Thermal Solutions APEX APX425C Installation, Operating And Service Instructions page 106

10
Operation
(continued)
185
Setpoint 180
175
100
Base Load
Rate
Default 40%
40
Range
(25 – 100%)
0
3
2
1
0
Lead Boiler
Optimized Boiler Modulation
Boiler firing rate is managed to increase
smoothly as boilers are started. For example,
when a second boiler is started the initial firing
rate is 100%/2 or 50%, when the third boiler is
started the firing rate starts at 200%/3 or 66%.
After the initial start, the Sequencer Master
develops a unison firing rate demand based
on its setpoint and header temperature.
Base Load Rate
During low loads, the Sequencer Master limits
firing rates to a Base Load Rate to ensure
modulating condensing boiler peak operating
efficiency. Lower firing rates boost efficiency by
helping increase the amount of flue gas water
vapor condensation. The control maintains
a Base Load Rate until the last lag boiler is
started. At this point, the Base Load Rate
is released to allow boilers to modulate as
required to meet heat load.
Customized Sequences
Normally, boilers are started and stopped in
numerical order. However, custom sequences
may be established to optimize the heat
delivery. For example, in order to minimize
boiler cycling, a large boiler may be selected
to run first during winter months and then
selected to run last for the remainder of the
year.
106
2
1
2
Fast
Load
Change
Start
Start
st
1 Lag
Figure 10-24: Multiple Boiler Sequence Diagram
(3 boiler system shown, typical for up to 8 boilers)
APEX Installation, Operating, & Service Instructions
3
4
Slow
Load
Change
Start Stop
nd nd
2 Lag 2 Lag
Multiple Demands
The Sequencer Master responds to Central
Heat, DHW and frost protection demands
similar to a stand-alone boiler. For example,
when DHW Priority Time is nonzero and DHW
priority is active, the Sequencer Master uses
DHW setpoint, Differential Above, Differential
Below and pump settings. However, the
Sequencer Master always uses the header
sensor and does not use the DHW sensor.
DHW Two boiler Start
When the DHW Pump parameter is set to
DHW Pump: Primary Loop Piped IWH and
the Sequencer Master DHW Two Boiler Start
parameter is set to Two Boiler, two boilers are
started without delay in response to DHW
demand. This feature allows rapid recovery
of large indirect water heaters (IWH's) and
multiple IWH's.
Shared or Isolated DHW Demand
When the DHW Pump parameter is set to
Primary Loop Piped IWH, the Sequence
Master sequences all required boilers to satisfy
the DHW demand. When the DHW Pump
parameter is set to Boiler Piped IWH, only
the individual slave boiler, with wired DHW
Demand or DHW Sensor and pump, fires to
satisfy DHW demand.
5
4
5
Stop Stop
st
1 Lag Lead Boiler
106308-05 - 3/23