Sequence Of Operation - A.O. Smith 100 Series Service Handbook

SEQUENCE OF OPERATION

EMC 5000 Control System
compares the temperature read
from the Controlling Probe (Inlet
Probe, Remote Temperature Probe
or Quad Thermistor Probe ) to its
Operating Setpoint.
If the temperature read is less than
the Operating Setpoint minus the
Setpoint Differential AND the
Enable/Disable circuit is closed a
call for heat is activated
OPERATIONAL NOTE
The events shown in this flow chart
are in sequential order. The EMC
5000 Control System performs some
functions simultaneously . Only key
events are shown in order to provide
a general understanding of how the
control operates. Event timings can
vary depending on actual conditions .
SERVICE NOTE
VF boilers utilize two flame sensors.
One is positioned closer to the
Burner to enable flame detection
during minimum firing rates.
The wiring from the two flame
sensors is joined and connects to a
single connection point on the MCB
circuit board.
When the "Flame" Error Message is
displayed remove, inspect, and
clean both flame sensors.
Call For Heat is Activated
EMC 5000 Control System performs diagnostic
system check. The normal state of all pressure
switches and the Manual Reset High Limit are
verified by the MCB. Blower Prover switch
contacts are verified open. Blocked Inlet, Blocked
Flue and High Limit switches are verified closed
YES
MCB energizes Pump Relay
Boiler Circulating Pump Starts
Flow Switch contacts verified closed by MCB
YES
MCB sends start & speed instruction to Electronic Speed
Control in Combustion Blower Assembly
Combustion Blower Starts
Pre-Purge high speed operation
Blower Prover Switch contacts verified closed by MCB
YES
MCB sends low speed instruction to Electronic Speed
Control in Combustion Blower Assembly.
Combustion Blower Reduces Speed
MCB energizes Igniter
MCB verifies minimum Igniter current of 2.7 AC amps
YES
MCB energizes Gas Valve
Fuel gas is drawn Into the Combustion Blower inlet
by the Venturi and mixed with combustion air .
Air/Fuel mixture is supplied to the Burner.
MCB verifies minimum flame sensing
current of 2.5 µA (DC micro amps)
from flame sensor(s).
YES
Water is heated to Operating Setpoint
MCB runs Combustion Blower and
Circulating Pump for Post-Purge period.
Control System enters Standby mode.
Servicing should only be performed by a Qualified Service Agent
Control System Locks Out Displays
A Specific Error Message:
"Blower Prover"
NO
"Blocked Inlet"
"Blocked Flue"
"MRHiLimit Fault"
Control System Locks Out
NO
Displays Error Message:
"No Flow"
Control System Locks Out
NO
Displays Error Message:
"Blower Prover"
Control System Locks Out
NO
Displays Error Message:
"Igniter"
After 1 or 3 failed trials for ignition
(depending on SW1 dipswitch #2 setting)
NO
Control System Locks Out
Displays Error Message:
"Flame"
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