Boiler runs unexpectedly in cold weather even when heating is off at the programmer.

Safety: Do not disable frost protection in areas at risk of freezing; this can cause major damage.

User checks:

• Locate any frost thermostat near the Grant boiler or in the loft/garage and check its setpoint (typically around 5°C).
• Adjust the setpoint slightly lower if it is set unreasonably high (e.g., above 10°C).

Engineer checks:

• Test the frost thermostat for correct open/close temperatures; replace if stuck.
• Verify that the frost protection wiring is correctly integrated with the main controls and not permanently calling for heat.