Viadrus Heating Demand / Diverter Fault (Combi) or External Valve Fault

Boiler produces hot water but no central heating, even when room thermostat is calling for heat.

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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

Faulty diverter valve on Viadrus combi boiler, Stuck 3-port or 2-port motorised valve on system boilers, Incorrect wiring of heating controls, Failed room thermostat

How to Fix / Troubleshooting

Safety first: Isolate power before working on valves or control wiring. Hot pipes can cause burns.

Step-by-step checks:

  • 1. Verify thermostat demand: Set the room thermostat high and ensure it clicks on. Check any programmer is set to HEATING ON.
  • 2. Feel pipes: On combi models, feel the flow pipe leaving the boiler when heating is requested. If it stays cold while DHW works, the diverter valve may be stuck in DHW position.
  • 3. Check motorised valves (system boilers): Locate the 3-port or 2-port valves. When heating is called, the valve head should move and the manual lever should feel loose. If stuck, the head or valve body may need replacement.
  • 4. Basic wiring check: Ensure all control cables are firmly connected at the boiler and junction box.

When to stop: Diverter and motorised valve replacement is best done by a heating engineer, especially where draining is required.

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Repair Difficulty

Medium 3/5

Required Part

Diverter Valve (Combi) or Motorised Valve
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