Centurion Water Heater- Edge [ii]: Install-Operation-Service
SECTION 9: O2 TRIM OPERATION
OMM-0153_A
• 8/25/2022 Technical Support • (800) 526-0288 • Mon-Fri, 8 am - 5 pm EST Page 95 of 131
9.2 OPERATION DETAILS
During operation, the O
2
Trim system will adjust the command voltage sent to the combustion
air blower within a limited range. The amount of voltage trim depends on the error between the
desired O
2
% (target %) and the current reading of the O
2
sensor (O
2
%) and also on high and
low limits of blower voltage for each valve position. The total amount of corrective voltage trim
is limited by the controller to insure safe and reliable operation of the system.
Figure 9-2 graphically shows the functional logic of the O
2
Trim system and how the blower
voltage (BV), O
2
Limits, and air/fuel ratio interact during an O
2
Trim operation. The limits are
fixed presets within the controller. The target range is adjustable within these limits to allow the
user to select the optimal air/fuel ratio for a particular Water Heater or application.
Figure 9-2 shows how the controller would react to an O
2
% reading above the upper limit. It will
reduce the blower voltage (BV) until the O
2
% reading is within the Target Range, provided the
BV adjustments are within the limits for that fire rate. The control will then store this as the new
BV calibration setting until changed manually or by another O
2
Trim function cycle.
Figure 9-2: O
2
Trim Logic
Once the system operational stability conditions are met, the O
2
Trim system will:
1. Lock the fire rate at the current position that needs adjustment to the air/fuel ratio.
2. The
Demand
LED will flash once per second to indicate that the trim function has started.
3. Check the oxygen levels inside the combustion chamber:
•
If oxygen levels are within the set range, O
2
Trim releases control.
•
If oxygen levels are outside the set range, O
2
Trim will adjust Blower Voltage to bring
the Water Heater back to the Target O
2
value.
This process repeats until the target oxygen range is achieved or the unit reaches the allowable
blower voltage limit.
