S.255 on a Vaillant heat pump: Fan unit 1: Air inlet temp. too high The compressor does not start because the outdoor temperature at the fan is above the application limits. Heating mode: > 43 °C. Domestic hot water mode: > 43 °C. Cooling mode: > 46 °C
Short answer
On a Vaillant heat pump, S.255 means: Fan unit 1: Air inlet temp. too high The compressor does not start because the outdoor temperature at the fan is above the application limits. Heating mode: > 43 °C. Domestic hot water mode: > 43 °C. Cooling mode: > 46 °C. The heat pump keeps running with some functions stopped. The wording differs by model range — each is shown below.
Reviewed 23 September 2026 · 2 sources
What it means
On the aroTHERM plus (VWL ../6 A)
The manual says: Fan unit 1: Air inlet temp. too high The compressor does not start because the outdoor temperature at the fan is above the application limits. Heating mode: > 43 °C. Domestic hot water mode: > 43 °C. Cooling mode: > 46 °C.
Part involved: Heat pump control (status/Live Monitor).
When it shows: Fan unit 1: Air inlet temp. too high The compressor does not start because the outdoor temperature at the fan is above the application limits. Heating mode: > 43 °C. Domestic hot water mode: > 43 °C. Cooling mode: > 46 °C.
Likely causes:
- Operating status reported by the heat pump control (not a fault in itself).
On the flexoTHERM exclusive (VWF ../4)
The manual says: Fan unit 1: Air inlet temp. too high.
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? | Partly |
|---|---|
| Severity | Information only |
| Engineer needed? | No |
The heat pump keeps running with some functions stopped.
Resetting itself: yes - status changes automatically with operating state.
Manual reset: not applicable.
What you can check
- Operating status only - no action needed. If heating or hot water is not being delivered for a long period, note the status code and tell your installer.
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.
- Intermittent fault 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.32; uniTOWER VIH QW 190/6 E 0020291525_05 p.57; 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.49 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026