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Viessmann · Heat pumps · Viessmann Vitocal 250-A (One Base, monobloc)

F.791Phase L1 in power supply of instantaneous heating water heater not present

Name in the manufacturer's documentation (service manual): Phase L1 in power supply of instantaneous heating water heater not present[1]

In plain words (editorial)

Phase L1 of the power supply to the instantaneous heating water heater is missing – reduced heater output. The manual says to check the heater's fuse, power supply and power cables.

When to call a service technician: Check that the heater's fuse has not tripped; otherwise contact your heating contractor.

✎ editorial – written from the cited sources, not a manufacturer statement. Status: draft.

The Polish page has more detail (plain-language explanation, safe checks, guides): Read in Polish →

Sources

Every statement has a footnote to the document and PDF page. “Quote verified” means the quote was found automatically in the text of that page.

  1. [1] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified F.791
  2. [2] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Check fuse for instantaneous heating water heater
  3. [3] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Check power supply on
  4. [4] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Check power cables
  5. [5] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Check instantaneous heating wa- ter heater
  6. [6] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Replace if required
  7. [7] Viessmann – Service instructions: System configuration and diagnosis for heat pumps with Viessmann One Base (6194799 GB, 3/2022) (6194799 GB (3/2022)) , p. 60 manufacturer document – copy from a distributor quote verified Reduced output, instantane- ous heating water heater