I guess I owe a story here. I went to Nissan dealer and one of advisor greeted me. I told him the problem and show him the air is just as cold as outside, that's after some 5 miles drive. The he started to turn the heat cold knob back and forth, which I did, but no effect. Then he turn the fan from I to IIII, all the suddent hot air started to come out, even later I set the fan back to I, the air is still hot. I stood there like I was a fool, maybe I was. But anyhow, no charge , no anything. Fix right on the spot.
I think the reason maybe the air control valve was stuck by some debris, then when he turn the fan to max, it must blow the debris away and free up the valve.