Viessmann VITOCAL 250-A - Page 152

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F.990 Short circuit, low loss header temperature sensor
F.990
System characteristics
Secondary circuit operating in unregulated mode
Cause
Only for heat pumps with 1 integral heating/cooling cir-
cuit:
Short circuit, low loss header temperature sensor
Measures
1. Check the sensor at the following connections:
■
Terminals X17.1/X17.2 on EHCU electronics mod-
ule
■
Wall mounted indoor unit:
6-pole connection socket on underside of appli-
ance on left, terminals 5 and 6
■
Floorstanding indoor unit:
Extra low voltage (ELV) terminal area < 42 V,
lower luster terminal, terminals 7 and 8
■
Check wires and plugs for correct seating.
■
Check connections for contact corrosion.
■
Check cables for mechanical damage, e.g. crush-
ing, kinking, chafing or breakage.
2. Check resistor R for NTC 10 kΩ on the disconnected
plug. If R < 500 Ω, replace the sensor.
3. Replace EHCU electronics module.
F.991 Lead break, low loss header temperature sensor
F.991
System characteristics
Secondary circuit operating in unregulated mode
Cause
Only for heat pumps with 1 integral heating/cooling cir-
cuit:
Lead break, low loss header temperature sensor
Measures
1. Check the following connections:
■
Terminals X17.1/X17.2 on EHCU electronics mod-
ule
■
Wall mounted indoor unit:
6-pole connection socket on underside of appli-
ance on left, terminals 5 and 6
■
Floorstanding indoor unit:
Extra low voltage (ELV) terminal area < 42 V,
lower luster terminal, terminals 7 and 8
■
Check wires and plugs for correct seating.
■
Check connections for contact corrosion.
■
Check cables for mechanical damage, e.g. crush-
ing, kinking, chafing or breakage.
2. Check resistor R for NTC 10 kΩ on the disconnected
plug. If R > 300 kΩ, replace the sensor.
3. Replace EHCU electronics module.
F.992 Short circuit, temperature sensor in heating/cooling water buffer cylinder
F.992
System characteristics
No heating/cooling of external heating/cooling water
buffer cylinder
Cause
Only for heat pumps with 1 integral heating/cooling cir-
cuit:
Short circuit, temperature sensor in heating/cooling
water buffer cylinder
Messages
Fault messages (cont.)
6218234
Diagnosis