6021980-UIM-A-1120
62
Johnson Controls Ducted Systems
Reheat mode sequence of operation
The MagnaDRY reheat mode of operation is designed to
remove latent heat (humidity) from a space when there are low
load conditions and the air conditioning unit is not used to cool
the space. The general sequence of operation of the patented
MagnaDRY reheat is outlined in the following paragraphs.
The user can select three different modes of operation from the
unit control board (UCB) within the Smart Equipment™
controller menu. The available modes are normal, alternate,
and aux. The following sections describe each mode.
Normal occupied operation mode
If the return humidity is greater than or equal to the hot gas
reheat humidity setpoint, and there is no demand for cooling,
the C1 output energizes and the AUX-HGR output energizes.
If there is a demand for one stage of cooling and the return
humidity is greater than or equal to the hot gas reheat (HGR)
humidity setpoint, the C1 output energizes but the AUX-HGR
output de-energizes.
Any additional cooling demands energize compressor outputs,
but do not change the status of the AUX-HGR output.
When the return humidity falls to 3% or more below the
setpoint, the C1 and AUX-HGR outputs de-energize.
Note:
If HGR enabled for unoccupied operation is enabled,
during unoccupied mode the control works the same as
described above, except it uses the HGR unoccupied
humidity setpoint instead.
Normal cooling mode
When there is a call for first stage cooling, with or without a call
for dehumidification, the UCB de-energizes the HGR relay de-
energizing SOL 2, SOL 3 (HGRH) and energizes SOL 1,
engaging cooling circuit #1 resulting in circuit #1 cooling mode
operation. The unit is now in first stage cooling without HGRH.
When there is a call for second stage cooling, the UCB
engages both circuit #1 and circuit #2 in 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.
On units with economizers, the unit does 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.
Normal reheat mode
When the UCB detects a need for dehumidification through the
field installed return/space humidity sensor and there is not a
call for cooling, the UCB energizes solenoids SOL 3 (HGRH),
SOL 2 and the reheat relay (RHR), which de-energizes SOL 1.
The unit then operates with refrigerant flow in the evaporator
reheat coil and condenser coil circuit # 1. See Figure 30.
Alternate mode
If the return humidity is greater than or equal to the hot gas
reheat humidity setpoint, and there is no demand for cooling,
C1 and AUX-HGR outputs energize, and C2 energizes.
If there is a demand for one stage of cooling and the return
humidity is greater than or equal to the hot gas reheat humidity
setpoint, C1 and AUX-HGR outputs energize, and C2
energizes.
If there is a demand for both first and second cooling stages
and the return humidity is greater than or equal to the hot gas
reheat humidity setpoint, C1 and C2 outputs energize and AUX-
HGR de-energizes.
Any additional cooling demands energize compressor outputs,
but do not change the status of the AUX-HGR output.
When the UCB detects a need for dehumidification through 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 ZT150, 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 on 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 does 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.
Table 25: 3 stage dehumidification sequence in normal
and alternate mode
Request
Normal mode
Alternate mode
HGR
C1
C2
C3
HGR
C1
C2
C3
Dehumidification
On
On
Off
Off
On
On
On
Off
One stage of
cooling (Y1)
Off
On
Off
Off
On
On
On
Off
Two stages of
cooling (Y2)
Off
On
On
Off
Off
On
On
Off
Three stages of
cooling (Y3)
Off
On
On
On
Off
On
On
On
