F.743 on a Vaillant heat pump: Building circuit: Return temp. too high
Short answer
On a Vaillant heat pump, F.743 means: Building circuit: Return 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: Building circuit: Return temp. too high.
Part involved: Return temp. too high.
Likely causes:
- Return temperature in the building circuit too high for the compressor to start Heating:; Return temperature > 55 °C to 60 °C (depending on the brine inlet temperature) Cooling:; Return temperature > 35 °C.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Building circuit inlet Return temperature too high.
Part involved: Building circuit inlet Return temperature too high.
Likely causes:
- Return temperature in the building circuit too high for the compressor to start Heating:; Return temperature > 55 °C Cooling:; Return temperature > 35 °C; General VWS 36/4.1 230V Flow/return heating connections, boiler side 22 mm diameter Boiler-side flow/return heat source connections 22 mm diameter Product dimensions, width 440 mm Product dimensions, height 720 mm Product dimensions, depth 435 mm Weight without packaging 59 kg Weight with packaging 67 kg Weight when ready for operation 66 kg Heating circuit/compressor rated voltage 1/N/PE 230 V / 50 Hz Control circuit rated voltage 1/N/PE ….
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). Check the system control settings (heating/hot water modes switched on, time programmes, target temperatures). Do not keep resetting the unit; report the code to your installer (the operating instructions for this product were not checked for a homeowner reset procedure).
What the engineer will check
On the flexoTHERM exclusive (VWF ../4)
- Cooling: Check that the 4-port valve functions correctly; Check the sensors.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Check the sensors; Checking the heating pump; Checking the non-return flap (if present); Checking the 2-zone set (if present) F Sensor characteristics for the VR 10 external temperature sensor Temperature (°C) Resistance (ohms) -40 87879 -35 63774 -30 46747 -25 34599 -20 25848 -15 19484 -10 14814 -5 11358 0 8778 5 6836 10 5363 15 4238 20 3372 Appendix Temperature (°C) Resistance (ohms) 25 2700 30 2176 35 1764 40 1439 45 1180 50 973 55 807 60 672 65 562 70 473 75 400 80 339 85 289 90 247 95 212 100 183 105 158 110 137 115 120 120 104 125 92 130 81 135 71 140 63 145 56 150 50 155 44 G Sensor ….
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
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