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Rev 2.1
2-10.3 4–20 mA Remote Setpoint
Remote setpoint control will be enabled from the factory with default setting of 40
⁰
F for 4 mA and 200
⁰
F for 20
mA. When the remote signal is connected to the boiler, setpoint is provided using the linear interpolation of the
4 mA and 20 mA input signal from the building automation controller. If the signal is disconnected, the boiler
will automatically use the local setpoint as operating setpoint. 4-20 mA setpoint range can be changed using the
Enhanced panel or the System Operator Interface panel, see section 4-21 for parameters.
For other related parameter and functions see chapter 4 for details.
2-11 Lead Lag Installation and Programming
Atlas A Boiler comes standard with embedded lead lag control. Lead lag function is disabled from the factory
but can be enabled with programming covered in this section. Also, necessary wiring connection must be
completed before Lead Lag System can operate.
Each unit contains the ability to be a standalone boiler or to operate as a part of lead lag system. Lead lag master
will send messages through the communication port to add or drop stage as well as send its firing rate and check
slave status.
Lead lag master will command its slaves to maintain the system header temperature (common outlet connection
of all boilers in the system). The sensor must be installed on pipe with immersion well and should be installed
on the pipe with system flow. Flow on each boiler should be adjusted to assure all boilers in the system have
same flow rates.
2-11.1 Sensor Wiring
Lead lag master’s multipurpose S5 sensor (Terminals H1 & H2) is used to read system header temperature. If
the lead lag master’s system header sensor fails for any reason, the lead lag master will use the average outlet
temperature of the lead lag slaves currently firing.
If lead lag system uses outdoor reset, outdoor sensor can be connected to one of any slave’s multipurpose S5
sensor (Terminals H1 & H2). Lead lag master will automatically use this input as outdoor temperature.
Use the single element NTC sensor (Ajax PN: CAA198799Z) for header and outdoor sensor.
2-12 Lead Lag Operation
2-12.1 Sequencing
Factory default setting for sequencing is
Equal Runtime Algorithm
where boilers will be sequenced according to
a runtime. Boiler with lower runtime will fire first to equalize the run time of all boilers in the system.
2-12.2 Rate Allocation Control
Factory default setting for rate allocation is the
Parallel Common Base Limited Rate Allocation
method to
modulate the slave boilers. The method will limit the firing rate of all slave stages to the
Common Base Load
Rate
(default 40%) until all stages are firing. Once all units are firing, there is no restriction on the slave’s
commanded firing rate. As load decreases, as long as all available stages are firing, there are no restrictions on
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