Viessmann VITOCAL 250-A - Page 154

154
Measures
1. Check the sensor and 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.995 Lead break, temperature sensor in heating water buffer cylinder
F.995
System characteristics
No heating of external heating water buffer cylinder
Cause
Only for heat pumps with 1 integral heating/cooling cir-
cuit:
Lead break, temperature sensor in heating water buf-
fer cylinder
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.996 Short circuit, temperature sensor in cooling water buffer cylinder
F.996
System characteristics
No cooling of external cooling water buffer cylinder
Cause
Only for heat pumps with 1 integral heating/cooling cir-
cuit:
Short circuit, temperature sensor in cooling water buf-
fer cylinder
Messages
Fault messages (cont.)
6218234
Diagnosis