28 on a Vaillant geoTHERM: Coolant (refrigerant) pressure too low - low pressure switch tripped (1.25 bar g)
Short answer
On a Vaillant heat pump, 28 means: Coolant (refrigerant) pressure too low - low pressure switch tripped (1.25 bar g). The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Coolant (refrigerant) pressure too low - low pressure switch tripped (1.25 bar g).
Part involved: Coolant.
When it shows: Temporary shutdown: compressor stops, heat pump remains in operation; becomes a fault-induced shutdown (60/62/81/83/84) if it occurs three times in a row.
Likely causes:
- Heat source providing too little heat: air in heat source circuit; brine pump defective/low output; unequal brine circuit flows; dirt strainers clogged; stop valves closed; TEV incorrectly adjusted/defective.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - compressor restarts automatically after 5 min (60 min after errors 27/29) once the cause is removed.
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
- Bleed heat source circuit; check brine pump; balance brine circuits; clean strainers; open stop valves; check refrigerant circuit.
Who to ask for: F-gas certified refrigeration engineer with heat pump experience (R290 flammable-refrigerant competence required for aroTHERM plus / GeniaAir).
Certification: F-gas Category I (Regulation (EU) 517/2014 as retained in GB) certificate; flammable (R290) refrigerant training for R290 units.
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.73; geoTHERM VWS/VWW 6-17 kW 400 V 0020057444_01 p.71; geoTHERM VWS/VWW 22-46 kW 0020051574_04 p.101; geoTHERM exclusive operating manual 0020071929_00 p.26 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026