50 Installation and maintenance instructions Compact 0020313973_01
Code/meaning Possible cause Measure
F.68
Fault: Unstable flame signal
Air in the gas supply (e.g. dur-
ing initial start-up)
▶ Reset the unit once.
Gas flow pressure too low ▶ Check the gas flow pressure.
Incorrect air ratio ▶ Check the CO
2
content at the flue gas analysis point.
Incorrect flue gas recirculation ▶ Check the flue gas recirculation.
Condensate discharge pipe
blocked
▶ Check the condensate discharge pipe.
F.70
Invalid device specific number
(DSN)
Device Specific Number not
set/is incorrect
▶ Set the correct Device Specific Number.
F.71
Flow temperature sensor fault
The flow temperature sensor
signals a constant value
▶ Check the positioning of the flow temperature sensor.
Flow temperature sensor in the
incorrect position
▶ Check the positioning of the flow temperature sensor.
Flow temperature sensor de-
fective
▶ Replace the flow temperature sensor.
F.72
Fault: Return temperature
sensor
Flow temperature sensor de-
fective
▶ Replace the flow temperature sensor.
Return temperature sensor de-
fective
▶ Replace the return temperature sensor.
F.73
Water pressure sensor signal in
the wrong range (too low)
Short circuit in the cable har-
ness
▶ Check the cable harness.
Interruption in the cable harness ▶ Check the cable harness.
Water pressure sensor defect-
ive
▶ Replace the water pressure sensor.
F.74
Water pressure sensor signal
outside correct range (too high)
Short circuit in the cable har-
ness
▶ Check the cable harness.
Interruption in the cable harness ▶ Check the cable harness.
Water pressure sensor defect-
ive
▶ Replace the water pressure sensor.
F.76
Fault: Thermal cut-out
Defective thermal fuse ▶ Check the heat exchanger for leakages. If the heat exchanger
has no leaks, bypass the thermal fuse. If you can then start
the product, replace the thermal fuse.
F.77
Fault: Flue non-return
flap/condensate pump
No feedback from the flue non-
return flap or the feedback is
incorrect
▶ Check that the flue non-return flap functions correctly.
Flue non-return flap defective ▶ Replace the flue non-return flap.
Condensate pump defective ▶ Replace the condensate pump.
F.78
Interruption: Domestic hot water
outlet temperature sensor with
external control
NTC sensor defective ▶ Replace the NTC sensor.
F.83
Fault: NTC temperature fluctu-
ation
System pressure too low ▶ Check the system pressure.
Flow temperature sensor: No
contact
▶ Check whether the flow temperature sensor is lying against the
flow pipe correctly.
Return temperature sensor: No
contact
▶ Check whether the return temperature sensor is lying against
the return pipe correctly.
Insufficient/no water in the
product.
▶ Fill the heating installation. (→ Page 29)
F.84
NTC temperature difference is
implausible
Flow temperature sensor in-
stalled incorrectly
▶ Check whether the flow temperature sensor has been installed
correctly.
Return temperature sensor in-
stalled incorrectly
▶ Check whether the return temperature sensor has been in-
stalled correctly.
Flow and return temperature
sensors inverted
▶ Check whether the flow and return temperature sensor has
been installed correctly.
F.85
Flow and return temperature
sensors have been installed
incorrectly (inverted)
Flow/return temperature
sensors have been installed on
the same pipe/wrong pipe
▶ Check that the flow and return temperature sensors have been
installed on the correct pipe.
F.86
External safety shutdown
Limit thermostat settings incor-
rect
▶ Check the limit thermostat settings.
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