start and stop times.
All optimization calculations and equipment control are performed in
the field panel where the SSTO and SSTOCO commands are
defined. Each program within a field panel may also perform SSTO
calculations for up to 5 zones.
CAUTION:
Be careful when designing your programs so that
functions do not conflict. If points used for SSTO are
also commanded by another application, such as
Duty Cycling (DC), one program may interfere with
the operation of the other if both functions are trying
to control a point during the same time period.
How SSTO Adjusts the Start and Stop Times
The following abbreviations are used in this section:
asp Adjusted stop time
ast Adjusted start time
ccoef4 Cooling auto-tune coefficient
csp Calculated stop time
cst Calculated start time
est Earliest start time
hceof4 Heating auto-tune coefficient
lst Latest start time
APOGEE PPCL User’s Manual
2-12 Siemens Building Technologies, Inc.
An adjusted start time (ast) and adjusted stop time (asp) are added
to or subtracted from the times calculated by SSTO. If the ast or asp
value reaches a certain limit, SSTO is forced to start or stop at the
earliest (est) or latest (lst) time.
The variables ast and asp may change to help tune the times. If
allowed to reach a high number, it is possible for the following to
occur:
• ast may adjust the calculated start time (cst) past the earliest start
time (est) or latest start time (lst).
• asp may adjust the calculated stop time (csp) past the latest start
time (lst) or latest stop time (lsp).
This means that the equipment always starts or stops at the earliest
or latest start or stop time.
Each day, the SSTO statement uses the heating auto-tune
coefficient (hceof4) and the cooling auto-tune coefficient (ccoef4)
defined in the SSTOCO statement to adjust the calculated start time
(cst) and the calculated stop time (csp).
Ast and Asp Initial Settings
Set the ast and asp variables to zero during initial setup of the SSTO
statement. If the SSTO statement has been set up properly, these
values typically stay below 5 times the variables hcoef4 or ccoef4.
If the ast or asp variables are continually incrementing or
decrementing the calculated times, then check the SSTO setup and
examine the SSTOCO coefficients.