All boilers that burn hydrocarbon fuels (gas, oil or wood) produce water vapor as a byproduct of combustion.

About half of the fossil fuel boilers currently sold in the U.S. are “conventional” boilers with heat exchangers constructed of cast iron, steel or copper tube. They’re designed with the intent of being operated at conditions that do not allow this water vapor to condense, on a sustained basis, within a boiler’s heat ex-changer. It’s the same story with nearly all current-generation cordwood gasification and pellet boilers available in North America. They are not intended to op-erate with sustained flue gas condensation.

The two italicized words in the preceding paragraph deserve further attention. Just because manufacturers intend their boilers to operate at conditions that prevent sus-tained flue gas condensation does not mean that these boilers cannot condense. Given suitable conditions, any boiler can be a “condensing boiler.” While such conditions are desirable in modern boilers using appropriately designed heat exchangers and suitable venting systems, they can quickly scale and corrode carbon steel boilers, destroy steel vent connector piping, and cause severe damage to masonry chimneys.