There are several reasons for this furnace-chimney incompatibility. First, the size of the chimney can be an issue. Modern, higher-efficiency furnaces transfer more heat into your home and less heat up the chimney than older, less-efficient units. While this means more efficiency for your energy dollar, it also means that the existing chimney might be too large for the new furnace. The result could be improper ventilation of flue products, which can cause condensation problems inside the chimney.
Other considerations include chimney height and location, proper lining and condition of the chimney. Building codes must also be kept in mind to ensure proper draft in the chimney for adequate ventilation.
For the best advice on how to configure your new furnace, contact Hamco!