All boilers that burn hydrocarbon fuels (natural gas, propane, oil, wood) produce water vapor as a byproduct of combustion.
About half of the 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 the boiler’s heat exchanger.