Control System Operations; Burner Control System; Overview - A.O. Smith 100 Series Service Handbook

BURNER CONTROL SYSTEM

The R7910A1138 is a burner control system that provide heat control, flame supervision, circulation pump control, fan control, boiler/
water heater control sequencing, and electric ignition function. It will also provide boiler/water heater status and error reporting.
Multiple boiler/water heaters can be joined together to heat a system instead of a single, larger burner or boiler/water heater. Using
boiler/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.
Control System consists of:
• R7910A1138 Control Device.
• S7999D Touchscreen Display—required for setup and ModBus communication but not required for the system to operate
once the R7910A1138 is programmed.
• S7910A Local Keyboard Display Module.
• Flame Sensor.
• Temperature Sensor, NTC Type 10KΩ at 77°F (25°C) or 12KΩ at 77°F (25°C).
• Limit Sensor, NTC Type 10KΩ at 77°F (25°C).
• 24V Digital I/O.

OVERVIEW

Functions provided by the R7910A1138 include automatic boiler/water heater sequencing, flame supervision, system status indication,
firing rate control, load control, CH/DHW control, limit control, system or self-diagnostics and troubleshooting.
The R7910 maximum version of the controller offers:
1. NTC-temperature sensor for:
• Outlet Limit And Temperature.
• Stack Temperature Limit and Temperature.
• Inlet Temperature.
• Outdoor Temperature (R7910 only).
2. Modulating output PWM-driven rotation speed controlled DC-fan for optimal modulation control.
3. Three Pump Outputs with 5 selectable operation modes.
4. 24VAC:
• Output control of gas valve (Pilot and Main) and External Ignition Transformer.
• Digital inputs for room limit control, high limit control, Gas pressure switch, low water cutoff.
5. External spark transformer.
6. Flame sensor.
7. Test jacks for flame signal measurement from a flame sensor.
8. Alarm Output.
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CONTROL SYSTEM OPERATIONS

FIGURE 26. R7910A1138 CONTROL SYSTEM
Servicing should only be performed by a Qualified Service Agent