M.200 on a Vaillant aroTHERM plus: Building circuit : Brine pressure low
Short answer
On a Vaillant heat pump, M.200 means: Building circuit : Brine pressure low. The heat pump keeps running. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Building circuit : Brine pressure low.
Part involved: Brine pressure low.
Likely causes:
- Pressure loss in the building circuit due to leakages or air pockets; Building circuit pressure sensor is defective.
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump keeps running.
Resetting itself: yes - message clears when the condition is resolved (maintenance message reset by engineer via Installer level > Resets).
Manual reset: engineer resets maintenance message after work.
What you can check
- 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.
What the engineer will check
- Check the building circuit for leaks, top up with heating water and purge; Check the plug contact on the PCB and on the wiring harness; check that the pressure sensor is working correctly and, if required, 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
- uniTOWER (VIH QW 190/6 E) operating, installation and maintenance instructions ↗ · 0020291525_05 · page uniTOWER VIH QW 190/6 E 0020291525_05 p.59; VWZ MEH 97/6 hydraulic station 0020291544_03 p.51; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.56 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026