Glossary · Heating, cooling & ventilation
AFUE (annual fuel utilization efficiency)
Also called: furnace efficiency · boiler efficiency · AFUE rating
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.
What it means
AFUE counts losses up the chimney or vent but not heat lost from leaky or uninsulated ducts, which can be large. Mid-efficiency furnaces are about 80%, and condensing furnaces reach 90% or more; older furnaces with standing pilot lights score lower.
Why it matters in an audit
Knowing your furnace's AFUE lets the auditor estimate what a replacement or a heat pump would save, and AFUE minimums decide what can legally be installed.
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 (made from Nov 19, 2015) | 80% | ecfr.gov ↗ |
| US amended standard (December 2023 rule) | 95% for furnaces made from Dec 18, 2028; on June 8, 2026 the Supreme Court vacated the appeals ruling that upheld it and sent the case back, the rule stays in effect meanwhile, and DOE is weighing a delay to Jan 1, 2030 | contractingbusiness.com ↗ |
In the United States
Most new US gas furnaces can still be 80% AFUE; as of mid-2026 the 95% rule's future was unsettled.