Thank you for the information. Based on the fact that there is a clicking noise when turning the the heating on and off using the app would indicate that the command is reaching the display from the app and from the display it's passed on to the Heat Link which then moves the internal relay with an audible click.
Usually we expect to see a similar behaviour in system boilers with a separate hot water tank, as they usually have a separate valve that gets stuck even though a command is issued from the Heat Link. This is using the 3rd Generation one.
Since your unit seems to be a 2nd Generation thermostat, based on the serial number, the Heat Link would only have heating control terminals. But, as mentioned, since it is clicking, it would indicate it's working as expected. Also the manual mode seems to be working as expected as well.
What I could suggest in this case is if you are comfortable in doing so or are able to get an installer to have a look at the unit, to do a continuity test. To check the continuity and not the voltage through the terminals to see if it behaves as expected. If the test looks ok, than it is something else in the system that is causing this behaviour and if it does not have the expected results, it could be the thermostat.
Do I do have to let you know that since it seems to be a 2nd Generation unit, a unit that has not been manufactured now for several years, in case that it is determine that something is going on with the thermostat a replacement will not be possible. As much as I'd like to, the warranty is most likely expired and parts for that generation are no longer available.
Should you wish I can send you the continuity steps over email, to the email connected to your account here.
