Thermostat displays F8 and cannot control heating zones via external zone module.

Safety: Turn off power to all control modules before working on bus wiring.

• Step 1 – Check zone module power:
  • Verify that the Protherm zone control module has power (status LED on).
  • If not, check its power supply and fuse.
• Step 2 – Inspect bus wiring:
  • Locate the communication bus between thermostat and zone module.
  • Ensure correct polarity and secure connections at both ends.
  • Repair any damaged cable sections.
• Step 3 – Verify addressing:
  • Check DIP switches or software settings for zone module address.
  • Ensure addresses do not conflict and match thermostat configuration.
• Step 4 – Power cycle system:
  • Turn off power to thermostat and zone module for 2 minutes.
  • Restore power and allow modules to reinitialize.
  • Check if F8 clears.
• Step 5 – Replace module if needed:
  • If communication still fails, the Protherm zone control module may be defective and require replacement by a qualified technician.