One of the considerations when replacing a boiler is whether to re-use the existing flue and chimney. While most installations work without problems, there are a couple precautions that you should consider.
For example, to lower installation costs, an installer and designer chose to re-use the existing Category III stack, which meant it was a noncondensing, positive static pressure flue. The new boiler called for a Category I, which was noncondensing with a negative static pressure. After about two heating seasons, the Category III flue collapsed into itself because it was not designed to operate in a negative condition. It was like a beer can being squeezed after consumption.