F.788 on a Vaillant heat pump: Building circuit: Pump fault
Short answer
On a Vaillant heat pump, F.788 means: Building circuit: Pump fault. 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
On the aroTHERM plus (VWL ../6 A)
The manual says: Building circuit: Pump fault.
Part involved: Pump fault.
When it shows: High-efficiency pump electronics detect dry running, blockage, overvoltage or undervoltage and lock the pump.
Likely causes:
- The electronics system of the high-efficiency pump has detected a fault (e.g. dry running, blockage, overvoltage, undervoltage) and has switched off and locked the pump.
On the flexoTHERM exclusive (VWF ../4)
The manual says: Building circuit: Pump fault.
Part involved: Pump fault.
Likely causes:
- The electronics system of the high-efficiency pump has detected a fault (e.g. dry running, blockage, overvoltage, undervoltage) and has switched off and locked the pump.
Is it safe?
On the aroTHERM plus (VWL ../6 A)
| 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 - engineer switches heat pump off for at least 30 s (no current) then restarts; reset button after cause eliminated.
On the flexoTHERM exclusive (VWF ../4)
| 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
On the aroTHERM plus (VWL ../6 A)
- Note the full code and text shown on the display (and the date/time it appeared). Check that radiator/underfloor zone valves and thermostatic valves are not all closed and that the system control is calling for heat. The operating instructions allow the reset button on the indoor unit (or a reset via the system control) to be pressed once; if the code returns, do not keep resetting - contact your installer or Vaillant.
On the flexoTHERM exclusive (VWF ../4)
- Note the full code and text shown on the display (and the date/time it appeared). Check that radiator/underfloor zone valves and thermostatic valves are not all closed and that the system control is calling for heat. 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 aroTHERM plus (VWL ../6 A)
- Switch the heat pump off for at least 30 seconds (no current); Check the quality of the plug contact on the PCB; Check that the pump functions correctly; Purging the building circuit; Check that the dirt filter that is in place is permeable.
Who to ask for: Vaillant-trained heat pump installer/engineer (MCS-certified installer).
Certification: MCS installer certification; Vaillant product training recommended.
On the flexoTHERM exclusive (VWF ../4)
- Switch the heat pump off for at least 30 seconds (no current); Check the quality of the plug contact on the PCB; Check that the pump functions correctly; Purging the building circuit.
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
What owners and engineers report
16 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- Keeps stopping / cutting out on the aroTHERM plus (VWL ../6 A): 6 reports
- Gurgling or rushing water noise on the aroTHERM plus (VWL ../6 A): 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.37; uniTOWER VIH QW 190/6 E 0020291525_05 p.62; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.60 · Reviewed 23 September 2026
- 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.50 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026