The Viva learning thermostat lets you divide your home into heat zones. Heat zones are groups of heating controllers (like radiator thermostat valves) that are controlled together from the thermostat tab in the app and use data from one or more temperature sensors in the heat zone.
By default, Viva groups all heat sources into one heat zone, but by selecting Settings->Heat zones from the menu, you can divide your home into several zones. Click "Add heat zone" and select the rooms you want to include in the heat zone. A new heat zone requires a minimum of 1 sensor and 1 heat source.
Because Viva requires a source of temperature data to control heating, there is currently no way to directly control a single heat source without an integrated sensor.