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Since the 92 change to NFPA 54 the venting standards have become more and more restrictive concerning 80 % equipment and masonry chimneys. Masonry chimneys are not suitable for venting gas equipment. the physics are all wrong. They are oversized (look at the cross sectional area, 8x8 chimney tile is about 56 nominal sq" vs 6" liner at about 28 sq"), when a gas expands it cools. When it cools it condenses. The condensate is corrosive. If the appliance is oversized, and it always is, the short burn cycles further aggravates the problem as the chimney does not get up to steady temp. as there is no steady state operation. If the top bricks are loose it was not the chimney liner guys fault, although it was his responsibility to point it out and suggest repair, IMHO. Chimney tops are where you can most easily identify venting problems. Just look at the older houses in your area and see how many of the chimneys are "the leaning towers of Pisa". Rather than spend the money to line the chimney however your friend would have been money ahead ditching the chimney and upgrading to a condensing unit. or at least a direct vent of higher efficiency