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Viessmann · Boiler troubleshooting

Lag boilers lose contact with lead boiler (cascade)

What is going on

After about 60 s of lost communication each boiler switches to emergency operation with its own flow setpoint.

Likely causes

  • CAN bus break between lead and lag boilers

Check in this order

  1. Check CAN bus cables and plugs.
  2. Emergency setpoint is set with parameter 2451 (disabled by default; default 60 C / 140 F, range 0 to 80 C / 32 to 176 F).
  3. Frost protection stays active below 5 C (41 F).

Applies to: Vitodens 200-W B2HE 85, 120, 150, 199

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Source: Viessmann Vitodens 200-W B2HE 85/120/150/199 Service Instructions 6167 813-24 (09/2026), p.13, 60. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.