91 on a Vaillant geoTHERM: Brine pressure too low (< 0.2 bar; restarts automatically above 0.4 bar)
Short answer
On a Vaillant heat pump, 91 means: Brine pressure too low (< 0.2 bar; restarts automatically above 0.4 bar). 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: Brine pressure too low (< 0.2 bar; restarts automatically above 0.4 bar).
Part involved: Brine pressure too low.
When it shows: Brine pressure too low (< 0.2 bar; restarts automatically above 0.4 bar).
Likely causes:
- Pressure drop in heat source system due to leakage or air lock; brine pressure sensor defective; L1 and N interchanged on PCB.
- Emergency mode (electric auxiliary heating): possible.
Is it safe?
| 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 automatically when pressure recovers (errors 90/91 need no reset).
Manual reset: not required.
What you can check
- Note the full code and text shown on the display (and the date/time it appeared). The geoTHERM operating manual says: do not attempt to remove the source of the malfunction yourself; only an expert technician can remedy the fault and reset the error code. Warnings (26, 36) and temporary faults can be noted and discussed at the next inspection.
What the engineer will check
- Check heat source system for leaks, top up brine, bleed; check pressure sensor and plug; check L1/N connections.
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
What owners and engineers report
1 owner and engineer report mentions this.
- No heating on the geoTHERM (VWS/VWW ../2, VWS ../3): 1 report
Sources
- Installation instructions geoTHERM VWS 6-10kW 230V (GB 08/2008) ↗ · 0020046310_01 · page geoTHERM VWS 6-10 kW 230 V 0020046310_01 p.76; geoTHERM VWS/VWW 6-17 kW 400 V 0020057444_01 p.74; geoTHERM VWS/VWW 22-46 kW 0020051574_04 p.105; geoTHERM exclusive operating manual 0020071929_00 p.27 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026