@Scott you made it awesome. The last code you wrote works exactly like I wanted. I knew that I am on the right way, and I was aware that I need to modify $rules['commentbutton'] statement at qa-include/pages/question-view.php however I havent heard about $parentpost variable before. Many thanks.
This modification is good, because if you set points for answering, then question owner sometimes answer its own question just to gain points. And this kind of stuff mess the site.
On the other hand, if any one can comment on answers a large conversation accumulates just on one answer, and it wont affect hotness. Because hotness is sensitive to answers, but not to comments. So, I recommend this modification.