Emerson E2 Manuel D’Utilisation

When the Heat/Cool receives an indication that the 

building is unoccupied, the control mode changes from the 
PID method described previously, and uses simple ON/
OFF control of the heating and cooling outputs.

The Heat/Cool application in unoccupied mode makes 

use of a hysteresis value to form a range of temperature 
values across which the heating and/or cooling will be 
applied. As shown in Figure 11-26, when the temperature 
drops to the Unoccupied Heating Setpoint or rises to the 
Unoccupied Cooling Setpoint, the heat or cool outputs go 
to 100% ON. They remain in this state until the tempera-
ture rises above the heating setpoint plus the hysteresis, or 
below the cooling setpoint minus the hysteresis, at which 
time the outputs will go to 0% ON.

HEATING
SET POINT

COOLING

SET POINT

0%

100%

HYSTERESIS

TEMPERATURE

OU

T

P

U

T

26512035

Optimum Start/Stop (OSS) is a feature that works 

alongside the Heat/Cool’s occupied and unoccupied tem-
perature control modes. OSS takes control of heating and 
cooling several minutes before the building is scheduled to 
change occupancy states, and prepares the building for the 
upcoming change in setpoints. As a result, when the occu-
pancy state changes, the temperature will be comfortably 
within the range of the new setpoint.

Figure 11-15 shows an example of how pre-starts and 

pre-stops work in a heating application. From unoccupied 
mode, the pre-start period ramps the temperature up 
slowly so that when the scheduled change from unoccu-
pied to occupied mode occurs, the temperature will 
already be at or near the occupied heating setpoint. During 
the pre-stop, which occurs before the Heat/Cool goes from 
occupied to unoccupied mode, heating is suspended and 
the temperature is allowed to “coast” down to the unoccu-

pied setpoint.

OCCUPIED
SET POINT

UNOCCUPIED

SET POINT

26512036

OSS is designed to handle pre-starts and pre-stops in 

the most energy-efficient manner possible. Every time a 
pre-start or pre-stop occurs, OSS measures the amount of 
time it takes to bring the temperature from the previous 
setpoint to within the “comfort zone” of the new setpoint 
(a user-defined range of values above and below the set-
point within which the temperature is considered accept-
able). This duration is used to determine the average rate 
of temperature change, called the K factor. 

The K factor is stored in the memory along with the 

average value of the outdoor air temperature during the 
pre-start or pre-stop. Over time, collected K factor data 
will be sorted and combined into a table. As a result, by 
constantly observing and recording the results of previous 
pre-starts and pre-stops, OSS will be able to intelligently 
guess how much time a pre-start or pre-stop mode should 
last based on the outside temperature.

Heat/Cools keep track of three different kinds of K fac-

tors:

• Heat K factor - used to guess pre-start durations for 

Heat/Cools operating in heating mode.

• Cool K factor - used to guess pre-start durations for 

Heat/Cools operating in cooling mode.

• Coast K factor - a measurement of the change in 

temperature when no heating or cooling is active. 
This is used to determine pre-stop durations for 
both heating and cooling Heat/Cools.

If desired, Heat/Cools may be configured with a Set-

point Reset that varies the value of the heating and/or 
cooling setpoints based on an analog value from a reset 
sensor. This is most often used to vary the value of a heat-
ing or cooling setpoint based on the outside air tempera-
ture.

To set up a setpoint reset for heating or cooling, the 

user must specify the minimum and maximum range of 
reset sensor values, and the maximum range of setpoint