Johnson Controls
5513350-JTG-1018
82
While the system is in mechanical cooling, the HGRH valve
is controlled to the effective DAT setpoint. This is neces-
sary because the compressors may be controlled to a tem-
perature lower than the effective cooling DAT setpoint to re
-
move additional moisture content from the air. This strategy
allows humidity to be removed from the air while attempting
to maintain normal cooling DATs.
While the system is idle, the HGRH valve is controlled so
that the DAT setpoint resets between the effective cooling
DAT setpoint and the RAT. This strategy allows humidity to
be removed from the air while attempting to keep the DAT
warm enough so that the system does not enter heating
mode.
The controller uses the humidity sensor as the basis for
the evaporator temperature setpoint. When the RA humid-
ity first rises above setpoint, the evaporator temperature
setpoint is the same as the low cooling DAT setpoint. As
humidity continues to rise, the evaporator temperature set-
point lowers to remove additional moisture from the air.
If the changeover sensor calls for heating or the OAT falls
below 54.0°F, dehumidification mode ends regardless of
humidity.
HGRH Bleed Valve
The HGRH bleed valve is a solenoid valve that connects
the HGRH coil to the suction line. The purpose of the
HGRH bleed valve is to bleed off any remaining or trapped
liquid in the HGRH coil when dehumidification mode is not
active. When the unit enters dehumidification mode, the
HGRH bleed valve closes. When the unit exits dehumidifi-
cation mode and after a 5-minute delay, the HGRH bleed
valve opens.
Humidification Output
If the humidity sensor falls below the humidification setpoint
and the unit is not in cooling mode, the unit is placed in hu-
midification mode. The humidifier or steam valve enables
and modulates to maintain the humidity setpoint as sensed
by the humidity sensor. The humidity high limit overrides
the output if necessary to prevent the DA humidity from ex-
ceeding the DA humidity high limit setpoint. Humidification
is not allowed in cooling mode.
Twinning
The application of having two or more separate rooftop units
tied into one common main duct trunk line is known as twin
-
ning. The application is compatible with BACnet. The twin
-
ning process allows for redundancy with the cooling/heat
-
ing load. All twinned units operate concurrently and are not
sequenced based on the cooling or heating load. It allows
for the user to design the system in an N+1 arrangement, in
which three units are sized to handle the load and the fourth
is available as a backup. The user is then able to shut down
one of the running units and enable the backup via the BAS.
Each rooftop unit requires an isolation damper with an end
switch to ensure the damper is fully open before the supply
fan can start. Each rooftop unit in a twinning application
also requires its own duct static pressure sensor to allow
the rooftop units to run independently if one or more units
are shut down for maintenance or BAS communication
is lost. Each unit has a manual reset high duct pressure
switch installed to prevent over pressurization of the duct.
In a twinning system, several features are synchronized
while others can operate independently. The method used
to synchronize these features is for each twinned unit in the
group to broadcast the key values so it can be used by the
other rooftop units in the group. The approach is referred to
as independent twinning.
The idea is that if each unit has the exact same values,
then each rooftop unit can execute the exact same control
algorithm. This has the advantage of being more robust to
communication errors or sensor failures and less complex
than a master and dependent unit arrangement. If one unit
fails or is manually shut down, the remaining units continue
to run without interruption. In the master and dependent
unit arrangement, if the master unit disables, the failover
logic to a different master is more complicated and may
result in a disruption in the system. Independent twinning
prevents such disruptions or failures in the system.
The synchronized features include supply fan control,
economizer suitability, occupancy, demand control ventila-
tion, exhaust fan control, and smoke control.
The supply fan control must be synchronized to maintain
DA static pressure between all units serving the same duct.
This requires all reliable DA static pressure values from the
rooftop units to be averaged before passing to the propor-
tional-integral-derivative (PID). The static pressure setpoint
must also be synchronized, and any change to one setpoint
must be shared with the other rooftop units. This allows the
PID in each rooftop unit to calculate the same output value
and run all the supply fans at the same speed.
During start-up of a previously shutdown rooftop unit, the
supply fan speed slowly ramps up until it matches the fan
speeds of the rooftop units currently in operation. When the
additional rooftop unit begins ramping, the static pressure
increases, causing the other rooftop unit fans speeds to slow
down to reach setpoint. Once the rooftop unit fan speed
matches the existing rooftop units, it releases into control.
Controls (Continued)
