JOHNSON CONTROLS
88
FORM 100.50-NOM12
ISSUE DATE: 04/02/2019
SECTION 5 – SEQUENCE OF OPERATION
• When more heat is needed, the IPU board sends
24 VAC to a relay in the heating section. This re-
lay closes a set of contacts that energize the high
fire solenoid of the gas valve.
• When more heat is still needed and the unit has
the available capacity, furnace 2 and 3 follow the
above sequence.
Modulating Natural/Propane Gas Heat
• Modulating natural gas heat can be ordered with
one of three heating capacities:
1. 375 MBH (1 furnace section)
2. 750 MBH (2 furnace sections)
3. 1125 MBH (3 furnace sections)
• The modulating gas heat option has a turndown
ratio of 24:1 with all three furnaces installed
•
On modulating gas heat, the first furnace section
is always divided into two separate halves: 1A and
1B.
• Each half of furnace section 1 has its own 2
stage gas valve.
• Each half of furnace section 1 has its own
ignition control.
• Furnace section 1A is also a modulating gas
valve.
• Furnace sections 1A and 1B shares a 2 speed
induced draft motor.
• The induced draft motor switches between
low and high speeds depending on the heat-
ing demand.
• Furnace 1A is always the first ON and last OFF;
Furnace 1B is always the last ON and first OFF.
When the unit has two or three furnace sec-
tions, furnaces 2 and 3 stage ON before furnace
1B stages ON.
• Furnace sections 2 and 3 have separate 2 stage gas
valves, single speed induced draft motors and igni-
tion controls, if applicable.
• Each furnace is able to operate independently of the
other furnaces, except for furnace section 1. When
either 1A or 1B is in a fault, both halves are not al-
lowed to operate. Furnace sections 2 and 3 are still
allowed to operate.
Modulating Natural Gas Heat Setup
• Set the heating system type to modulating gas un-
der the OPTIONS-HEATING menu.
• Set the gas heat capacity to the appropriate selec-
tion under the OPTIONS-HEATING menu.
• Ensure that Heating System is User Enabled un-
der the PROGRAM-HEATING menu.
1. SZVAV
• 1st and 2nd stage heating setpoints must be
entered under the SETPOINTS-HEATING
menu.
• Occ and unocc zone heating setpoints must
be entered under the SETPOINTS-HEAT-
ING menu. This is only when zone control
method is set for wired or comm zone tem-
perature.
• If using Unocc mode, night set-back must be
User Enabled.
2. VAV
• RAT heating setpoint must be entered under
the SETPOINTS-HEATING menu.
• SAT heating setpoint must be entered under
the SETPOINTS-HEATING menu.
• For Unocc mode, the IPU board controls to
the unocc zone heating setpoint.
• For Unocc mode, night set-back must be
User Enabled.
TABLE 35 -
MODULATING GAS FURNACE STATUS
OFF
No demand for heat
PURGE
Induced draft motor is running for 30
seconds
TRY FOR
IGNITION
Ignition control is attempting to
ignite the burner (7 seconds)
ON-LOW
Furnace is operating normally in
Low-Fire mode
ON-HIGH
Furnace is operating normally in
High-Fire mode
FAULT
Ignition control did not prove flame,
or other safety switch opened
FAULT-LOCKOUT
Multiplexor output to I/O board is not
accurate