basically you're saying, if i set the central air system to a particular temperature, it will draw current/energy till it reaches that temp and then barring much need for reheating (assuming lack of drafts), it won't draw much current to keep it at that temp?
Initially you need enough energy to heat the room from T1 to T2 AND counter the losses to the environment. After you have reached T2 you will need to supply enough energy to keep it at T2, so the heat that goes into the room will need to equal the losses to the environment (heat leaving the room through the walls - heat flux because of the temperature difference between the room and the environment).
Actually what happens is, once you reach the specified temperature T2, the heating system will be turned off. Then the temperature will start dropping because of the heat losses; the temperature sensor will detect that and turn the system on again and so on. Those systems are equipped with controller elements and there is actually a range of temperature. You specify 20C and when it reaches 22C it stops then starts again when ot drops to 18C. Or something like that.