About Lead / Lag - Bradford White Brute BNTH Installation And Operation Instructions Manual

Page 44
About Lead/Lag Operation
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 35 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
Touch
Screen
Lead Lag
Addr
Master
Boiler 1
Figure 36. " L ead/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.
Addr Addr
1 2 3
Boiler 2 Boiler 3
the starting order for the boilers. This means
that each time the system starts up, a different
boiler may start first.
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
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.
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
both modulate
together between 20%
and 65%
Nearing max. demand -
Once both reach 65%,
then they are both
allowed to go over
65%
Figure 35. Lead / Lag Operation, 2 boilers.
Addr Addr
4 5
Boiler 4 Boiler 5 Boiler 6
B
RADFORD
Caution
will not work.
WARNING
First
boiler
Addr Addr Addr
6 7 8
Boiler 7 Boiler 8
W
HITE