- Well, that's the way thermostats do
Let's say that you feel that 70 degrees is your favorite room temperature.
Traditional thermostats have a second slider or knob (a variable resistor, I believe) that controls how much the temperature is allowed to swing before switching. So if x represents the amount of swing in degrees, if you set the temperature to 72, the heat will go on until the temperature is 72+x, and then switch off until the temperature is 72-x, and then switch on again. So your average
temperature is somewhere around 72.
In your concept, you're saying that rather than 72 degrees be the mean temperature of the room, it becomes the boundary -- if you're x degrees below 72, turn on the heat until you reach 72, or if you're x degrees above 72, turn on the air conditioning. The problem with this is that you'll find you're rarely at the temp you set-- you'd be on average 72-x/2 degrees in the winter and 72+x/2 degrees in the summer, which means that you're back to adjusting the temperature between seasons. Likewise, the system could even turn on the heat in the summer or the air conditioning in the winter (possibly damaging the unit).
To avoid turning on the wrong system and get back to a situation where the temperature you set is the mean, the system could instead factor in the outside temperature to figure out what it's fighting against.