A.O. Smith Boiler 3400 User Manual

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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 ame signal measurement from either a ame
rod or UV ame sensor.
8. Alarm Output.
FIGURE 48. GENERAL R7910A1138 BOILER SCHEMATIC
The above gure shows two loops of heat control: Central
Heating (CH), and an optional second loop for Domestic Hot
Water (DHW) can be congured on each R7910A. The DHW
loop transfers heat from the boiler outlet to hot water appliances
in conjunction with the primary system heat loop. Priority
assignment to each heat loop can be congured to specify which
loop gets serviced rst.
CONTROL SYSTEM
BURNER CONTROL SYSTEM
FIGURE 47. R7910A1138 CONTROL SYSTEM
The R7910A1138 is a burner control system that provide
heat control, ame supervision, circulation pump control,
fan control, boiler control sequencing, and electric ignition
function. It will also provide boiler status and error reporting.
Multiple boilers can be joined together to heat a system instead
of a single, larger burner or boiler. Using boilers in parallel is
more efcient, costs less, reduces emissions, improves load
control, and is more exible than the traditional large boiler.
Control System consists of:
• R7910A1138 Control Device.
• S7999B 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 Rod .
• 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
sequencing, ame supervision, system status indication, ring
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.

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