F8 error and thermostat cannot detect boiler status or flow temperature

Safety first: Isolate power to the boiler and controls before touching any wiring. OpenTherm is low voltage but may be adjacent to mains terminals.

Steps:

• 1. Verify wiring type: Confirm that the thermostat is wired to the boiler's OpenTherm or digital bus terminals, not to 230 V switched live terminals. Mis‑wiring can damage the thermostat.
• 2. Check polarity and continuity: At the boiler, ensure the two OpenTherm wires are firmly connected and not reversed if the boiler requires polarity. Test continuity of the cable between boiler and thermostat.
• 3. Inspect for damage: Look for crushed cable, rodent damage, or signs of overheating. Replace damaged cable with suitable low‑voltage flex.
• 4. Reset boiler and thermostat: Power cycle both devices. Some Ideal boilers require a menu setting to enable OpenTherm; verify this in the boiler's installer menu.
• 5. Test with another controller: If available, connect a known‑good OpenTherm controller to the boiler. If communication still fails, the boiler's OpenTherm interface may be faulty.