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XI Control System Wiring
WARNING
All wiring and grounding must be done in accordance with the authority having jurisdiction or, in the absence of such
requirements, with the National Electrical Code (ANSI/NFPA 70).
WARNING
OPERATING CONTROL CONTACTS MUST BE “DRY” (ELECTRICALLY ISOLATED FROM ALL OTHER
CIRCUITS IN THE OPERATING CONTROL). IF OPERATING CONTROL CONTACTS ARE NOT DRY, THE
BOILER SAFETY DEVICES COULD BE DEFEATED RESULTING IN PROPERTY DAMAGE, INJURY OR
LOSS OF LIFE.
1) Provide the boiler with a dedicated branch circuit with a fused disconnect. The minimum rating of this circuit
must be 15A.
2) Wire the boiler following the appropriate wiring diagram in Figures 35 or 36. Use 14 gauge wire for all 120 volt
boiler wiring. Use wire with a temperature rating of 105°C for wiring routed along gas manifolds. Install the
junction box provided on one of the end panels. This junction box is to become the hub for all limit and low water
cut-off wiring. All 120 volt boiler wiring must be enclosed in conduit. Route wiring to base modules in flexible
conduit along the gas manifolds. See Figure 34 for typical wire routing.
3) Make sure that all single pole switches and safety controls are in the “hot” (ungrounded) side of the circuit.
4) The “operating control” shown in Figures 35 and 36 is not supplied. It is the control that turns on the boiler in
response to a call for heat. If a 24 volt thermostat is to be used as the operating control a 24 volt relay center,
such as the Honeywell R845, may be used to cycle the boiler in response to a call for heat. An R845A may also be
used to start a system circulator if necessary as long as it’s contact rating is not exceeded.
On water boilers with multiple base modules, the bases may be “staged” by using a set of operating control
contacts to break the 24 volt “hot” (ungrounded) lead connecting the base transformer to the ignition module. Do
not stage steam boilers with multiple bases.
Sequence of Operation – Standard Boiler Wiring
1) All low water cut-offs, limit controls, and the operating contacts are wired in series so that the boiler will not
operate if any of these controls are open. Under normal circumstances, only the operating control contacts will be
open.
2) Upon a call for heat, the operating control will make and 120 volts will be applied to all base module transformers,
which are wired in parallel.
3) 24 volts will then appear across the “24V” and “24V (GND)” terminals on the ignition modules.
4) Upon application of voltage across the “24V” and “24V (GND)” terminals, the ignition module will start an ignition
spark at the pilot and apply 24 volts across the pilot valve.
5) Once the pilot is established, the pilot flame will act as a diode, converting the AC current at the electrode to a half
wave DC current at the pilot’s ground strap. This DC current flows through the boiler to the “GND (BURNER)”
connection on the ignition module.