LP-446-r4 Rev. 10.12.16
41
boiler outlet temperature and stand-by, waiting for a demand
for hot water. If the temperature of the tank sensor falls below
the tank set point minus the tank differential temperature, a
demand for hot water is generated by the control.
When a demand for hot water is received, the control begins
the following demand sequence. The boiler first turns on the
pump (if it is not wired to run continuously). Once the pump is
running, the control will display LOW WATER FLOW and wait
for the water flow in the system to increase to an acceptable
level determined by the flow switch on the outside of the
boiler. (NOTE: This step may happen very rapidly. If flow is
adequate, LOW WATER FLOW may never display.)
Once flow through the boiler is adequate, the control will
measure the supply temperature. If it is below the set point
temperature minus the ignition differential set point, the
control will ignite the burner. After the burner is lit, the
control modulates the firing rate to control the supply water
temperature at the set point temperature plus the supply
offset temperature (installer #4) above the tank set point
temperature. When the tank temperature is equal to the
tank set point temperature, the control will extinguish the
burner and run the combustion fan to purge gasses from the
combustion chamber. In addition, the pump will run for a
pump post purge interval. The control will then be in standby,
waiting to process the next demand for heat.
During this process, the control will extinguish the burner
if it senses a dangerous or unsafe condition. If the control
determines that a dangerous or unsafe condition has occurred,
the control may lock out the boiler and prevent it from igniting
until a maintenance person diagnoses the problem, repairs
it, and resets the control. In the event that the control goes
into lockout, it will show a diagnostic code on the display,
illuminate the LED fault indicator, and close the alarm relay
contacts to aid in recognition of the condition, diagnosis, and
repair.
F. Setting Up a Cascaded System
If the boiler is part of a cascaded system the operation is
somewhat different. The control of each boiler in a cascaded
system completes its own power up system check as described
above. One of the boilers in the cascade system is designated
as the master boiler. After the master boiler completes its
power up sequence, it checks the communication bus to see if
any other boilers are present. If other boilers are present, the
master control determines these follower boiler addresses.
The master boiler control will recheck the bus every few
seconds as long as it is powered up to update the status of the
connected boilers. The control in the master boiler processes
all heat demands and dictates which of the follower boilers
should light and what firing rate the followers should try to
achieve.
When the master boiler receives a demand for heat, it
determines which boiler is first in the firing sequence and
sends that boiler a command to begin a demand sequence.
That boiler will then begin a demand sequence as described
above. Once the boiler ignites, the master boiler control will
increase the firing rate command to that boiler until the system
sensor temperature is at the tank set point temperature plus
the supply offset temperature (installer #4), or that boiler is
at high firing rate. If the command from the master boiler
control gets to the high firing rate of the follower boiler,
but the system sensor is below the required temperature, the
master boiler control will then tell the next boiler in the firing
sequence to begin its demand sequence. The master boiler
control will then begin to ramp up the firing rate command of
that boiler. This process will continue while there is a demand
until all boilers in the cascade system are at high fire or the
desired temperature of the system sensor is reached. If the
system sensor temperature reaches tank set point and installer
#4 before all boilers are at high fire, the master control will
modulate the cascade command signal to maintain the system
sensor at tank set point and installer #4 until the demand is
complete. When the tank temperature is equal to the set point
temperature, the demand is complete, and the master boiler
control will extinguish all boilers that may be lit. If the demand
decreases, the firing rate command and amount of boilers lit will
decrease exactly opposite as described above.
Whenever the master boiler control needs to fire a follower
boiler, it sends a firing rate command to that boiler. The
follower boiler will respond to the command until its supply
sensor temperature gets to be 5
o
F above the tank set point
temperature plus the supply offset temperature (installer #4),
at which point the individual boiler will modulate on its own so
as not to overheat. As a result, it is not uncommon to see the
cascade output at maximum but individual boilers firing at less
than their maximum firing rate.
G. Lockout Condition
If any boilers, including the master boiler in the cascade system,
are in a lockout condition, the master control will recognize the
lockout condition and skip over the boiler in the firing sequence.
Each boiler in the cascade system is responsible for its own
safety functions. So, if any individual boiler control senses an
unsafe condition, it will extinguish the burner and, if necessary,
go to a lockout condition. In this way, every boiler in the system
has its individual safety controls intact and operational, even if
the firing of the boiler is under control of the master boiler.
In the event that the system sensor fails, all boilers in the
system will ignite simultaneously when there is a demand, and
each boiler will individually regulate firing rates to maintain
the master set point temperature (tank set point + installer
#4) at the individual supply sensors built into the boiler. If this
should happen, the master boiler will display an E03 fault code,
indicating that the supply sensor has failed.
H. Cascade System Programming
1. If the boiler is used alone, skip this section.
2. Programming the Master Boiler:
a. Make sure there is no demand for heat being supplied to
the boiler.
b. Apply power to the boiler.
c. Enter the system setting program navigation following
instructions in Part 10 of this manual.
d. Verify that cascade address function 15 is set to 0. This
makes the master boiler address 0. NOTE: The Master Boiler
MUST be addressed as 0. This establishes the unit as the
master in a cascaded system.
e. Change Cascade Mode function 23 from VISION3 to ALL
926 if cascaded system IS NOT common vented. If cascade
system IS common vented, set Function 23 to the total
number of boilers in the system (4 BOILERS for a four boiler
system, 6 BOILERS for a six boiler system, etc.)
Summary of Contents for MODCON500
Page 35: ...LP 446 r4 Rev 10 12 16 35 Figure 29 Cascade Master and FollowerWiring...
Page 57: ...LP 446 r4 Rev 10 12 16 57 Figure 35 500 Model Combustion System Replacement Parts...
Page 58: ...LP 446 r4 Rev 10 12 16 58 Figure 36 700 850 Models Combustion System Replacement Parts...
Page 60: ...LP 446 r4 Rev 10 12 16 60 Figure 38 Cabinet Replacement Parts All Models...
