This heat exchanger came from an $800,000 house that the installing contractor botched, they not only botched this house but all others in the development. This 12 year old 80k BTU 3 ton furnace had 2 8x20 returns, both using only the wall cavity one of which was completely blocked with insulation. The supply duct was 10x11... This heat exchanger didn't have a chance. When I showed up for a maintenance I fired up the furnace and felt a massive amount of heat coming from the blower compartment, you could fry an egg on the firewall (downflow). At that point I was sure the heat exchanger was blown, pulled the blower and found the cracks. Anyway, just wanted to show the homeowners here what happens when you don't make an informed, non-price driven decision when it comes to your HVAC system, cracks like these are bad for your health!