OMM-0153_E
• 5/11/2023 Technical Support • (800) 526-0288 • Mon-Fri, 8 am - 5 pm EST Page 88 of 124
CENTURION Installation, Operation & Maintenance Manual
SECTION 9: O2 TRIM OPERATION
SECTION 9:
O
2
TRIM OPERATION
Advanced combustion control systems need to maintain precise air/fuel ratios to maximize
efficiency. Gas Water Heaters often deviate from the ideal air-fuel ratio due to environmental
variations such as humidity, atmospheric pressure, filter dust loading, delivered gas energy
content and other factors. If the Water Heater is operating with fixed blower/damper positions,
the air/fuel ratio will normally vary within an acceptable level but will not be fully optimized for
efficiency and reliability.
The O
2
Trim system is designed to measure and maintain an ideal air-fuel ratio in Centurion
Water Heaters, thus maximizing efficiency and reliability while minimizing emissions.
Figure 9-1: Simplified O
2
Trim Diagram
W A R N I N G :
O
2
Trim and combustion calibration can both alter the voltage sent to the blower and can
thus interfere with each other. If a change is made to any calibration point during combustion
calibration, you must make a corresponding change to the same calibration point in O
2
Trim.
If you fail to make the change in O
2
Trim, it may ignore the combustion calibration value and
adjust the O
2
to the O
2
Trim value instead.
NOTE:
For full instructions and menu options, see OMM-0161 Section 6.6.1.
9.1 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.
