Glossary · Heating, cooling & ventilation

Condensing furnace (high-efficiency furnace)

Also called: 90% furnace · 95% furnace · condensing boiler · high-efficiency boiler

A gas furnace or boiler that captures extra heat from its exhaust by condensing the water vapor in it, reaching 90% efficiency or more.

What it means

By cooling the exhaust until its water vapor condenses, these units capture heat that older furnaces send up the chimney. They vent through plastic pipe, drain acidic condensate and often draw combustion air from outdoors (see sealed-combustion appliance).

Why it matters in an audit

Moving from a mid-efficiency to a condensing furnace saves fuel, but an audit may show that a heat pump or dual-fuel system saves more.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
US minimum from Dec 18, 2028 (2023 rule, future in doubt)	95% AFUE for non-weatherized gas furnaces, a level only condensing furnaces reach	ecfr.gov ↗

Sources

Related terms

AFUE (annual fuel utilization efficiency) The share of a furnace's or boiler's fuel that ends up as heat in your home over a year; a 95% furnace loses about 5% up the vent.
Dual-fuel (hybrid) system A heat pump paired with a gas, propane or oil furnace or boiler; controls switch to the fuel when it's colder or cheaper to do so.
Draft (chimney draft) The upward pull in a chimney or vent that carries exhaust out of the house; weak or reversed draft lets combustion gases spill indoors.
Backdrafting and spillage When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.
Mid-efficiency furnace A non-condensing gas or oil furnace, around 80% efficient, that sends its exhaust up a chimney; 80% AFUE is still the US minimum for most gas furnaces.
Sealed-combustion (direct-vent) appliance A furnace, boiler, water heater or fireplace that brings in combustion air from outdoors through its own pipe and vents exhaust straight outside.

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