20 on a Vaillant geoTHERM: Frost protection heat source monitoring source outlet (T3-T8 difference too large)
Short answer
On a Vaillant heat pump, 20 means: Frost protection heat source monitoring source outlet (T3-T8 difference too large). 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: Frost protection heat source monitoring source outlet (T3-T8 difference too large).
Part involved: Frost protection heat source monitoring source outlet.
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 pump defective; sensor T8 or T3 defective; heat source flow too low; dirt filter missing/full; air in brine circuit (deactivated as standard - 20 K).
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
- Check heat source flow rate; check sensors (VR 11 characteristics) and plugs; check brine/well pump flow (approx. 3-5 K spread); fit/clean dirt filter; bleed brine circuit.
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.72; geoTHERM VWS/VWW 6-17 kW 400 V 0020057444_01 p.70; geoTHERM VWS/VWW 22-46 kW 0020051574_04 p.99; 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