F.742 on a Vaillant heat pump: Environment circuit: Inlet temp. too high
Short answer
On a Vaillant heat pump, F.742 means: Environment circuit: Inlet temp. too high. The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the flexoTHERM exclusive (VWF ../4)
The manual says: Environment circuit: Inlet temp. too high.
Part involved: Inlet temp. too high.
Likely causes:
- Inlet temperature in the environment circuit too high for the compressor to start; Brine inlet temperature > 50 °C; Feed-in of external heat into the environment circuit.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Environment cir. inlet temperature too high.
Part involved: Environment cir. inlet temperature too high.
Likely causes:
- Inlet temperature in the environment circuit too high for the compressor to start Heating:; Environment circuit inlet temp. > 25 °C.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: no - the fault remains displayed until the cause is eliminated.
Manual reset: yes - after the cause is eliminated, press the reset button (indoor unit) or reset via the system control; if it recurs after several resets contact customer service.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). Contact your installer or Vaillant Service; do not attempt any repair.
What the engineer will check
On the flexoTHERM exclusive (VWF ../4)
- Heating: Check that the 4-port valve functions correctly; Check the environment circuit; Check the sensors; Reduce or stop the external heat that is entering.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS ground-source installer; F-gas certificate for refrigerant work.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Check the environment circuit; Check the sensors; Checking the environment circuit pump; Checking the cooling mixer; Checking the isolation valve in the environment circuit (if present).
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS ground-source installer; F-gas certificate for refrigerant work.
What you might notice
Sources
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.54; flexoTHERM 230 V 0020213395_04 p.49 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.39 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026