Glossary · Heating, cooling & ventilation
Mid-efficiency furnace
Also called: mid-efficiency · 80% furnace · non-condensing furnace · mid-efficiency boiler
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.
What it means
Mid-efficiency furnaces were the standard upgrade before condensing models: they don't recover the heat in their exhaust's water vapor, so roughly a fifth of the fuel's energy goes up the flue. The federal minimum for most new gas furnaces in the US is still 80% AFUE, so they're still sold alongside condensing models rated 90% or more. After major air sealing a mid-efficiency furnace runs less often, so its chimney can get colder between firings and draft poorly.
Why it matters in an audit
If your mid-efficiency furnace is nearing replacement, an audit helps you compare a condensing furnace, a heat pump or a dual-fuel system for your home.
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| US minimum, non-weatherized gas furnaces | 80% AFUE (made from Nov 19, 2015); 95% for furnaces made from Dec 18, 2028 under the 2023 rule, whose future is in doubt | ecfr.gov ↗ |