A.16 on a Viessmann One Base: Insufficient flow rate
Short answer
On a Viessmann heat pump, A.16 means: Insufficient flow rate. 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: Insufficient flow rate.
Part involved: Insufficient flow rate.
When it shows: System characteristics:; Refrigerant circuit off: Inverter and compressor cannot be switched on.; Instantaneous heating water heater is switched off. The instantaneous heating water heater will not start.
Likely causes:
- Minimum flow rate undershot.
Is it safe?
| Does the heat pump stop? | Yes |
|---|---|
| Severity | Warning |
| Engineer needed? | Yes |
The heat pump stops until the fault clears or is reset.
Resetting itself: Not stated per code in Viessmann document 6199893 GB 01/2026 (VSA00009).
Manual reset: User may acknowledge via HMI after noting the code (6249162 p.57); the message returns if the cause persists.
What you can check
- On the HMI, tap the message and "?" to read the system characteristics and Viessmann's user tips; note the code (e.g. F.xxx) and text for your contractor (operating instructions 6249162 p.57).
- Acknowledge the message once via the HMI; do not acknowledge fault messages several times in quick succession (6249162 p.58) - notify your contractor.
- Check the heat pump breaker/isolator in the consumer unit is ON (do not repeatedly reset a breaker or RCD that keeps tripping).
- Check the system pressure shown on the HMI "System overview" (6249162 p.35) or pressure gauge and report a low reading; make sure the filling loop is closed.
What the engineer will check
- Open shut-off valves in the direction of flow. Remove any blockages.
- Clean the heating water filter. Clean the dirt separator if necessary.
- Check the integral secondary pump/heating circuit pump in heating/cooling circuit.
- Check the impeller. Remove the rotor if necessary. Remove dirt. Flush the system if necessary.
- Check connection of flow sensor (if installed) at X19.1 to X19.4 on the EHCU electronics module:.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
What you might notice
Sources
- Service instructions for contractors: Heat pumps in monoblock design incl. hybrid – System configuration and diagnostics (Vitocal 150-A/151-A/250-A/252-A, IDU-A, AWMOF-151/251, AquaSnap 30AWH, BI-BLOC R290) (6199893 GB 01/2026) ↗ · 6199893 GB 01/2026 (VSA00009) · page 202 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026