S.252 on a Vaillant heat pump: Fan unit 1: Fan blockedIf the fan speed is 0 rpm, the heat pump is switched off for 15 minutes and then restarted. If the fan does not start up after four unsuccessful restarts, the heat pump is switched off and the fault message F.718 …
Short answer
On a Vaillant heat pump, S.252 means: Fan unit 1: Fan blockedIf the fan speed is 0 rpm, the heat pump is switched off for 15 minutes and then restarted. If the fan does not start up after four unsuccessful restarts, the heat pump is switched off and the fault message F.718 …. 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: Fan blockedIf the fan speed is 0 rpm, the heat pump is switched off for 15 minutes and then restarted. If the fan does not start up after four unsuccessful restarts, the heat pump is switched off and the fault message F.718 ….
Part involved: Heat pump control (status/Live Monitor).
When it shows: Fan unit 1: Fan blockedIf the fan speed is 0 rpm, the heat pump is switched off for 15 minutes and then restarted. If the fan does not start up after four unsuccessful restarts, the heat pump is switched off and the fault message F.718 ….
Likely causes:
- Fan unit 1: Fan blockedIf the fan speed is 0 rpm, the heat pump is switched off for 15 minutes and then restarted. If the fan does not start up after four unsuccessful restarts, the heat pump is switched off and the fault message F.718 ….
On the flexoTHERM exclusive (VWF ../4)
The manual says: Fan unit 1: Fan blocked.
Part involved: Heat pump control (status/Live Monitor).
When it shows: See official description.
Likely causes:
- Fan unit 1: Fan blocked.
Is it safe?
| Does the heat pump stop? | Partly |
|---|---|
| Severity | Warning |
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
- Note the status code. Check the outdoor unit's air inlet and outlet are not blocked by leaves, debris, snow or ice build-up and clear any loose debris from around it (do not open the casing, poke the fins or use tools).
What the engineer will check
- Read Live Monitor status and fault history; if the status persists or precedes an F-code, follow the corresponding F-code diagnostics in the installer fault table.
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