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351579-YTS-B-0608
(optional) are disabled, so cooling is disabled and the
optional economizer outside air dampers are closed.
3.
When the return air temperature climbs above the MWUP
setpoint, heating is disabled, VAV and the economizer are
enabled.
4.
A call for first stage cooling initiates the cooling sequence.
The optional economizer is enabled, and drives the
damper to the minimum open position. If free cooling is
available, dampers open to satisfy setpoints. If there is no
economizer, the control powers terminal C1 to energize
compressor 1.
The control provides a minimum of 30 seconds between
each cooling stage call [see Lead-Lag], a 4 minute mini-
mum ON time for each cooling stage and a 5 minute mini-
mum OFF time for each cooling stage.
5.
A call for Compressor 1 powers the high pressure safety
switch (HPS1), the low pressure switch (LPS1), the coil of
contactor 6M (condenser fan 1), and the coil of contactor
7M (contactor, condenser fan 2) through condenser fan
cycling switch PS5 (PS5 disables condenser fan 2 opera-
tion based upon the discharge pressure of the Compressor
1). If the system pressures are within HPS1 (high refriger-
ant pressure) and LPS1 (low refrigerant pressure) switch
settings is not tripped, contactor 1M is engaged to start
scroll Compressor 1.
6.
A call for Compressor 2 powers contactor 2M (contactor,
compressor 2) if HPS2 and LPS2 (high and low refrigerant
pressure) contacts are closed.
7.
30 and 40 ton only (all R410a): A call for Compressor 3
powers the coils of contactors 8M (contactor, condenser
fan 3) and 9M (contactor, condenser fan 4 - 40 ton only). If
HPS3 and LPS3 (high and low refrigerant pressure) con-
tacts are closed, power is applied to the coil of Contactor
3M, which starts compressor 3.
8.
40 Ton Only (all R410a): A call for Compressor 4 powers
the coils of contactor 4M if HPS4 and LPS4 (high and low
refrigerant pressure) contacts are closed.
9.
When the internal time clock matches the setting in the hol-
iday and daily schedules, the unit enters Unoccupied
mode. The control disables the supply fan, the optional
power exhaust, cooling, heating, and the economizer. The
optional outside air dampers are closed with the optional
inlet guide vanes.
10. When the space temperature falls below the heating set-
point, the control powers contactor 5M from terminal FAN
to engage the supply fan motor. If the supply fan operates
correctly (proper rotation) the pressure difference between
the heating section and the supply fan section increases
and APS closes to enable heating operation. If heat safe-
ties are satisfied, the VAV speed signal (VFD or Inlet Guide
Vanes) goes to maximum flow [the customer-installed con-
nection will drive the VAV boxes to full open]. The control
engages unit heat (optional) at full capacity. Until heat is
22
energized, duct pressure control is activated. The optional
power exhaust system is activated.
In heat mode, cooling is disabled and the optional econo-
mizer outside air dampers are closed. This operation is
maintained until the space temperature rises above the
heating setpoint or Occupied mode is engaged.
11. Gas Heat Single Stage Module: Each module installed
has it own ignition control and its own set of safeties. When
there is a call for heat from the unit control board (UCB) for
each module, the ignition control board (ICB) for that mod-
ule checks the state of the flame sense, the roll out switch,
the pressure switch and the primary temperature limit
switch circuits for that module. If they are in the expected
state, then the ICB energizes the draft motor and verifies
that the pressure switch closes. After the pressure switch
closes, a 30 second heat exchanger purging period is com-
pleted. After this purging period, the ICB will energize the
gas valve and the ignition coil for 10 seconds. Once the
flame sensor senses a flame is present, the ignition coil is
de-energized. The ICB monitors flame stability for 10 sec-
onds. The ICB will retry 3 times to obtain a stable flame. If
the furnace fails to light 3 times during the same call for
heat, then the ICB will lock out operation of that module
until the next call for heat exists. The ICB monitors the
module's safety devices during that module's operation.
When the UCB de-energizes the call for heat for a given
module, the ICB closes the gas valve for that module and
performs a 30 second purging of the heat exchanger by
continuing the operation of the draft motor.
Gas Heat Operation Errors - During furnace operation,
the ICB monitors the flame sense circuit, the pressure
switch, the primary limit switch and the roll out switch. If
a signal from any of the inputs moves to a fault state,
then the ICB immediately closes the main gas valve.
The ICB will determine the device that is signaling a fault
and flash a code for that device (Table 7). A primary limit
trip, pressure switch trip or flame sense fault triggers a
temporary lock out. A roll out switch trip requires manual
reset. Some UCB's installed on Millennium also monitor
the primary limit and/or gas valve for fault conditions to
set warning flags.
Temperature Limits - The primary limit is located such
that a temperature sensitive switch can sense the tem-
perature of the heat exchanger tubes. The limit is
mounted just above the inlet of the heat exchanger
tubes on the left side. If a primary limit fault occurs (the
primary limit opens due to excessive heat exchanger
temperature), then the ICB will flash the appropriate
code (Table 7) and monitor the primary limit. The ICB will
energize the draft motor during the time the primary limit
is open. When the primary limit closes and the call for
heat still exists, the ICB will start the ignition sequence
over.
Gas Valve - The gas valve is of the redundant type such
that if either one of two valves inside the gas valve fails
to shut off the gas flow then the other valve will shut off
the flow.
Johnson Controls Unitary Products
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