Vaillant
Domestic Hot Water Control Fault
Hot water does not heat according to the Vaillant smart thermostat’s hot water schedule.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Cylinder sensor not configured or assigned, Vaillant 3‑port or 2‑port valve not opening, Hot water priority settings misconfigured, Boiler DHW mode disabled
How to Fix / Troubleshooting
Safety: Isolate power before working on valves or wiring. Cylinder surfaces and pipes may be hot.
- 1. Check hot water schedule: Confirm that DHW schedules are enabled and set to the desired times and temperatures.
- 2. Verify cylinder sensor: Ensure the Vaillant cylinder sensor is installed, wired, and recognised in the controller configuration.
- 3. Observe valve operation: During a DHW call, check that the cylinder zone valve or 3‑port valve moves to the hot water position. Replace the actuator if it does not move.
- 4. Check boiler DHW settings: On the boiler, ensure DHW mode is enabled and not limited by eco or holiday settings.
- 5. Technician check: If wiring and valves appear correct but DHW still does not heat, have a professional check the boiler and control wiring.
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Repair Difficulty
Hard
4/5
Required Part
Vaillant cylinder sensor / DHW zone valve actuator
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