F.723 on a Vaillant heat pump: Building circuit: Pressure too low
Short answer
On a Vaillant heat pump, F.723 means: Building circuit: Pressure 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 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Building circuit: Pressure too low.
Part involved: Pressure too low.
When it shows: Filling pressure falls below the minimum pressure; the heat pump is switched off automatically.
Likely causes:
- Filling pressure of the heating installation below the minimum pressure; heat pump switched off automatically (operating instructions).
On the flexoTHERM exclusive (VWF ../4)
The manual says: Building circuit: Pressure too low.
Part involved: Pressure too low.
Likely causes:
- Pressure loss in the building circuit due to leakages or air pockets; Building circuit pressure sensor defective.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Building circuit: Pressure too low.
Part involved: Pressure too low.
Likely causes:
- Pressure drop in the building circuit due to leakages or air pockets; Building circuit pressure sensor defective.
Is it safe?
On the aroTHERM plus (VWL ../6 A)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes - heat pump restarts once the filling pressure is above the minimum.
Manual reset: not required once pressure is restored.
On the flexoTHERM exclusive (VWF ../4)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes - restarts when pressure above minimum (0.5 bar min, 0.7 bar operating).
Manual reset: not required once pressure restored.
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: yes - restarts when pressure above minimum (0.5 bar min, 0.7 bar operating).
Manual reset: not required once pressure restored.
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). Read the heating system pressure on the display (Menu > Live Monitor > building circuit pressure) or the pressure gauge: it should be about 1.0-1.5 bar when cold. Vaillant's operating instructions say to inform your competent person so that they can top up the heating water. Check the filling loop valves are closed and look for visible water leaks or a dripping pressure-relief pipe.
On the flexoTHERM exclusive (VWF ../4)
- Note the full code and text shown on the display (and the date/time it appeared). Read the building/environment circuit pressure in Live Monitor. The flexoTHERM operating instructions say to inform your competent person so that they can top up the heating water (F.723) or brine fluid (F.714).
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
- Note the full code and text shown on the display (and the date/time it appeared). Read the building/environment circuit pressure in Live Monitor. The flexoTHERM operating instructions say to inform your competent person so that they can top up the heating water (F.723) or brine fluid (F.714).
What the engineer will check
On the aroTHERM plus (VWL ../6 A)
- Inform the competent person so they can top up the heating water; engineer checks the building circuit for leaks/air, expansion vessel pre-charge and pressure sensor.
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)
- Check the building circuit for leaks; Top up with water, purge; Check the quality of the plug contact on the PCB and on the cable harness; Check that the pressure sensor is working correctly; Replace the pressure sensor.
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 building circuit for leaks; Top up with water, purge; Check the quality of the plug contact on the PCB and on the cable harness; Check that the pressure sensor works correctly; Replace the pressure 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
What owners and engineers report
16 owner and engineer reports mention this.
- No heating on the aroTHERM plus (VWL ../6 A): 8 reports
- No hot water on the aroTHERM plus (VWL ../6 A): 5 reports
- Pressure keeps dropping on the aroTHERM plus (VWL ../6 A): 3 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.10; uniTOWER VIH QW 190/6 E 0020291525_05 p.11; VWZ MEH 97/6 hydraulic station 0020291544_03 p.11; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.11 · 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.52; flexoTHERM 230 V 0020213395_04 p.47 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.38 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026