Why do you think Montag runs to Faber's house in Fahrenheit 451?
I assume that you are talking about what Montag does after he kills Beatty.
I think that Montag runs to Faber's house at this point in the book because he has no other possibilities. So far, in the book, there are only two people other than Montag who seem to have any inclination to rebel against the society. These two are Faber and Clarisse. So you would think that Montag would turn to one of them to hide him or help him escape the Hound.
Of course, by this time, Clarisse is dead so Montag cannot get any help from her. It makes sense, then, to go to Faber. Besides, Faber is something of a mentor to Montag so it makes sense that Montag would think that he could help. After all, Faber is the one who gave him the "bullet" and who was going to help him rebel against the society.
So, Montag runs to Faber because Faber is his mentor and because Faber is the only one left who might help him.
Montag runs to Faber's house after killing Beatty because he has no one else to turn to. Obviously, he cannot turn to anyone that he knew before, such as his fellow firemen or Mildred's friends. Everyone else he knows are quite content with this society and would turn in him if he showed up at their doorsteps. Faber is the only one who would hide and protect Montag from the law. Montag needs somewhere to hide because he has committed murder and he knows that the Hound will be set on him soon. He realizes that he must go on the run now and leave the city, but before he does so he needs a rest stop, and the only rest stop available to him is Faber's home.