A.O. Smith VB/VW- 1000, VB/VW- 750 instruction manual Control Components

Control Components

The Control System

The control system consists of four basic components:

1)Modulation Control Board (MCB); 2) Power Distribution Board (PDB); 3) Variable Frequency Drive (VFD), see Figure

2; User Interface Module, see Figure 20. The Modulation Control Board and the Power Distribution Board are located in the control box and can be accessed by opening the front door of the unit. The User Interface Module is attached to the front door panel. Every system will have one Modulation

Control Board (MCB), one Power Distribution Board (PDB), and one User Interface Module (UIM).

The MCB contains dipswitches which are used to configure the boiler for several different control options, see the Control System Section.

flue of the unit is restricted or blocked. The BFS is a normally closed switch that opens when positive pressure is placed on the switch because of any restriction to the exhaust venting. If the BFS is activated check and clear any obstructions causing the restriction.

Low Gas Switch

This VF boiler is available with a low gas pressure switch which meets the CSD-1 code requirements, see Figure 5.

The Low Gas Pressure Switch (LGPS) is normally closed and remains closed unless the pressure falls below the preset pressure.

FIGURE 5.

LOW GAS PRESSURE SWITCH.

FIGURE 2.

Hot Surface Igniter

The Hot Surface Igniter is a device that ignites the main burner by high temperature­ (>1800°F) [982°C], see Figure 3. The igniter is made of recrystallized silicon carbide, and when 120 VAC is applied to the igniter, suf­fi­cient heat is generated to ignite the main burner. Although improvements have been made to strength­en the igniter, it is still fragile and care must be taken in handling the igniter to prevent breakage.

Water Flow Switch

The water flow switch is installed at the boiler outlet to prevent burner operation in the event of inadequate water flow through the boiler. It is a normally open switch that will close its contacts when increasing water flow rate is detected. The water flow switch is factory-set. The contacts will open when the flow rate drops below the factory setting causing the gas valve to close which will turn off the gas to the burner, see Figure 6. Under no circumstances shall the flow switch be tampered with or bypassed. Doing so may cause damage to the heat exchanger not covered under the warranty.

FIGURE 3. FIGURE 4.

Pressure Switches

This control system has 3 pressure switches that are standard. Blocked

Inlet Pressure Switch (BIS), Blower Prover Switch (BPS) Blocked Flue

Switch (BFS).

The BPS on this model is a normally open switch that closes on increased vacuum. Once the blower moves enough air to create a vacuum across theVenturitheBPSisactivated.Iftheblowerfailsorcannotmovesufficient air a soft lockout will occur. Inspect the blower for correct operation.

The Blocked Inlet Switch (BIS) will activate is the intake is blocked only during the heating cycle. The BIS is a normally closed pressure switch that opens when the air intake is blocked. If the BIS is activated check and clear the intake of any obstructions.

The blocked flue pressure switch (BFS) activated when the exhaust

FIGURE 6. WATER FLOW SWITCH.

Flame Sensor

Each Boiler is equipped with two flame senors coupled together to detect the presence of the burner flames at high and low fire conditions. These flame sensors work together as one to sense the  flame. If no flame is sensed, the gas valve(s) will close automatically. If no flame is sensed on three ignition trials, the boiler will lock out. In the event of a lockout, depress the SELECT button on the display board to restart the boiler.

8