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.