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Viessmann Vitodens 100-W fault 980

No DHW heating (minimum system pressure too low before start)

Gas boiler

What it means

Minimum system pressure before the start of DHW heating is too low, preventing DHW heating from starting.

Likely causes

  1. Shut-off valve closed or too restricted
  2. Limescale build-up
  3. Sludge
  4. Incorrect hydraulic configuration
  5. Circulation pump faulty
  6. Air in heating circuit
  7. System pressure unstable or too low

Where to start

  1. Check that the cylinder flow and return are open
  2. Ensure that all quick-action air vent valves on the appliance side are permanently open
  3. Run the entire venting program of the heating circuit again (select via service menu) - quick-action air vent valve in boiler circuit must be permanently open

The full answer, free

  • 15 more checks in order
  • The readings and component tests from the manual
  • How to put it right, step by step
  • The manual page for this fault (page 88), right under the answer
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Safety: Isolate gas and power before you open the case. If you have been into the gas side, do a tightness test, and a combustion and flue check before you hand back. Gas Safe (UK) or RGI (Ireland) work only.

From the Viessmann manual for the Vitodens 100-W. Check the model on the data plate before you order parts.

Other Viessmann Vitodens 100-W fault codes