F9 displayed and thermostat backlight flickers or unit reboots randomly

Safety first: Isolate mains power before removing the thermostat or accessing any wiring.

Steps:

• 1. Confirm supply type: Check the Ideal thermostat installation manual to verify whether your model requires 230 V AC or 24 V AC/DC. Ensure it is not connected to the wrong supply.
• 2. Tighten terminals: Remove the thermostat from its backplate and inspect the live, neutral, and earth connections. Tighten any loose screws and ensure no copper is exposed.
• 3. Measure voltage: With power restored and using a multimeter, measure the voltage at the thermostat backplate. It should be within the specified range (e.g. 230 V ±10% or 24 V ±10%). If outside this range, investigate the supply circuit or transformer.
• 4. Check shared transformer loads: If a 24 V transformer feeds multiple devices (zone valves, other stats), ensure the total VA rating is not exceeded. If overloaded, fit a higher‑rated transformer as per Ideal's guidance.
• 5. Replace thermostat if regulator faulty: If supply is stable but F9 persists and the thermostat reboots, the internal power regulator may be failing and the thermostat should be replaced.