The temperature variance is between whatever temperature the house is warmed to when the CH is on, and, given time, the outside temperature, which is what it would eventually fall to with the CH off. But that's without taking into account the effect of sunlight entering through a window, which can make a difference to the temperature in a room. This all makes the variance at any time so unpredictable, and you'll only find out by going round the house with a thermometer, and taking notes.