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Figure 30. Lead / Lag Operation, 2 boilers.
Low demand -
The first boiler in
sequence fires at
less than 65%
First
boiler
Second
boiler
Demand increases -
Once the first boiler
reaches 65%,
the second boiler
switches on, and
both modulate
together between 20%
and 65%
Nearing max. demand -
Once both reach 65%,
then they are both
allowed to go over
65%
9.7.17 Lead Lag Slave Configuration
About Lead/Lag Operation -
If an installation includes two or more boilers, they
may be set up for “Lead/Lag” operation. One boiler
will be set up as the “Master”, and the others will
operate as “Slaves.” Figure 31 shows an installation
with eight boilers. (A Lead/Lag system can include up
to eight boilers.) The boiler controllers are connected
in a “daisy chain” using a Modbus connection.
A single System sensor is used to monitor the demand
for heat. The input from this sensor is used by the
Master controller to control the modulation rates of all
of the operating boilers.
Lead/Lag Modulation Cycle -
Note - We will explain the modulation cycle here, in
case you need to understand how the Lead/Lag system
actually operates. If you are installing the unit(s)
and want to skip this section, just remember that, as
the heating demand increases, the Lead/Lag system
puts more boilers on-line. As the heating demand is
reduced, the Lead/Lag system shuts off some of the
boilers.
The signal from the System sensor is sent to the unit
operating as the Lead/Lag Master. A Run sequence is
initiated when the system temperature falls to the Lead/
Lag Central Heat setpoint. (Actually this is the LL
CH setpoint less the On Hysteresis value. The default
setting for On Hysteresis is 5°F, but this is adjustable.)
Notice that, when a boiler is operating as part of a
Lead/Lag system, it does not use the normal CH
setpoint – it uses the special LL CH setpoint on the
Lead/Lag Master.
The unit acting as the Lead/Lag Master
decides which boiler is assigned to start first. This
assignment is rotated across all of the available
boilers so that any one boiler does not run
significantly longer than the others. The Lead/Lag
controller tracks the run times for all of the boilers,
and uses this to calculate the starting order for the
boilers. This means that each time the system starts
up, a different boiler may start first.
Figure 31. “ Lead/Lag” Operation in a System with Eight Boilers.
Note: The Displays on Boilers 2 thru 8
will display information pertaining only to that specific boiler.
Touch
Screen
Lead Lag
Master
Addr
1
Addr
2
Addr
3
Boiler 4
Addr
4
Boiler 5
Addr
5
Boiler 6
Addr
6
Boiler 7
Addr
7
Boiler 8
Addr
8
Boiler 3
Boiler 2
Boiler 1
Caution
You should set the Modbus addresses before
you connect the Modbus wiring. If the wiring
is attached before the Modbus addresses on
the controls are changed, there will be multiple
controls with the same address, and the system
will not work.
WARNING
If the Modbus addresses are not assigned
properly, the system could fail to operate
correctly, or it might operate in an unsafe
manner. This could lead to property damage,
personal injury or death.
