Bradford White Brute MagnaTech BMGV1600 Installation And Operation Instructions Manual page 70

Page 66
9.7.17 Lag (Slave)
About Lead Lag Operation -
If an installation includes two or more units, they may
Figure 42 shows an installation with eight units (A
Lead Lag system can include up to eight units.) The
a Modbus connection.
NOTE: Lead & Lag within the touchscreen
display is referred to Master & Slave. These are
industry terms and are used interchangably by
different control system manufacturers.
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 units.
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 units on-line. As the heating demand is reduced,
the Lead Lag system shuts off some of the units.
The signal from the System sensor is sent to the unit
operating as the Lead. 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 unit 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 unit.
The unit acting as the Lead decides which unit
This assignment is rotated
Touch
Screen
Lead Lag
Addr
Master
1
Boiler 1 Boiler 2
Figure 42. " Lead Lag" Operation in a System with Eight Units.
Addr Addr Addr
2 3
Boiler 3 Boiler 4
across all of the available units so that any one unit
The Lead Lag controller tracks the run times for all
of the units, and uses this to calculate the starting
order for the units. This means that each time the
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
Low demand -
The first boiler in
sequence fires at
less than 65%
Demand increases -
Once the first boiler
reaches 65%,
the second boiler
switches on, and by
default, both modulate
together between 20%
and 65% (this can be adjusted up to 20% and 85%).
Nearing max. demand -
Once both reach 65%,
then they are both
allowed to go over
65%
Figure 41. Lead Lag Operation, 2 boilers.
Addr Addr
4 5 6
Boiler 5 Boiler 6
Caution
will not work.
First
boiler
Addr Addr
7 8
Boiler 7 Boiler 8