W-FAST-FLASH on a Technibel S.A.S. Technibel PHRIE / PHIE inverter monobloc air-to-water: Low water temperature at heat exchanger outlet (TW2 <16 C during defrost in heat mode, <4 C in cooling); manual reset after 4th event in an hour
Short answer
On a Technibel S.A.S. heat pump, W-FAST-FLASH means: Low water temperature at heat exchanger outlet (TW2 <16 C during defrost in heat mode, <4 C in cooling); manual reset after 4th event in an 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: Low water temperature at heat exchanger outlet (TW2 <16 C during defrost in heat mode, <4 C in cooling); manual reset after 4th event in an hour.
Part involved: CWC2 (A2) board / water circuit.
Likely causes:
- Water volume too low.
- Low water flow.
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 system water volume.
- Check flow rate/pump setting, valves, filter.
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