34 on a Viessmann Vitotronic 200: Evaporation temperature too low
Short answer
On a Viessmann heat pump, 34 means: Evaporation temperature too low. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 2 sources
What it means
The manuals for this range word it in different ways:
- Evaporation temperature too low.
- Hot gas temp. 2: Compressor may shut down as a result of changed refrigerant circuit conditions (only heat pumps without electronic expansion valve).
Part involved: Evaporation temperature too low; Hot gas temp. 2.
When it shows: Type "Note" (H) - triggers "07 Refrigerant circuit" on the heat pump control unit. Heat pump characteristics: Cooling mode:; Compressor OFF Heating mode:; Compressor remains in operation. Compressor may shut down as a result of changed refrigerant circuit conditions (only heat pumps without electronic expansion valve).
Likely causes:
- Evaporation temperature too low.
- Hot gas temperature sensor short circuit, heat pump stage 2.
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 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 reversible suction gas temperature sensor: Check resistance value (Pt500A) at connection X25.15/X25.16 of the controller and sensor PCB: See "Controller and sensor PCB".
- Check resistance value (Pt500A) at terminals X25.17/ X25.18 (see "Controller and sensor PCB"); replace sensor if required.
Who to ask for: Viessmann-trained heat pump engineer with F-gas refrigeration certification.
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 · page 184 · Reviewed 23 September 2026
- Service instructions Vitotronic 200 WO1C (5790974 GB, 2023-09) – brine/water & split variants ↗ · 5790974 VSA00002 · page 69 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026