Afr Rod Function - A.O. Smith Water Heater 520 Direct Vent Indoor Service Manual

12-4. 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 520 models adjusts the fan motor speed to ensure that air and
fuel are always at a proper mixture ratio, minimizing emissions.
There are 3 types of FM speed adjustments, depending on the model & gas type, as shown the table
below.
<AFR normal range of values>
Type
Model
Gas type
MAX
Combustion
mode
MIN
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 burning 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 "10" and "99" error codes>
Type A Type B Type C
FM FM FM
+15% +20% +18%
+13.5% +18% +16.2%
0% 0% 0%
-9% -12% -10.8
A
520 Indoor
LPG Natural gas
25 to 50 31 to 48.5
3 to 10
Stage
"991" error code
16
15
"101" error code
6 Default condition
0
520 Indoor / 520 Outdoor Service Manual
LPG
20 to 46
2 to 10 3 to 18
(Condition of AFR stage)
0 to 5: Air-rich
In order to solve the "air excess"
problem, the fan motor speed is
automatically
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
combustion stage goes up.
15: "101" error code
16: "991" error code
64
CONFIDENTIAL
Ver. 1.00
B C
520 Outdoor
Natural gas
19 to 40
2 to 15
decreased and
increased and
(Unit: μA)
AFR
AFR