Afr Rod Function - A.O. Smith T-H3J-DV Service Manual

12-5. AFR rod function

<Function>
The AFR rod checks flame conditions during combustion. When the AFR rod detects unexpected flame
conditions, the computer board of the water heater adjusts the fan motor speed to ensure that air and
fuel are always at a proper mixture ratio, minimizing emissions.
< AFR normal range of values >
240 (T-H3J) models
Combustion
340 (T-H3J) models
Combustion
540 (T-H3) models
Combustion
• If the detected AFR value is higher than the normal value, the FM speed is increased.
• If the AFR value is lower than the normal value, the FM speed is decreased.
• These values are read with the front cover close.
• Measure the values after the water heater for ten minutes.
*The AFR stage can be confirmed in "Diagnostic Mode". (Section 12-1)
<The relationship between the AFR stage and the "101" and "991" error codes>
FM Stage
+ 15 %
16
+ 13.5 %
15
0 %
6
- 9 %
0
240 (T-H3J), 340 (T-H3S) and 540 (T-H3) models Service Manial
Installation type
Gas type Propane
MAX 12.0 to 35.0 23.6 to 58.0
mode
MIN 5.0 to 20.0
Installation type
Gas type
MAX 19.0 to 52.0 24.6 to 65.0
mode
MIN 5.0 to 20.0
Installation type
Gas type Propane
MAX 20.0 to 57.0 26.2 to 78.0
mode
MIN 5.0 to 20.0
"991" error code
"101" error code
Default condition
66
Direct vent
Natural gas
3.5 to 11.0
Direct vent
Propane Natural gas
3.5 to 11.0
Direct vent
Natural gas
3.5 to 11.0
(Condition of AFR stage)
• 0 to 5: Air-rich
• In order to solve the "air excess" problem, the
fan motor speed is automatically decreased
and AFR combustion stage goes down.
• 6: Default
• 7 to 16: Gas-rich or Air shortage
• In order to solve the "air deficiency"
problem, the fan motor speed is
automatically increased and AFR
combustion stage goes up.
• 15: "101" error code
• 16: "991" error code
• When the "101" or "991" error code appears,
see section 12-4.
Page
CONFIDENTIAL
Ver. 1.00
Outdoor
Propane Natural gas
6.0 to 41.0 17.1 to 55.0
3.0 to 15.0 3.8 to 19.0
Outdoor
Propane Natural gas
7.0 to 47.0 21.4 to 56.0
3.0 to 15.0 3.8 to 19.0
Outdoor
Propane Natural gas
7.0 to 48.0 22.3 to 60.0
3.0 to 15.0 3.8 to 19.0