FLOW-RATE-MIN on a Heliotherm Basic Comfort air split: Temperature difference between heat outlet and inlet too high - insufficient flow/energy for defrost
Short answer
On a Heliotherm heat pump, FLOW-RATE-MIN means: Temperature difference between heat outlet and inlet too high - insufficient flow/energy for defrost. 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: Temperature difference between heat outlet and inlet too high - insufficient flow/energy for defrost.
Part involved: Flow rate min. (Heat.inlet-Heat.outlet temperature difference monitoring).
Likely causes:
- Too low heating-side flow.
- Circulation pump defective.
- Pump not at full load.
- Heating valves closed.
- UFH regulating valves too closed.
- Heating system temperature too low.
Is it safe?
Switch the heat pump off at its isolator and call an engineer.
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Fault — needs a manual reset once the cause is fixed |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Manual reset: Fault clearance function in the controller main menu (web control 321 manual p.27, intended for the Heliotherm competence partner).
What you can check
- Note the error text shown on the controller and contact your Heliotherm competence partner.
- Check the heating system pressure gauge reading.
What the engineer will check
- Check valves.
- Replace circulation pump.
- Check PWM pump control - set to full load; check flow via vortex sensor.
- Use buffer/hydraulic separator.
- Activate 2nd step.
Who to ask for: heat pump / refrigeration (Heliotherm competence partner).
What you might notice
Sources
- Heliotherm Installation Instructions Air Split Heat Pump Basic Comfort Series (Version 1.0, 28.03.2019) - via ManualsLib 3389544 ↗ · Version 1.0 · page 36 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026