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Vaillant · Heat pumps · Vaillant aroTHERM plus with uniTOWER plus

S.276Building circuit: Lockout contact S20 open

Name in the manufacturer's documentation (service manual): Building circuit: Lockout contact S20 open[1]

In plain words (editorial)

The lockout contact S20 on the heat pump's main PCB is open – the manual names an incorrect limit thermostat setting or a flow temperature sensor reading too low. Its fix concerns the maximum flow temperature, the limit thermostat set value and the sensor values.

When to call a service technician: Ask a heating engineer to check the settings.

✎ 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] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified S.276 Building circuit: Lock
  2. [2] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified Adjust the maximum flow temperature for the direct heating circuit via the system control
  3. [3] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified Adjust the set value for the limit thermostat
  4. [4] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified Check the sensor values
  5. [5] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified Contact S20 is open at the heat pump's main PCB. Incorrect
  6. [6] Vaillant uniTOWER plus VIH QW 190/6 E – Operating, installation and maintenance instructions (0020291525_05) (0020291525_05) , p. 57 official manufacturer document quote verified Contact S20 is open at the heat pump's main PCB