OK. You may remember I posted photos of my options to install the NC-30 either through an old lined boiler flue or through the basement fireplace flue. Because the old boiler flue was lined with galvanized I opted to re-line the fireplace thinking it would be less work than removing old galvanized liner. I may have been wrong about that. The fireplace was one of the old steel ones with the cold air intakes below the hearth step and the hot air exhaust above the fireplace. This meant I had to cut out those pipes that intersected the flue along with cutting lots of other metal in places the human body was not meant to reach. I also found that the basement flue has a dogleg to the right - good thing I got flex liner. So anyway . . . on the bottom of the liner I have an elbow with 18" of rigid SS liner going into the room. I had to bust out a hole in the basement wall above the hot air vent. In the process of busting out this hole I caused some hairline cracks along the mortar seams of the nearby cinder blocks. I also encountered another layer of block behind that. and busted that out too. I'm hoping the steel fireplace did not actually offer any structural support as I cut a lot out. There was fiberglass insulation between steel and block so I'm guessing this meant there was no load on the steel. These hairline cracks - are they of concern? I'm thinking once I remove the vent and fill it in with block and mortar the weight will be supported again and the cracks are just cosmetic. Otherwise I could try removing those blocks up to the cracks and replace with new mortar. My intention is to fill in the fireplace opening with block as well as that rectangular hot air vent above the fireplace opening.