F.1049 on a Viessmann One Base: Fault, 3/2-way mixing valve for dual mode operation
Short answer
On a Viessmann heat pump, F.1049 means: Fault, 3/2-way mixing valve for dual mode operation. The manual does not say whether the heat pump stops. An engineer is needed to fix the cause.
Reviewed 23 September 2026 · 1 source
What it means
The manual says: Fault, 3/2-way mixing valve for dual mode operation.
Part involved: Fault, 3/2-way mixing valve for dual mode operation.
When it shows: System characteristics: External heat generator not ready for operation Further message: A.109 if set flow temperature is not reached within 60 min.
Likely causes:
- Malfunction of the 3/2-way mixing valve for dual mode operation.
Is it safe?
| Does the heat pump stop? | Not stated |
|---|---|
| Engineer needed? | Yes |
The manual does not say whether the heat pump stops.
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 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
- Check configuration of external heat generator.
- Check the following connections to the HIO electronics module:.
- 52.1/52.2/52.4.
- TS1.1/TS1.2.
- Check wires and plugs for correct seating.
- Check connections for contact corrosion.
- Check cables for mechanical damage, e.g. crushing, kinking, chafing or breakage.
- Check whether the external 3/2-way mixing valve opens and closes.
Who to ask for: Viessmann-trained heat pump installer/service engineer.
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 181 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026