I have a 7 year old 100 btu model fg8 80% furnace. The fan motor failed and when I pulled the blower I figured what the heck I will peak at the hx. I could not believe my eyes two out of the four cells were cracked. The next day I pulled the hx and found a 3/8 wide by 4" crack that I did not see untill I pulled it. I could not beleive that one of the worst looking hx ers I have ever seen was in my own house. It is a factory defect in that the crack is due to no air hitting the outside cell. The outside cell sits right above the slide channel for the blower so no air could hit it. It also cracked on the other side of that cell and cracked the cell next to it. I know it was firing fine since I have clocked the input and check the temp rise. I bet it is a waste of time for waranty other than the hx. I did not evan attemp checking into the hx since it would just crack again.