Brian immediately diagnosed the problem as an old collapsing liner and not just blockage in the chimney, so saved us money in the long run not doing work that would not have solved the cause of the problem. He pulled out the old liner, which was essentially disintegrating, inserted a new liner and insulation to prevent any vibrations, put on the new cap, and fully caulked the top, and recemented the hole in the furnace room, from having to chip out the old cement to enable the removal of the old liner and blockage. FYI, Brian told us aluminum liners for gas furnaces have a useful life of 10 years or so (ours was 20+) and should be replaced to avoid long-term problems.