W-SLOW-FLASH on a Technibel S.A.S. Technibel PHRIE / PHIE inverter monobloc air-to-water: Water flow fault in heat pump: no flow detected by flow switch while pump runs (>10 s stops HP; stored if >1 min or >3 times/hour)
Short answer
On a Technibel S.A.S. heat pump, W-SLOW-FLASH means: Water flow fault in heat pump: no flow detected by flow switch while pump runs (>10 s stops HP; stored if >1 min or >3 times/hour). The heat pump stops until the fault clears or is reset. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Water flow fault in heat pump: no flow detected by flow switch while pump runs (>10 s stops HP; stored if >1 min or >3 times/hour).
Part involved: CWC2 (A2) board / water circuit.
Likely causes:
- Pump not running.
- Closed valves.
- Blocked filter.
- Low system pressure.
- Flow switch.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — clears by itself when the condition goes |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Automatic then manual (A2 RESET 5 s, system OFF, or power off).
What you can check
- Note the code/LED pattern and the model from the rating plate, then contact a heat pump/refrigeration engineer.
- Check the heat pump's breaker/isolator at the consumer unit is on.
- Check the heating system pressure gauge reads in the normal range stated by the installer (usually about 1-1.5 bar cold) and that the filling loop is closed.
What the engineer will check
- Check circulating pump.
- Check valves open.
- Check water filter.
- Check water pressure.
- Check flow switch (§2.3.2).
Who to ask for: refrigeration / heat pump engineer.
Certification: Electrical competence for work inside the unit.
What you might notice
Sources
- Technibel PHRIE / PHIE inverter monoblock air to water heat pump, medium temperature, R410A - Service Manual (Jun. 2015, 10 12 211 - GB - 06) ↗ · 10 12 211 - GB - 06 · page 112-114 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026