How to simply turn heating on/off with VRT 340f thermostat

I called a number of Central Heating contractors and also Uponor, the company that makes the controller.

I was told that the clicking noise comes from the soft-fuses inside the controller trying to reset. They said box has failed and needs to be replaced by an electrician. Since the controller is the simplest model in their range, there are no diagnostic LEDs or other features that indicate a fault status, except the clicking noise of the soft-fuse.

The repair should only be about an hours labour plus cost of a new controller box.

The Nest can only control one zone at a time, but that's ok, because it sounds like there's only one "control zone" in your system.

Your heating system may work better if its split into three sections, but since the will all come on at the same time, the Nest should be able to control all three at the same time. What the Nest can do is control each zone separately.

Typical control systems have a "hot" wire and a "common" wire. Just tie all the commons together and all the hots together and then run a single common and hot to the Nest for control.