Viessmann VITOTRONIC 200 Service Instructions Manual page 39

Messages
(cont.)
Message
Code/system characteris-
tics
21 Return sensor secndry
Operation with tempera-
ture value of flow temper-
ature sensor in secon-
dary circuit minus 5 K. If
both temperature sen-
sors (secondary flow and
return) are faulty, the
heat pump is shut down
(message A9).
22 Return sensor sec. 2
Operation with tempera-
ture value of flow temper-
ature sensor in secon-
dary circuit minus 5 K. If
both temperature sen-
sors (secondary flow and
return) are faulty, the
heat pump is shut down
(message A9).
28 Flow sensor secondary
Operation with tempera-
ture value of return tem-
perature sensor in sec-
ondary circuit plus 5 K. If
both temperature sen-
sors (secondary flow and
return) are faulty, the
heat pump is shut down
(message A9).
Cause
Short circuit, secondary
circuit return temperature
sensor.
Short circuit, return tem-
perature sensor, secon-
dary circuit, heat pump
stage 2 (if installed).
Lead break, secondary
circuit flow temperature
sensor
Troubleshooting
Measures
Check resistance (Pt 500)
at plug-in connection F9 or
terminals X5.10/X5.11
(see page 172); replace
sensor if required.
Check resistance (Pt 500)
at plug-in connection F18
or terminals X6.6/X6.7
(see page 172); replace
sensor if required.
Check resistance (Pt 500)
at plug-in connection F8 or
terminals X5.8/X5.9 (see
page 172); replace sen-
sor if required.
39