Thermostat shows F.12 and hot water control is unavailable in the app

Safety: Isolate power to the boiler and wiring center before opening covers. The cylinder sensor circuit is low voltage but runs near hot pipework.

Diagnosis:

• Locate the Glow‑worm cylinder sensor on the hot water cylinder (usually strapped to the side or inserted into a dry pocket).
• Inspect the sensor cable for damage, especially near clips and entry points into junction boxes.
• With power off, disconnect the sensor at the wiring center and measure its resistance with a multimeter. Compare to the Glow‑worm NTC chart for the approximate cylinder temperature.
• Check that the sensor is connected to the correct low‑voltage sensor terminals on the wiring center or boiler.

Remedy:

• Replace the cylinder NTC sensor if its resistance is out of range or intermittent when the cable is flexed.
• Re‑route or replace damaged cable runs and ensure all terminations are tight.
• After replacement, restore power and verify that the thermostat now displays a realistic cylinder temperature and that F.12 clears.