Thanks for letting us know @CEB
Could you tell me please when did the situation start occurring first and when was the thermostat installed?
Also, please answer to the following questions:
Are there any lights on the Heat Link? Does the boiler activate when calling for heat by pressing the Heat Link's button to activate Manual Mode?
Are you seeing any wiring report or error on the thermostat and/or the app?
Have you had any recent power cuts in your area? If yes, has the power been restored?
Have you recently had service or maintenance on your electrical, plumbing, or heating system?
If yes, how long was the power off for? (Be specific.)
Could you tell us please if you've checked on the breaker switch and fuse box?
Any blown fuse in there? Have you tried to turn the breaker switch off and on again? Any difference?
