The way the nest works , it leans all the time about how quickly your house heats up and say you want your house at 20 C at 7 AM the nest knows what the outside temp is and will calculate how long it will take to bring your house to that temp and will bring it on early, the alloted time changes with outside temperatures and existing temp within the room that the nest is located, it doesnt mean that if you set it at 20 c at 7 that the boiler will come on at 7