Heartbeat module comments collapsed by default
In this case you can use Collapsible Comment module. This module will help you to collapse the threads of the comments.

Now if you want to collapse entire comment region in collapsible form then you can do this using javascript.

Create a js file and add below javascript in a js file and add it to your theme or module.

(function($) {$(function() {
      var $comments = $('#comments');
      if (!$comments.size()) return;
      var count = $comments.find('.comment').size();
      // if (!count) return; // Do nothing when there is no comment.
      var label = Drupal.formatPlural(count, '1 comment', '@count comments');
      var $legend = $(document.createElement('legend')).html('&lt;span class="fieldset-legend"&gt;' + label + '&lt;/span&gt;');
      var fsetclass = 'comments collapsible' + (location.href.search(/#(comment|new)/) == -1 ? ' collapsed' : '');
      var $fset = $(document.createElement('fieldset')).addClass(fsetclass);
      var $fsetwrap = $(document.createElement('div')).addClass('fieldset-wrapper');
      $fset.insertBefore($comments).append($legend).append($fsetwrap);
      $fsetwrap.append($comments);
      var attach = function() {Drupal.behaviors.collapse.attach($fset.parent(), Drupal.settings);};
      Drupal.behaviors.collapse ? attach() : $.getScript(Drupal.settings.basePath + 'misc/collapse.js', attach);
    });})(jQuery);


The above code will help you to collapse comments. Enjoy!!! :-)