Johnson Controls ZL Series Installation Manual page 67

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 39: Dehumidification sequence in normal
and alternate mode
Request HGR
Normal mode
Dehumidification On
One stage of cooling Off
(Y1)
Two stages of cooling Off
(Y2)
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 set up 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 .
Gas heating operation
1. Heating stages are controlled by the W1
through W2 thermostat inputs. A W1 or W2
input energizes a H1 or H1/H2 output.
2. When the pre-ignition process is complete the
ignition module energizes the gas valve and
provides a 24 V input to the MV terminal on
the UCB.
C1 C2 HGR C1 C2
Alternate mode
On Off On On On
On Off On On On
On On Off On On
Johnson Controls Ducted Systems
3. The FAN ON HEAT DELAY timer starts as soon
as 24 V is present on MV terminal. When the
timer expires the FAN output for the indoor
fan operation energizes. If 24 V is not received
on the MV terminal within 5 minutes, an alarm
appears.
4. When the thermostat heat inputs are lost and
the 120 second Minimum Heat Run Timers
have expired, heating outputs stage off. The
FAN OFF HEAT DELAY timer starts when 24 V
is removed from the MV terminal. When the
timer expires, the FAN output for the indoor
fan operation de-energizes.
Note: If 24 V is lost on the MV terminal
during the same heat cycle, an alarm
appears and the fan output energizes
and remains On until 24 V is present
again on the MV terminal.
Note: If 24 V is present on the MV
terminal without a call for heat, an alarm
appears and the fan output energizes.
If this condition occurs for 6 minutes an
alarm appears, and remains, until the
alarm condition is cleared.
5. At any time, if 24 V is lost on the LIMIT
terminal, the FAN output for indoor fan
operation is energized. If 24 V is lost on
the LIMIT input 3 times in 1 hour, an alarm
appears and the FAN output is energized. The
heating H1 and H2 outputs are de-energized
until the alarm is cleared.
Gas heat ignition control board
function
Ignition control board on standby
The ignition control board (ICB) has all outputs de-
energized and monitors the thermostat and flame
sense. The ICB resets ignition trial and flame loss
counters. The ICB begins a call for heat when W1 is
energized at the Unit Control Board (UCB). The ICB
ignores W2 until ignition has been established.
Call for heat
The ICB checks to see if the pressure switch is open.
If the pressure switch is closed, the ICB flashes 3
on the LED and waits indefinitely for it to open.
When the pressure switch is sensed as open, the ICB
begins pressure switch proving period. If the call for
heat is lost, the ICB goes back to Standby.
Installation Manual ZL Series 7.5- 12.5 Ton 60 Hertz R-410A 67