Figure; Control Components; The Control System; Hot Surface Igniter - A.O. Smith VB-500, VB-750 Instruction Manual

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.

HOT SURFACE IGNITER

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

FIGURE 3.

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
the Venturi the BPS is activated. If the blower fails or cannot move sufficient
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

CONTROL COMPONENTS

FIGURE 2.
FIGURE 4.
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.
LOW GAS PRESSURE SWITCH.

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 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
FIGURE 5.