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Service Manual
Singular Combi Boiler - BTC 439002301 A (12.2021)
7.6 High Limit Cutout HEX T2 (302°F)
Part No.
7738007253
Function
Due to the excessively high temperatureof the Heat Exchanger (HEX), it will
trip and stop combusting.
Failure Event
Unable to detect the excessively high temperature if the cutout fails.
Effects
Unable to stop combusting if the heat exchanger temperature exceeds
302°F (150°C)
Error Code
A4
Diagnostic
Visual inspection: Connection or breakage of wires.
Voltage check: Check range of voltage shown below.
Color/Number of Wires
Red-Red: DC 5 V
Table 9
7.7 High Limit Cutout Supply T3 (203°F)
Part No.
7738007275
Function
Due to the excessively high supply water temperature, it will trip and stop
combusting.
Failure Event
Unable to detect the excessively high supply water temperature if the
cutout fails.
Effects
Unable to stop combusting if the water temperature from the heat
exchanger exceeds 203°F (95°C)
Error Code
A4
Diagnostic
Visual inspection: Connection or breakage of wires.
Voltage check: Check range of voltage shown below.
Color/Number of Wires
Red-Red: DC 5 V
Table 10
7.8 Primary Heat Exchanger
Part No.
7738007256*
Function
Main part for heat transfer from the burner
It has multiple paths of water tubes on the heat exchanger surface as
well as inside the combustion chamber.
Failure Event
Water and/or exhaust gas leakage through a crack
Improper heat transfer can cause the water in the heat exchanger to
boil due to possible scale formation
Effects
Leakage of exhaust gas
Excessive heating of water that produces boiling noises.
Error Code
AA, A4
Diagnostic
Visual inspection : Check if there is a crack on the surface of heat
exchanger.
Sound inspection : Check if boiling occurs inside the unit
Color/Number of Wires
N/A
Table 11
* part number is for fully assembled heat exchanger unit.