Johnson Controls
5513350-JTG-1018
73
Controls (Continued)
Schedule
At the early start period, prior to the scheduled occupancy,
the supply fan turns ON for a 5-minute stabilization period
and the RAT control sensor compares to the warm-up set
-
point. If the value is equal to or below the warm-up setpoint,
heating is controlled to the warm-up DAT setpoint. If the
value is above the warm-up setpoint, the control goes into
occupied mode. Cool down operates opposite of warm-up,
using cooling instead of heating stages.
Optimal Start
The optimal start control algorithm is a function of the differ-
ence between space temperature and occupied setpoint, the
OAT, and the amount of time prior to scheduled occupancy.
When unoccupied, the controller calculates an early start
time based on control temperature deviation from setpoint.
This ensures the space is at conditioned levels when the
occupied period starts.
Historical data determines when optimal start begins, but
the early start time can be user limited. The optimal start
time is always within the same calendar day. The warm-up
or cool down DAT setpoints function until the demand is
satisfied. The control uses the heating or cooling setpoint
to determine when the demand is satisfied. The control
goes into occupied mode thereafter.
Coast
When enabled, 2 hours (adjustable) prior to the transition
from occupied to unoccupied, the cooling/heating setpoints
begin to ramp up/down linearly at 1.0°F per hour (adjust
-
able) towards the unoccupied cooling/heating setpoints.
During this time, the fans continue to run and the ventilation
sequence remains active.
Multiple Unit Staggered Start
When enabled, the start-up delay utilizes the minimum OFF
time plus a delay time. This is used to prevent all units on
a site from restarting simultaneously after a power failure.
The delay time can be manually set or randomized be-
tween 0–60 seconds.
Additional functionality includes applying a start-up delay
when switching into the occupied mode. This only happens
if the supply fan command is currently OFF when the oc
-
cupancy mode switches into occupied. This staggers start-
up for multiple rooftop units if all are commanded from the
same occupied command. After the timer has expires, the
rooftop unit can start.
Automated Demand Response Methods
The controller is capable of three automated demand re-
sponse methods: demand shed, load shed, and capacity limit.
Demand Shed
This strategy operates by setting the operating cooling/
heating temperature setpoint up or down when the unit
controller receives a demand shed signal from a binary
hard wired input or BAS command. The up/down setpoint
is adjustable. The strategy also provides an adjustable rate
limit that allows the user to control the rate of change of the
temperature setpoint as it is set up and down.
Load Shed
This function shuts down all cooling and electric heat for
an adjustable time period in response to a BAS command.
Capacity Limit
This strategy operates by limiting total allowed mechanical
cooling or electric heat capacity in proportion to the value of
an analog BAS capacity limit signal (0–100%).
Supply Air System
The supply fan is used to circulate air to condition the
temperature of the space and to provide ventilation to the
space. Supply fan starts and operating hours are measured
under all operating conditions. Two user-selectable supply
air modes are available: VAV and SZVAV.
Variable Air Volume (VAV)
Duct Pressure Control
When the fan energizes, the output from the controller main-
tains the supply duct pressure to the duct pressure setpoint.
If the duct pressure is greater than the setpoint, the control-
ler output decreases.
If the duct pressure is below the setpoint, the controller out
-
put increases. If the duct pressure reaches the duct pres-
sure shutdown setpoint, the fan and all other outputs of the
unit de-energize.
Note:
If the unit is in a heating mode, the unit controller con-
tinues to control duct pressure to the duct static pressure
setpoint. Therefore, in any VAV heating mode, all VAV boxes
must be commanded open, to maximum flow, in order to get
adequate airflow to support the heating function and to pre
-
vent the heat section high temperature limit switches from
opening.
