if (scrollView.contentOffset.y <= - 65.0f && !_loading) {

// when refresh condition is met (you scrolled beyond the bottom boundary)
[self performSelector:@selector(yourRefreshMethod) withObject:nil afterDelay:2.0];

// when refresh condition is not met (you scrolled back to the normal boundary)
[NSObject cancelPreviousPerformRequestsWithTarget:self];