H12
Buderus
External Control Signal Fault
Error H12 appears; heater may not respond to external demand signals.
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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
Open or shorted external demand contacts, Incorrect wiring of external call-for-heat, Faulty external controller or relay, Misconfigured control parameters
How to Fix / Troubleshooting
Safety: Turn off power before working on control terminals.
- Identify external controls: Determine what external device (e.g., building management system, relay, thermostat) is connected to the heater's demand terminals.
- Inspect wiring: Check for loose, broken, or shorted wires at the heater and at the external device.
- Test contacts: Use a multimeter to verify that the external contacts close when there is a demand and open when there is none.
- Bypass test (if allowed): Temporarily jumper the demand terminals at the heater to simulate a call for heat. If the heater responds normally, the issue is with the external device or wiring.
- Check control settings: In the installer menu, confirm that the external control function is enabled and configured correctly.
- Restore and test: After correcting wiring or replacing faulty external controls, restore power and verify that H12 clears.
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Repair Difficulty
Medium
3/5
Required Part
External Control Interface / Demand Terminals
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