This unit is in my attic, the heat works but the problem is that is not hot enough, the blower start too soon and won't let the heat exchanger get hot. Somebody help me here, thank you. I change the speed in the blower to medium low still the same.

Why is it in your attic? Hot air rises so it defeats the purpose unless you have a good forced air system. Is something wrong with the forced air system ?

My house have two furnace one in the basement and the other one in the attic that handle the second floor.