S.34 on a Vaillant heat pump: Heating mode: Frost protection If the measured outdoor temperature falls below XX °C, the temperature of the heating circuit's flow and return is monitored. If the temperature difference exceeds the set value, the pump and compressor are started without a heat demand
Short answer
On a Vaillant heat pump, S.34 means: Heating mode: Frost protection If the measured outdoor temperature falls below XX °C, the temperature of the heating circuit's flow and return is monitored. If the temperature difference exceeds the set value, the pump and compressor are started without a heat demand. The heat pump keeps running. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 3 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Heating mode: Frost protection If the measured outdoor temperature falls below XX °C, the temperature of the heating circuit's flow and return is monitored. If the temperature difference exceeds the set value, the pump and compressor are started without a heat demand.
Part involved: Heat pump control (status/Live Monitor).
When it shows: Heating mode: Frost protection If the measured outdoor temperature falls below XX °C, the temperature of the heating circuit's flow and return is monitored. If the temperature difference exceeds the set value, the pump and compressor are started without a heat demand.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
On the flexoTHERM exclusive (VWF ../4)
The manual says: Heating mode: Frost protection.
Part involved: Heat pump control (status/Live Monitor).
When it shows: See official description.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
On the geoTHERM mini (VWS 30/4, VWS 36/4.1)
The manual says: Heating mode: Frost protection.
Part involved: Heat pump control (status/Live Monitor).
When it shows: See official description.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
Is it safe?
| Does the heat pump stop? | No |
|---|---|
| Severity | Information only |
| Engineer needed? | No |
The heat pump keeps running.
Resetting itself: yes - status changes automatically with operating state.
Manual reset: not applicable.
What you can check
- Normal frost-protection operation: the pump/compressor may run without a heat demand in cold weather. No action needed.
What the engineer will check
- None required unless heating/hot water is not delivered; then review status history, settings and application limits.
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
2 owner and engineer reports mention this.
- Outdoor unit running when heating is off on the aroTHERM plus (VWL ../6 A): 2 reports
Sources
- Heat pump control interface module (VWZ AI) operating and installation instructions ↗ · 0020291573_01 · page VWZ AI appliance interface 0020291573_01 p.31; uniTOWER VIH QW 190/6 E 0020291525_05 p.56; VWZ MEH 97/6 hydraulic station 0020291544_03 p.49; aroTHERM split hydraulic station VWL ../5 IS 0020257319_05 p.54 · Reviewed 23 September 2026
- flexoTHERM exclusive VWF 57/4 - 197/4 400V installation and maintenance instructions ↗ · 0020213394_05 · page flexoTHERM 400 V 0020213394_05 p.48; flexoTHERM 230 V 0020213395_04 p.43 · Reviewed 23 September 2026
- geoTHERM mini VWS 36/4.1 230 V installation manual ↗ · 0020250181_00 · page geoTHERM mini 0020250181_00 p.35 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026