23 on a Viessmann Vitotronic 200: Flow sensor sec. 2: Operation with temperature value of return temperature sensor in secondary circuit of heat pump stage 2, plus 5 K.; If the secondary circuit flow and return …
Short answer
On a Viessmann heat pump, 23 means: Flow sensor sec. 2: Operation with temperature value of return temperature sensor in secondary circuit of heat pump stage 2, plus 5 K.; If the secondary circuit flow and return …. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Flow sensor sec. 2: Operation with temperature value of return temperature sensor in secondary circuit of heat pump stage 2, plus 5 K.; If the secondary circuit flow and return ….
Part involved: Flow sensor sec. 2.
When it shows: Operation with temperature value of return temperature sensor in secondary circuit of heat pump stage 2, plus 5 K.; If the secondary circuit flow and return temperature sensors are faulty simultaneously, the message "A9 Heat pump" appears and the heat ….
Likely causes:
- Secondary circuit return temperature sensor short circuit, heat pump stage 2 (if installed).
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated per code; Vitotronic messages remain until the cause is removed and are acknowledged with OK (6150133 p.82). Unremedied acknowledged faults are redisplayed the following day.
Manual reset: Acknowledge with OK on the programming unit (user level permitted per 5817913 p.56-57) after the cause is remedied.
What you can check
- Note the 2-character message code and message text shown on the Vitotronic 200 display (press OK for details) and give them to your heating contractor (operating instructions 5817913 p.56).
- You may acknowledge the message once with OK as described in the Vitotronic 200 operating instructions; if it reappears, do not keep acknowledging it - call your contractor.
- Check the heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).
- Check the heating system pressure gauge reading and report a low reading to your contractor; make sure the filling loop is closed.
What the engineer will check
- Check resistance value (Pt500A) at plug F27 or at terminals X25.19/X25.20: See "Controller and sensor PCB". Replace sensor if required.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
Sources
- Service instructions Vitotronic 200 WO1C (5790974 GB, 2023-09) – brine/water & split variants ↗ · 5790974 VSA00002 · page 66-67 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026