I'm hoping someone has experience with heat exchangers in Whisperheat G20's. I've read that they can be prone to cracking, so I was inspecting with a flashlight through the damper door, and in 3 of the cells I can see a line just left of the seam where the cell curves up to the right. I'd say the lines are about 2 to 3 inches long and one has a very slight amount of rust along it. I've read in a couple of places that this could be a closed crack and I also read a comment by someone saying that this line is just there and it's normal, not a crack. Anyone have experience with this? I realize that getting a pro in would be the proper thing to do, but I'm just curious at this point.