A.O. Smith 3400 - Manual (Page 47)

47
LEAD LAG
Burner Control System devices contain the ability to be a stand-
alone control, operate as a Lead Lag Master control (which also
uses the burner control function as one of the slaves), or to
operate solely as a slave to the lead lag system.
Control System devices utilize two ModBus™ ports (MB1
and MB2) for communications. One port is designated to
support a system S7999D display and the other port supports
communications from the LL Master with its slaves.
The Lead Lag master is a software service that is hosted by a
Control System. It is not a part of that control, but is an entity
that is “above” all of the individual burner controls (including the
one that hosts it). The Lead Lag master sees the controls as a
set of Modbus devices, each having certain registers, and in this
regard it is entirely a communications bus device, talking to the
slave burner controls via Modbus.
The LL master uses a few of the host Burner Control's sensors
(header temperature and outdoor temperature) and also the
STAT electrical inputs in a configurable way, to provide control
information.
LEAD LAG (LL) MASTER GENERAL OPERATION
The XB Boiler is a multiple burner application and it works on
the basis of the Lead Lag Operation. The XB Boiler is factory
configured for Hydronic/Central Heating application. The LL
master coordinates the firing of its slave Control Systems. To do
this it adds and drops stages to meet changes in load, and it
sends firing rate commands to those that are firing.
The LL master turns the first stage on and eventually turns the
last stage off using the same criteria as for any modulation
control loop:
•
When the operating point reaches the Setpoint minus the
On hysteresis, then the first Control System is turned on.
• When the operating point reaches the Setpoint plus the Off
hysteresis then the last slave Control System (or all slave
Control Systems) are turned off.
The LL master PID operates using a percent rate: 0% is a
request for no heat at all, and 100% means firing at the maximum
modulation rate.
This firing rate is sent to the slaves as a percentage, but this is
apportioned to the slave Control Systems according to the rate
allocation algorithm selected by the Rate allocation method
parameter.
For some algorithms, this rate might be common to all slave
Control Systems that are firing. For others it might represent the
total system capacity and be allocated proportionally.
For example, if there are 4 slaves and the LL master's percent
rate is 30%, then it might satisfy this by firing all four slaves at
30%, or by operating the first slave at 80% (20% of the system’s
capacity) and a second slave at 40% (10% of the system’s
capacity).
The LL master may be aware of slave Control System’s minimum
firing rate and use this information for some of its algorithms,
but when apportioning rate it may also assign rates that are less
than this. In fact, the add-stage and drop-stage algorithms may
assume this and be defined in terms of theoretical rates that are
possibly lower than the actual minimum rate of the Burner Control
System. A Control System that is firing and is being commanded
to fire at less than its minimum modulation rate will operate at its
minimum rate: this is a standard behavior for a Burner control
system in stand-alone (non-slave) mode.
GENERAL OPERATIONAL SEQUENCE
INITIATE
The R7910 enters the Initiate sequence on Initial Power up or:
• Voltage fluctuations vary less than 20VAC or greater than
30VAC.
• Frequency fluctuations vary +/-5% (57 to 63 Hz).
• If Demand, LCI, or Stat interrupt (open) during the Prepurge
Period.
• After the reset button is pressed or fault is cleared at the
displays.
The Initiate sequence also delays the burner motor from being
energized and de-energized from an intermittent AC line input or
control input.
If an AC problem exists for more than 240 seconds a lockout will
occur.
HYDRONIC/CENTRAL HEATING (XB BOILER)
Start-up sequence central heating request (system in standby):
1. Heat request detected (On Setpoint - On Hysteresis).
2. The CH pump is switched on.
3. After a system Safe Start Check, the Blower (fan) is switched
on after a dynamic ILK switch test (if enabled).
4. After the ILK switch is closed and the purge rate proving fan
RPM is achieved (or High Fire Switch is closed) - prepurge
time is started.
5. When the purge time is complete, the purge fan RPM is
changed to the Lightoff Rate or if used, the damper motor is
driven to the Low Fire Position.
6. As soon as the fan-rpm is equal to the light-off rpm (or the
Low Fire Switch closes), the Trial for Ignition or Pre-Ignition
Time is started.
7. Pre-Ignition Time will energize the igniter and check for
flame.
8. Trial for Ignition. Specifics for timings and device actions are
defined by the A. O. Smith.
9. The ignition and the gas control valve are switched on.
10. The ignition is turned off at the end of the direct burner
ignition period, or for a system that does use a pilot, at
the end (or optionally at the middle) of the Pilot Flame
Establishing Period (PFEP). For an interrupted pilot system
this is followed by a Main Flame Establishing Period (MFEP)
where the pilot ignites the main burner. For an intermittent
pilot there is no MFEP.
11. The fan is kept at the lightoff rate during the stabilization
timer, if any.
12. Before the release to modulation, the fan is switched to
minimum RPM for the CH Forced Rate and Slow Start
Enable, if the water is colder than the threshold.
13. At the end of the CH-heat request the burner is switched off
and the fan stays on until post purge is complete.
14. A new CH-request is blocked for the forced off time set by
the Anti Short Cycle (if enabled).
15. The pump stays on during the pump overrun time.
16. At the end of the pump overrun time the pump will be
switched off.