A.O. Smith VB/VW- 750 User Manual

8

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.

The Hot Surface Igniter is a device that ignites the main burner by high 

tempera ture (>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 cient 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.

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

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.

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.

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. 

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.