to form in a furnaces heat exchanger?
In the normal course of furnace operation, the heat exchanger experiences repeated heating and cooling, which causes a corresponding expansion and contraction of the metal. Cracks will tend to form in areas of the heat exchanger that are weak, such as the areas that have been welded together at the factory. More weak spots and holes will form as a result of corrosion. Corrosion of the heat exchanger often signifies problems with the exhaust venting. What happens is that water in the exhaust, which contains carbonic acid, cools too quickly and condenses out, draining back down the chimney and onto the heat exchanger where the acid eats away at the metal.
