Lead Lag - A.O. Smith 100 Series Service Handbook

Domestic Hot Water
Start-up sequence DHW-request (system in standby):
1. Heat request detected (Tank Sensor below Setpoint).
2. The 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 (depending on configuration).
7. Pre-Ignition Time will energize the ignitor and check for flame.
8. Trial for Ignition. Specifics for timings and device actions are defined by the OEM or installer.
9. The ignition and the gas 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 DHW Forced Rate and Slow Start Enable, if the
water is colder than the threshold.
13. At the end of the DHW-heat request the burner is switched off and the fan stays on until post purge is complete.
14. The pump stays on during the pump overrun time.
15. At the end of the pump overrun time the pump will be switched off.

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 System function as one of the slaves), or to operate solely as a slave to the lead lag system.
Burner 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 Burner 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
Buner Control Systems via Modbus.
The LL master uses a few of the host Burner Control System'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 XP product 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, whereas the XWH Water Heater is factory configured for Domestic Hot Water
application. The LL master coordinates the firing of its slave Burner 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 Burner Control System is turned on.
• When the operating point reaches the Setpoint plus the Off hysteresis then the last slave Burner Control (or all slave Burner
Controls) 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 Burner Controls according to the rate
allocation algorithm selected by the Rate allocation method parameter.
For some algorithms, this rate might be common to all slave Burner Controls 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).
42 Servicing should only be performed by a Qualified Service Agent