D6 on a Viessmann Vitotronic 200: Flow switch: Compressor stops
Short answer
On a Viessmann heat pump, D6 means: Flow switch: Compressor stops. 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 switch: Compressor stops.
Part involved: Flow switch.
When it shows: Compressor stops.
Likely causes:
- Minimum flow rate in secondary circuit is undershot: See heat pump specification. Heat pump specification Heat pump installation and service instructions.
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 secondary circuit flow rate.
- Check secondary pump. If no flow switch is installed, insert a jumper across X3.3/X3.4: See "Luster terminals" or "Cross connect PCB". The flow switch signal ….
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
Sources
- Service instructions Vitotronic 200 type WO1C heat pump control unit (air/water split & monobloc, brine/water) (6150133, 2023-07) ↗ · 6150133 VSA00004 · page 107-108 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026