Boiler Controls; The Heating Cycle; Primary System Control - A.O. Smith 100 Series Service Handbook

BOILER CONTROLS

Boiler controls that are improperly installed or configured can cause serious operational and service related
problems such as short cycling. This section provides information for how various controls can work together
or independently to provide proper boiler and system control.

THE HEATING CYCLE

Two conditions must be met before the VF Boiler can start a heating cycle:
1. The actual temperature sensed by the Control System from the designated "Controlling Probe" (page 7)
must be lower than the Operating Setpoint minus the Setpoint Differential, see Adjusting User Settings
on page 43.
2. The Enable/Disable circuit must be a closed circuit. There are two wires provided in the wiring junction
box on the back of the Boiler for this circuit, see Figure 1.
NOTE: See the Sequence Of Operation on page 45.

PRIMARY SYSTEM CONTROL

All VF Boiler installations require a "Primary System Control" that senses and reacts to water temperature
inside the storage tank on domestic water applications or in the return line on primary/secondary hydronic
heating systems. When installed and configured properly the Primary System Control will activate and
deactivate boiler heating cycles based on its setpoint compared to current system water temperature. There
are three suitable methods to configure a Primary System Control. One of the following three methods MUST
BE used.
120 VAC
INPUT
POWER
SUPPLY
FACTORY SUPPLIED REMOTE
TEMPERATURE PROBE OR QUAD
THERMISTOR PROBE
FIELD SUPPLIED
EXTERNAL CONTROL
1. The Primary System Control can be the Boiler's EMC 5000 Control System working with the factory
supplied Remote Temperature Probe or the optional Quad Thermistor Probe installed inside the storage
tank on domestic water applications or in the return line on primary/secondary hydronic heating systems.
This application ensures the Boiler's ability to fully modulate its firing rate. See Temperature Probes on
page 15 and Figure 33 on page 65.
2. Alternatively, the iCOMM™ system hardware working with an onsite BACnet compliant BMS (Building
Management System) can be used as a Primary System Control. This application ensures the Boiler's
ability to fully modulate its firing rate and allows BACnet compliant controls to utilize outdoor temperature
6
POWER SUPPLY AND SYSTEM CONTROL - FIELD WIRING
BACK OF BOILER
120 VAC HOT
120 VAC NEUTRAL
GND - EARTH GROUND
CONTROL WIRING
COM
NO
Servicing should only be performed by a Qualified Service Agent
FACTORY INSTALLED WIRING
JUNCTION BOX
FIELD INSTALLED WIRING
HIGH VOLTAGE
WIRING
BLACK - 120 VAC HOT
WHITE - 120 VAC NEUTRAL
GREEN - GROUND
LOW VOLTAGE
YELLOW - 5 VDC
YELLOW - 5 VDC
BLACK/WHITE STRIPE - 24 VAC
YELLOW/VIOLET STRIPE - 24 VAC
Figure 1
REMOTE
TEMP
PROBE
ENABLE
DISABLE
CIRCUIT