F.787 on a Vaillant flexoTHERM exclusive: Environment circuit: Pressure mon. open
Short answer
On a Vaillant heat pump, F.787 means: Environment circuit: Pressure mon. open. 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: Environment circuit: Pressure mon. open.
Part involved: Pressure mon. open.
Likely causes:
- Pressure loss in the environment circuit due to leakages or air pockets; Environment circuit pressure switch defective; Cable not connected either between X110B and X110 or X110 and X110A for the power supply PCB. No 230 V voltage is present at X131. This is interpreted as an input contact being open; Bridge at X131 (as supplied). Larger voltage fluctuations in the power supply can lead to a fault message; T4 fuse defective.
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). The brine (ground-loop) circuit must only be topped up by a competent person; check for visible leaks at the unit and tell your installer.
What the engineer will check
- Check the environment circuit for leaks; Top up the medium (brine-to-water), purge; Check the screwed contact on the PCB; Check that the pressure switch works correctly; Replace the pressure switch; Check that the wiring of X110B with X110 or X110A with X110 is correct; Eliminate any voltage fluctuations in the network, e.g. via the site power supply; Check the T4 fuse and replace if necessary.
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.55; flexoTHERM 230 V 0020213395_04 p.49 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026