User Manual

Sequence of Operation
Staging Control
12
Siemens Industry, Inc.
Application Note, App 14900
140-1231
2020-02-11
ChgOvrDB: Change Over Deadband restricts how close CoolingSp and HeatingSp
can be set. A minimum deadband (default is 5°F) must to be maintained. To separate
the CoolingSp and HeatingSp, one setpoint change may change the other setpoint to
maintain a gap between them.
Staging Control
The RTU is controlled by a hysteresis loop (Step Control). Hysteresis prevents the
compressor/heater from frequently switching on and off. When the room temperature
reaches the corresponding switching on setpoint of a certain cooling/heating stage,
that stage will be turned on. When the room temperature reaches the corresponding
switching off setpoint of a certain cooling/heating stage, that stage is turned off.
For example, in Heating mode the thermostat switches the heater on when the
temperature drops below A, but does not turn it off until the temperature rises above B,
where B is greater than A.
If there is a call to turn on/off multiple stages, an Inter-stage delay will apply between
the two stages being turned ON/OFF. For more information, see the Compressor
Operation section.
Compressor Operation
When the HVAC Mode is in Cooling mode, the cooling staging logic controls the
compressor.
If a compressor has been ON, it will not shut OFF until its minimum ON timer has
expired. Likewise, if a compressor has been OFF, it will not turn ON until its minimum
OFF timer has expired.
In addition to the above rules, there is a time-based separation of cooling stages --
Cooling Inter-stage Delay applies to the unit with multiple cooling stages. There is also
a time-based separation of heating stages -- Heating Inter-stage Delay which applies
to the unit with multiple heating stages.
The following paragraphs explain compressor staging:
If CoolStg = 0, the application does not control COMPRESSOR 1 (Y1).
If CoolStg ≥ 1, the application controls COMPRESSOR 1 (Y1) as follows:
● If IndoorTmpEff is greater than the switching on point of the first cooling stage
(CoolingSp + 1/2 CoolingDB), and the first compressor has been OFF for at least
the time set in MIN OFF, COMPRESSOR 1 (Y1) is turned ON.
● If IndoorTmpEff is less than the switching off point of the first cooling stage
(CoolingSp – 1/2 CoolingDB), and the first compressor has been ON for at least
the time set in MIN ON, COMPRESSOR 1 (Y1) is turned OFF.
If CoolStg ≤ 1, the application does not control COMPRESSOR 2 (Y2).
If CoolStg = 2, the application controls COMPRESSOR 2 (Y2) as follows:
● If IndoorTmpEff is greater than the switching on point of the second cooling stage
(CoolingSp + 1/2 CoolingDB + StgDifCl), the second compressor has been OFF
for at least the time set in MIN OFF, and the Inter-stage Delay timer has expired,
COMPRESSOR 2 (Y2) is turned ON.
● If IndoorTmpEff is less than the switching off point of the second cooling stage
(CoolingSp – 1/2 CoolingDB + StgDifCl), and the second compressor has been
ON for at least the time set in MIN ON, COMPRESSOR 2 (Y2) is turned OFF.