Annual Fuel Utilization Efficiency — the percentage of your fuel that becomes usable heat over a season. An 80% furnace wastes 20 cents of every gas dollar up the flue; modern condensing furnaces reach 95%+.
Heating terms here point to gas furnace vs heat-pump trade-offs through Maryland cold snaps, AFUE efficiency, heat exchanger safety, and keeping older systems reliable in the region's mixed housing stock.