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Heat pumps · Error codes · Viessmann
F.808Viessmann

F.808 on a Viessmann heat pump: Fault, lower fan

Short answer

On a Viessmann heat pump, F.808 means: Fault, lower fan. The heat pump keeps running with some functions stopped. An engineer is needed to fix the cause. The wording differs by model range — each is shown below.

Reviewed 23 September 2026 · 2 sources

What it means

On the One Base

The manual says: Fault, lower fan.

Part involved: Fault, lower fan.

When it shows: System characteristics: Limited operation of the heat pump.

Likely causes:

  • Lower fan not running; outdoor unit may be in an unfavourable location.

On the One Base Split

The manual says: Wrong signal for number of connected fans.

Part involved: Wrong signal for number of connected fans.

When it shows: System characteristics: Limited operation of the heat pump.

Likely causes:

  • Wrong signal for number of connected fans.

Is it safe?

On the One Base

Does the heat pump stop?Partly
Engineer needed?Yes

The heat pump keeps running with some functions stopped.

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.

On the One Base Split

Does the heat pump stop?Partly
Engineer needed?Yes

The heat pump keeps running with some functions stopped.

Resetting itself: Not stated per code in Viessmann document 6199961 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 outdoor unit is not blocked by leaves, debris or snow.

What the engineer will check

On the One Base

  • If required, take steps to protect outdoor unit from wind, e.g. with an air diverter in front of the fans.
  • Isolate the outdoor unit from the power supply.
  • Check fan and fan motor bearings for damage and blockages: Turn slowly by hand. Remove any blockages. Replace fan motor if required.
  • Check 6.3 A fuse at X2 (L1) of VCMU refrigerant circuit controller. Replace fuse if required.
  • If the fan does not start automatically, check connection X17 on the VCMU refrigerant circuit controller:.
  • 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 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).

On the One Base Split

  • Ensure that only 1 fan is connected to plug-in connection P601 on the ODUC refrigerant circuit controller. If necessary, remove the additional fan.

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).

What you might notice

Sources

A code explains what the controller detected, not a confirmed root cause.

Reviewed: 23 September 2026