5167531-YIM-J-1019
Johnson Controls Ducted Systems
53
Figure 32: Reheat operation piping schematic - circuit #1 shown
Alternate mode
When the Unit Control Board (UCB) detects a need for
dehumidification via the field installed return/space humidity
sensor and there is not a call for cooling, the UCB energizes
SOL 3, SOL 2, and de-energizes SOL 1. (In the ZR150, SOL 4
is only energized when the discharge pressure in circuit #1
rises above 400 psig and de-energizes SOL 4 after the
discharge pressure falls below 320 psig). The unit then
operates with circuit #1 in reheat mode and circuit #2 in cooling
mode.
When there is a call for first stage cooling while there is still a
call for dehumidification, no operational change is made. The
call for cooling is ignored and the unit continues to operate with
circuit #1 in reheat mode and circuit #2 in cooling mode.
When there is a call for second stage cooling, the UCB de-
energizes the HGR, which de-energizes SOL 3 and SOL 2, and
energizes SOL 1. Both circuits now operate in the cooling
mode.
The indoor blower operation is always initiated upon a call for
first stage cooling, second stage cooling or dehumidification
(HGRH). The unit does not operate in the reheat mode if there
is any call for heating or two stage cooling.
On units with economizers, the unit will not operate in the
reheat mode if there is a call for cooling and the economizer is
operating as first stage of cooling. All safety devices function as
previously described.
Optional Aux mode
The Aux mode available with hot gas reheat units introduces an
operating mode that considers the dry bulb temperature in the
space when choosing hot gas reheat staging. The Aux mode
reduces the amount of over cooling while maintaining humidity
control in the space. The Aux mode is only applicable when the
unit is setup in the Alternate reheat mode.
If there is a call for dehumidification and no call for cooling the
unit automatically reverts back to the Normal hot gas reheat
mode only allowing refrigerant stage one to run in reheat mode
and refrigerant stage two remains off.
If there is a call for dehumidification and a call for cooling the
unit remains in the Alternate hot gas reheat mode allowing
refrigerant stage one to run in hot gas reheat or
dehumidification mode and refrigerant stage two runs in
cooling.
Electric heating sequence of operations
The following sequence describes the operation of the electric
heat section.
Evaporator
Coil
TXV
Check
Valve
Condenser
Coil
Airflow
Airflow
Reheat
Coil
Closed
Solenoid
Valve
(SOL 1)
Accumulator
Compressor
Reheat Valve
(SOL 3)
Check
Valve
Open
Solenoid Valve
(SOL 2)
Refrigerant Flow
Table 25: Dehumidification sequence in normal and
alternate mode
Request
Normal mode
Alternate mode
HGR
C1
C2
HGR
C1
C2
Dehumidification
On
On
Off
On
On
Off
One stage of cooling (Y1)
Off
On
Off
On
On
On
Two stages of cooling (Y2)
Off
On
On
Off
On
On
