F.820 on a Viessmann One Base: Lead break, electronic expansion valve 2
Short answer
On a Viessmann heat pump, F.820 means: Lead break, electronic expansion valve 2. 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: Lead break, electronic expansion valve 2.
Part involved: Lead break, electronic expansion valve 2.
When it shows: System characteristics: The refrigerant circuit will not start.
Likely causes:
- Lead break, electronic expansion valve 2.
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 6171807 GB 10/2021.
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 connections on VCMU refrigerant circuit controller.
- Check electronic expansion valve 2 via actuator test.
- Isolate the system from the power supply. Restart the system. If the fault persists, replace electronic expansion valve 2.
Who to ask for: Viessmann-trained heat pump engineer with F-gas certification and R290 (A3 flammable refrigerant) competence.
Certification: F-gas certificate (UK F-gas Regulations; EU 2015/2067 Cat. I cited by Viessmann) plus A3/R290 competence per EN 378-4 or IEC 60335-2-40 Annex HH (6202263 p.2).
Sources
- Installation and service instructions Vitocal 150-A, AWO(-M)-E-AC/AWO(-M)-E-AC-AF 151.A (6171807, 2021-10) ↗ · 6171807 VSA00001 · page 110 · Reviewed 23 September 2026
A code explains what the controller detected, not a confirmed root cause.
Reviewed: 23 September 2026