T-H2-DV/T-H2-OS Service Manual

Ver. 1.01

65

CONFIDENTIAL

12-4. AFR rod 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.

Type Type A Type B Type C

Model T-H2-DV T-H2-OS

Gas type LPG Natural gas LPG Natural gas

Combustion

mode

MAX 25 to 50 31 to 48.5 20 to 46 19 to 40

MIN 3 to 10 2 to 10 3 to 18 2 to 15

・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”. (Refer to p. 58.)

16

15

6

0

(Unit: μA)

(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

+15%

FM

0%

-9%

・

+13.5%

“991” error code

Stage

+18%

FM

0%

-10.8

・

+16.2%

+20%

FM

0%

-12%

・

/

・

+18%

Type A Type B Type C

“101” error code

Default condition

・

Brand	A.O. Smith
Model	T-H2-DV
Category	Water Heater
Language	English

A.O. Smith T-H2-DV User Manual

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Questions and Answers:

A.O. Smith T-H2-DV Specifications

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