Exhaust
The condensate expelled from a condensing boiler is acidic, with a pH about the same as an orange (between 3 and 4). Condensing boilers require a drainpipe for the condensate produced during operation. This consists of a short length of inexpensive polymer pipe with a vapour trap to prevent exhaust gases from being expelled into the building. Though the mildly acidic nature of the condensate poses no health risk to occupants, it may be corrosive to older cast iron plumbing waste pipes and concrete floors. A neutralizer is employed in such cases, typically consisting of a plastic container filled with marble or limestone aggregate or "chips" (alkaline) to raise the pH to acceptable levels. If a gravity drain is not available, then a small condensate pump must be installed to lift it to a proper drain.
The primary and secondary heat exchangers are constructed of materials that will withstand this acidity, typically aluminum or stainless steel. Since the final exhaust from a condensing boiler has a lower temperature than the exhaust from an atmospheric boiler 38°C (100°F) vs. 204°C (400°F) a mechanical fan is always required to expel it, with the additional benefit of allowing the use of low-temperature exhaust piping (typically PVC in domestic applications) without insulation or conventional chimney requirements. Indeed, the use of conventional masonry chimney, or metal flue is specifically prohibited due to the corrosive nature of the flue products, with the notable exception of specially rated stainless steel and aluminum in certain models. The preferred/common vent material for most the condensing boilers available in North America is PVC, followed by ABS and CPVC. Polymer venting allows for the added benefit of flexibility of installation location including sidewall venting saving unnecessary penetrations of the roof.
One UK company, Atmos Heating Systems, has patented a system which does not need a drainpipe.
Read more about this topic: Condensing Boiler
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