This document serves as a comprehensive user manual for the A.O. Smith VF Series 200/201 gas-fired power burner copper boilers, specifically models VB/VW-500, 750, and 1000, designed for hydronic heating and hot water supply. It covers installation, operation, maintenance, and warranty information, emphasizing safety throughout.

The boiler system is equipped with an advanced EMC modulation control system, a fully integrated electronic control system. This system includes sensors, output devices, a power switch, a 24VAC transformer, and several printed circuit boards: the Modulation Control Board (MCB), Power Distribution Board (PDB), and User Interface Module (UIM). The MCB is central to master control and flame control, with dip switches for system configuration. The UIM provides a user-friendly interface with touchpads and a 4-line, 20-character LCD display for operational information. The PDB handles input power, water pump connections, and system fuses, also distributing power. The MCB also features an external communications system, allowing connection to a PC, modem, EMS system, or for linking multiple boilers.

The system's operation is divided into four modes: Initialization, Standby, Running/Modulation, and Service. Upon power-up, it enters initialization, displaying an A.O. Smith screen, performing calibration, recalling stored values, reading dip switch configurations, and checking for pending faults. After initialization, it transitions to standby (yellow "Standby" LED on) or service mode (red "Service" LED on) if a fault is detected. In standby, the temperature screen is displayed; in fault mode, the current error screen appears. The system continuously monitors temperature from controlling probes (inlet or tank) against the setpoint. If the temperature falls below the setpoint minus the differential temperature and the thermostat input is closed, a "call for heat" is initiated, and the system enters run mode (green "Running" LED on).

The heating sequence begins with applying power to the pump, followed by a 30-second pre-purge period with the blower on. The igniter is then activated. Once the igniter reaches a minimum current (2.8 amps), the gas valve is energized. After one second, the system verifies flame presence via the flame rod. If flame is not detected within 4 seconds, the gas valve shuts off, followed by a 15-second inter-purge period. The system then re-attempts ignition up to three times if configured. If flame is still not detected after the third try, an error is declared, requiring a control reset. The boiler continues running until the setpoint is satisfied, after which the blower runs for a 15-second post-purge period. The pump continues for a programmed post-circulate cycle. The control then returns to the idle state (yellow "Standby" LED). Fault detection during standby or running modes will halt heating and shift the system to service mode, displaying the detected fault.

The boiler employs a hysteresis-type control algorithm. It begins heating when the temperature sensed by the control probe (inlet or tank) falls below the operating setpoint minus the differential setpoint and stops heating when the temperature reaches the operating setpoint. The operating setpoint range is 70°F (21°C) to 190°F (88°C) for VW models and 70°F (21°C) to 220°F (104°C) for VB models, depending on the probe selection. The factory default is 145°F (49°C). The High Limit setpoint ranges from 90°F (32°C) to 210°F (99°C) for VW and 90°F (32°C) to 235°F (113°C) for VB. A "soft lockout" occurs if the outlet temperature exceeds the High Limit setpoint, automatically clearing when the temperature drops below the high limit setpoint minus the differential. The High Limit Differential Setpoint, adjustable from 1°F to 50°F, determines this differential.

The UIM screens provide various information:

Menu Screen: The main selection point for other screens.
Temperature Screen: Displays sensed temperatures from Outlet, Inlet, and Tank probes, along with the calculated Delta T (Outlet minus Inlet).
System Status Screen: Shows the status of switch inputs and output states, indicating "True" conditions with an asterisk (*).
Control Status Screen: Displays the MCB micro states, which sequence from Idle to Pre-Circulate, Heat Igniter, Check for Flame, Heating, Inter-Purge, Post-Purge, and back to Idle. Faults can cause a jump to Hard or Soft Fault states.
User Settings Screen: Allows adjustment of operating setpoint, high limit, high limit differential, temperature units (°F or °C), post-circulate delay time (45, 90, 180 seconds, or continuous), and network address.
Configuration Settings Screen: Displays the status of dipswitches.
Log & System Info Screen: Shows elapsed hours of operation, running minutes, kBtu rating, and software revision level.
Current Error Screen: Displays the last detected error with a timestamp. Errors are cleared by pressing the "select" key.
Error History Screen: Lists the last 9 errors with timestamps.
Reload Defaults Screen: Restores factory default values for adjustable configurations.

The UIM features five touch switches:

Menu Key: Activates the menu screen.
Select Key: Selects screens, items, and accepts changes.
Up/Down Keys: Move through screens, change setpoints, and user settings with auto increment/decrement.
Help Key: Displays helpful information about the current screen or general interface usage.

Three LEDs on the UIM indicate system status:

Red LED: Indicates a fault (continuous for soft faults, flashing for hard faults).
Yellow LED: Indicates standby mode (ready for heat call).
Green LED: Indicates heating mode (continuous) or initialization/calibration (flashing).

The VF boiler also displays the current firing rate (25% to 100%) on the UIM. The EMC 5000 system performs extensive self-diagnostics, displaying approximately 80 different faults. Hard lockouts require manual system restart, while soft lockouts can be reset by the user or automatically after 60 minutes. Auto Reset lockouts monitor the fault cause and reset if it clears.

Preventative maintenance is crucial for efficient and safe operation. The main burner should be checked every three months for proper flame characteristics, ensuring complete combustion, rapid ignition, quiet operation, and no excessive flame lifting. If characteristics are not met, check for lint or foreign material in air openings. Cleaning involves shutting off gas and electricity, allowing the unit to cool, removing the main burner, cleaning ports with a vacuum cleaner (without distorting), and reinstalling. Good combustion and ventilating air flow must be maintained. The ignition system safety shut-off devices should be checked by closing the manifold valve; the main burner should extinguish within four seconds.

The combustion air filter, if installed, must be cleaned periodically per manufacturer instructions to prevent dust and dirt from entering the burner and heat exchanger. The blower compartment should be cleaned annually to remove dirt and lint that can cause excessive motor loads. The condensate removal system, including drains and tubes, needs monthly inspection to ensure free flow, especially in cold weather. Blockages can trigger a "blocked flue" error. Venting maintenance involves checking intake and exhaust piping every six months for dust, condensate leakage, deterioration, and carbon deposits. A metal brush should be used for cleaning to minimize explosion risk.

Heat exchanger preventive maintenance is essential to prevent scale and lime accumulation, which can reduce efficiency and equipment life. A regular deliming schedule (every two to six months, depending on water conditions and usage) is recommended. A change of approximately 5°F (3°C) in the normal temperature rise indicates scaling. Tube cleaning involves removing the return header casting, inspecting tubes for scale, and cleaning with a stiff wire brush or mechanical tube cleaner to bare metal. The heat exchanger itself does not need to be removed.

The manual provides detailed troubleshooting for ignition system faults, including "Display Fail," "Comm. Fail Stg.," "Low AC Voltage," "Low 24VAC," "Brown Out Reset," "Timeout Reset," "FCB Comm Timeout," "No Flow," "Blower Prov Stg," "Blocked Flue," "Blocked Inlet," "High Limit," "MRHL fault," "Low Water," "Low Gas," "Powered Accessory," "High Gas Stg," "Inlet Probe," "Outlet Probe," "Tank Probe," "Igniter Stg," "Igniter Pwr Stg," and "Igniter Hdwr Stg." Each fault description includes the red LED status (continuous or flashing) and lockout status (Auto Reset, SOFT, or HARD), along with potential causes and troubleshooting steps.

A.O. Smith VB-500 - Lighting and Operating Instructions

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