At this point you need to put up or shut up, i.e. either sue the installers for whatever damages you feel entitled to or get over it and move on.
As someone with some experience of technical disputes I would recommend the second course of action. As lawyers are fond of saying, a bad settlement is better than a good case. Your case is weak unless you commission an expert report that quantifies your hypothetical damage to the heat exchanger. Such a report, assuming you can find someone willing to do the work, will cost considerably more than just replacing the HX on spec. Keep in mind that there's no guarantee that you'll get the costs involved back even if you end up winning the case.