This feature can only be activated via the web app :
Call for Heat allows your Lightwave Link to detect when any of your room control devices need heat, and will tell the device which can provide heat to switch on or off accordingly. The device that you would like to switch on or off based on the requirements of others is called the parent device.
Now you have selected your parent device, you can now choose the devices which will trigger the parent to switch on or off. These will follow their normal schedule, with the added benefit of receiving heat from their parent when necessary. These are called children devices.
You should not pair TRVs to a Thermostat. You simply pair the Thermostat to the Boiler Switch, add the Thermostat and all your TRVs to the app.
Now you Activate the call for heat, choose the Thermostat as your parent, and your TRVs as Children, so if a child is cold and needs heat then it will ask the Thermostat (parent ) for heat, the Thermostat then will send a command to the boiler switch to turn the heating ON.
The Call for heat follows each TRV schedule. SO you must set schedules for each of your TRVs based on when you want them to call for heat.
Call for Heat will set the temperature of your parent controller to either 40C or 0C depending on whether heat is desired or not desired. If your heating was set to 40C after your power cut, then one of the child devices must've been under temperature and needed heat. These temperatures are chosen as maximum and minimum temperatures to ensure the heating will go on or go off as per the childs requirements.
If you would not want 40C and 0C to be used, then you would need to turn Call for Heat off.