Boiler/Water Heater Controls; Primary System Control - A.O. Smith 100 Series Service Handbook

BOILER/WATER HEATER CONTROLS

The 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/water
heater and system control.

PRIMARY SYSTEM CONTROL

All 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. The Primary System Control will activate and
deactivate boiler/water heater heating cycles based on its setpoint and current system water temperature. There are three suitable
methods to configure a Primary System Control. One of these three methods must be used.
1. The Primary System Control can be the boiler/water heater's control system working with the factory supplied Header Sensor,
installed inside the storage tank on domestic water applications or in the return line on primary/secondary hydronic heating
systems for XB.
2. Alternatively, on XWH water heater the Burner Control system can be used as a Primary System Control. It will also provide
water heater status and error reporting. Multiple water heaters can be joined together to heat a system instead of a single, larger
burner or boiler/water heater. Using boilers/water heaters in parallel is more efficient, costs less, reduces emissions, improves
load control, and is more flexible than the traditional large boiler/water heater.
3. MB2 and COM2 ports can be used for Building Management Systems.
FIELD WIRING
120 VAC Power Supply Wiring
A dedicated, single phase, 30/60 amp circuit breaker with a grounded neutral should be provided to supply power to the boiler/water
heater(s). Use #10 AWG wire for the 120 VAC power supply to the boiler/water heater. All 120 VAC power supply connections must
be made as shown in Figure 1. These connections should be made at the rear of the boiler/water heater where a wiring junction box
is provided. Field installed power supply wiring to the boiler/water heater should be installed in conduit. This conduit and wiring should
be separate from any other conduit/wiring to guard against EMI (electromagnetic interference).
Power Supply Check
To reduce the possibility of electrical interference with the control system, the power supply voltage, polarity and ground must be
checked. Using an AC volt meter check the 120 VAC power supply wiring from the breaker prior to making power supply connections
at the boiler/water heater. Confirm the power supply voltage & polarity are correct and that an adequate ground connection is present
by performing the three voltage tests below. See Figure 1.
Confirm RMS voltage between:
• H and GND = 108 VAC minimum, 132 VAC maximum.
• N and H = 108 VAC minimum, 132 VAC maximum.
• N and GND = < 1 VAC maximum.
FIGURE 1. FIELD WIRING
Servicing should only be performed by a Qualified Service Agent 7