90 on a Vaillant geoTHERM: Heating system pressure too low (< 0.5 bar; restarts automatically above 0.7 bar)
Short answer
On a Vaillant heat pump, 90 means: Heating system pressure too low (< 0.5 bar; restarts automatically above 0.7 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: Heating system pressure too low (< 0.5 bar; restarts automatically above 0.7 bar).
Part involved: Heating system pressure too low.
When it shows: Heating system pressure too low (< 0.5 bar; restarts automatically above 0.7 bar).
Likely causes:
- Pressure drop in heating system due to leakage, air lock or defective expansion vessel; rear screw couplings or 3-way valve compression fittings leaking.
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). Read the heating system pressure gauge (should be above 0.7 bar for restart). The geoTHERM manuals say only an expert technician should remedy faults - ask your installer to top up and find the leak. Look for visible leaks at the heat pump connections.
What the engineer will check
- Check heating system for leaks, top up water, bleed, check expansion vessel; retighten couplings/fittings.
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
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.75; geoTHERM VWS/VWW 6-17 kW 400 V 0020057444_01 p.73; 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