A.O. Smith Boiler VB/VW- 1000 User Manual

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8
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congurethe
boiler for several different control options, see the Control System
Section.
FIGURE 2.
HOT SURFACE IGNITER
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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.
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
theVenturitheBPSisactivated.Iftheblowerfailsorcannotmovesufcient
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uepressureswitch(BFS)activatedwhentheexhaust
ue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.
WATER FLOW SWITCH
The water ow switch is installed at the boiler outlet to
preventburneroperationintheeventofinadequatewaterow
through the boiler. It is a normally open switch that will close
itscontactswhenincreasingwaterowrateisdetected.The
waterowswitchisfactory-set.Thecontactswillopenwhen
theow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 ow 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amesenorscoupledtogether
todetectthepresenceoftheburneramesathighandlowre
conditions.Theseamesensorsworktogetherasonetosense
theflame.Ifnoflame issensed,thegasvalve(s)willclose
automatically.Ifnoame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.
CONTROL COMPONENTS

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