Viessmann Vitodens 100-W fault 981
No DHW heating (minimum system pressure too low during DHW heating)
Gas boilerWhat it means
Minimum system pressure is too low during DHW heating, preventing continued DHW heating.
Likely causes
- Shut-off valve closed or too restricted
- Limescale build-up
- Sludge
- Incorrect hydraulic configuration
- Circulation pump faulty
- Air in heating circuit
- System pressure unstable or too low
Where to start
- Check that the cylinder flow and return are open
- Ensure that all quick-action air vent valves on the appliance side are permanently open
- 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
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
- 416Flue gas temperature sensor placement fault during commissioning
- 694Burner in a fault state, signal comparison deviation, flue gas high limit safety cut-out
- 799No DHW heating, no central heating, central heating circuit pump electrical fault
- 980No DHW heating (minimum system pressure too low before start)