21 on a Viessmann Vitotronic 200: Inadequate evaporation pressure (low pressure fault)
Short answer
On a Viessmann heat pump, 21 means: Inadequate evaporation pressure (low pressure fault). 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 manuals for this range word it in different ways:
- Inadequate evaporation pressure (low pressure fault).
- Return sensor secndry: Operation with temperature value of flow temperature sensor in secondary circuit, minus 5 K.; If the secondary circuit flow and return temperature sensors are faulty at the ….
Part involved: Inadequate evaporation pressure (low pressure fault); Return sensor secndry.
When it shows: Type "Note" (H) - triggers "07 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Compressor OFF Type "Note" (H) - triggers "07 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Both compressors OFF Operation with temperature value of flow temperature sensor in secondary circuit, minus 5 K.; If the secondary circuit flow and return temperature sensors are faulty at the same time, the heat pump is switched off. After 24 h the message ….
Likely causes:
- Inadequate evaporation pressure (low pressure fault).
- Short circuit, secondary circuit return temperature sensor.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated in 6150133.
Manual reset: Refrigerant circuit messages cannot be acknowledged in the message history (6150133 p.155); acknowledge the resulting 05/07/A9 message on the control unit.
What you can check
- This code is only visible to engineers in the service menu (Diagnosis > Refrigerant circuit > Message history); homeowners see message 05, 07 or A9 - note that code and call your contractor.
- 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
- As for "D3 Low pressure" : See "Message overview" for the heat pump control unit.
- Check resistance value (Pt500A) at terminals X25.11/ X25.12: See "Controller and sensor PCB".
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification; Viessmann-trained heat pump installer/service engineer.
Certification: F-gas certificate (UK F-gas Regulations; Viessmann: refrigerant work by authorised refrigeration engineers only).
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 · pages 157, 167, 174, 162, 181, 184, 87 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026