5121835-TIM-J-1118
52
Johnson Controls Ducted Systems
3.
A low-pressure switch to protect against loss of refrigerant
charge, (opens at 50 ± 5 psig).
The above pressure switches are hard-soldered to the unit. The
refrigeration systems are independently monitored and
controlled. On any fault, only the associated system will be
affected by any safety/preventive action. The other refrigerant
system will continue in operation unless it is affected by the
fault as well.
The unit control board monitors the temperature limit switch of
electric heat units and the temperature limit switch and the gas
valve of gas furnace units.
Compressor protection
In addition to the external pressure switches, the compressors
also have inherent (internal) protection. If there is an abnormal
temperature rise in a compressor, the protector will open to shut
down the compressor. The UCB incorporates features to
minimize compressor wear and damage. An
Anti-Short Cycle
Delay (ASCD)
is used to prevent operation of a compressor too
soon after its previous run. Additionally, a minimum run time is
imposed any time a compressor is energized.
The ASCD is initiated on unit start-up and on any compressor
reset or lock-out.
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 being
required to cool the space. The general sequence of operation
of the patented MagnaDRY reheat is outlined in the following
paragraphs.
The unit control board (UCB) allows the user to select three
different modes of operation within the Smart Equipment™
Controller menu. Those modes consist of a Normal, Alternate
and, Aux each mode is described below.
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 will 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 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
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 32.
On the ZKT12 product only, 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.
Figure 31: Cooling operation piping schematic - circuit #1 shown
Evaporator
Coil
TXV
Check
Valve
Condenser
Coil
Airflow
Airflow
Reheat
Coil
Closed
Solenoid Valve
(SOL 2)
Accumulator
Compressor
Reheat Valve
(SOL 3)
Check
Valve
Open
Solenoid
Valve
(SOL 1)
Refrigerant Flow
