A.O. Smith 500 User's Information Manual page 10

• Stored values are recalled from memory.
• Configuration dipswitches are read.
• Pending faults are recalled
• Micros on all boards start running (indicated by a flashing
Yellow LED near each micro)
• Input sensors are read
• Communications between micros and boards is
established
• FCB's are configurated with the number of ignition trials to
run.
3. After initialization is complete (approximately 10 seconds)
the system turns the green LED off and goes to the standby
mode (yellow "Standby" LED on), unless a previously stored
fault has been recalled, which will send the system into the
service model (red "Service" LED on). In standby mode the
display shows the temperature screen and in fault mode
the current error screen is displayed.
4. The system then compares the temperature read from the
controlling probe (inlet or tank) to the setpoint temperature.
If the temperature is less than the operating setpoint minus
the differential temperature and the thermostat input is
closed then a call for heat is established and the system
shifts to the run mode (green "Running" LED turns on).
5. The heating sequence begins by applying power to the pump
and, if selected, the powered vent and the IRI gas valve.
6. After a few seconds the High Speed Blower (on all stages
with the blower dipswitch turned on) are turned on to perform
a cold purge of the chamber.
7. After cold purging is complete the blowers are turned off
and the stage 1 blower is turned on.
8. The stage 1 igniter is turned on.
9. After 18 seconds the system checks that the igniter has
turned on. If this is ok then the system turns on the gas
valve.
10. After 1.5 seconds the system checks the status of the flame
sensor. Note: If the "Ignition Tries" dipswitch is set for 3
tries the system will not declare an error until it tries the
ignition sequence three times. If it is set to 1 try then the
system will declare an error anytime a fault is detected.
11. The system now activates the other FCB stages depending
upon a control algorithm scheme that is described below.
For this example it is assumed that all four stages of heat
are required.
12. The stage 2 blower is turned on to purge the chamber.
13. After approximately 10 seconds the stage 2 igniter is turned
on.
14. After 18 seconds the system checks that the igniter has
turned on. If this is ok then the system turns on the gas
valve.
15. After 1.5 seconds the system checks the status of the flame
sensor.
16. Steps 12 through 15 are then repeated for stage 3 and 4.
17. The system is now in the heating mode with all four stages
on and will remain in this mode until the call for heat is
satisfied or a fault occurs.
NOTE: In standby and running modes the system constantly
monitors the signals and the internal operation for faults. Any
detected fault will halt the heating sequence and shift the system
to the service mode, where the detected fault will be displayed.
Temperature Setpoints (System Control Algorithm)
The boiler has a hysteresis type control, which means that it will
begin heating the water when the temperature sensed by the
control probe (inlet or tank) falls below the operating setpoint
minus the differential setpoint for stage 1. It will stop heating the
water when the temperature rises to the operating setpoint. If
the system has multiple stages then the differential setpoint for
each stage is also subtracted from the operating setpoint. The
following examples will further explain this operation.
Setup: 2 stage system, operating setpoint - 140, stage 1 to 2
differential setpoints = 10.
Example 1. Temperature begins at 150 and drops to 90, see
Figure 2A. At 140 the system remains in idle mode. As the
temperature drops to 130 (140-10) stage 1 turns on and stage 2
remains off. At 120 stage 2 also turns on.
Example 2. Temperature begins at 90 and rises to 150, see
Figure 2B). At 100 both stages are on. (This is the case when a
boiler is first started and the controlling temperature is below
the operating setpoint minus all of the differential setpoints. At
that time both stages are turned on, in sequence from 1 to 2. At
130 stage 2 turns off. At 140 both stages are off.
Example 3. Boiler is initially started and the controlling
temperature is at 95, see Figure 2B). Both stages will turn on in
sequence from stage 1 to stage 2.
Example 4. Boiler is initially started and the controlling
temperature is at 125, see Figure 2B).
Stages 1 and 2 will turn on in sequence from 1 to 2.
FIGURE 2A.
UIM Operating Procedures
FIGURE 3. UIM
The UIM receives commands from the user and displays
operational information to the user via an LCD (liquid crystal
display) up to eleven LED's, and five touch switches. The LCD
provides information to the user by the use of 10 menu-activated
screens. Within each of the screens, helpful information can be
displayed by pressing the "Help" button. The LED's visually
inform the user about the mode the system is in. The touch
switches allow the user to control the operation of the system.
10
FIGURE 2B.