Condensing furnaces come with some of the highest AFUE (annual fuel utilization efficiency) ratings, typically between 90-98%. But what makes them “condensing?”
All furnaces use burners which convert a fuel such as natural gas into heat, which then delivers this heat to the circulating air via the heat exchanger. This conversion process creates water vapor as a byproduct. Condensing furnaces then have a secondary heat exchanger which further extracts warmth from this water vapor. This increases the overall AFUE of the system.
A couple things to keep in mind… condensing furnaces should be used in crawl space or basement applications only, as freezing or leak issues can occur in attics. Condensing furnaces require PVC instead of metallic b-vent flue piping.
Craig Sopo 