Thermostat shows E20 and indicates bus power or communication overload on multi-zone systems.

Safety first: Isolate power to the wiring centre and all connected modules before working on bus wiring. Even low-voltage buses can be damaged by shorts.

Steps to diagnose and fix:

• 1. Inspect bus wiring: Trace the bus cable between thermostat, wiring centre, and any expansion modules. Look for crushed cables, staples, or signs of damage.
• 2. Disconnect segments: With power OFF, disconnect all bus branches at the wiring centre. Reconnect them one by one, powering up each time, to identify which branch triggers E20.
• 3. Check for incorrect connections: Ensure that no external power supplies are connected directly to the bus lines. Only the Danfoss wiring centre should power the bus.
• 4. Count devices: Verify that the number of connected thermostats and modules does not exceed the maximum supported by the wiring centre. If necessary, add an additional wiring centre and split the load.
• 5. Replace damaged module: If a particular module or thermostat causes E20 when connected, its bus interface may be shorted. Replace that device.

When to call a professional: Complex bus diagnostics and multi-module systems should be handled by a heating control specialist.