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Furnace Efficiency 101

Posted: November 5, 2018

Fixing the furnace

Efficiency is one of the most important considerations when it comes time for a furnace upgrade, since it will go a long way toward telling you how much heating bang you’re likely to get for your energy buck.

A key measure of efficiency in heating equipment is AFUE, or Annual Fuel Utilization Efficiency rating (on newer models, you will find a unit’s AFUE rating on a yellow sticker on your equipment; older models may not have an AFUE rating).

An AFUE rating tells you how much heat the furnace will produce over the course of a heating season compared to the amount of energy it consumes during the same period. In a furnace with a 90 AFUE rating, for example, 90 percent of the fuel it uses will end up heating your living room; the rest of that energy will be lost along the way.

How to identify high, low, and mid-efficiency furnaces

Not all furnaces have AFUE stickers – some older furnaces were never AFUE rated; on others, the sticker simply fell off. But you can still approximate a system’s efficiency using its features. Here are the U.S. Department of Energy’s guidelines for identifying a low-, medium-, and high efficiency furnace:

Old, low-efficiency heating systems (56-70 AFUE)

Mid-efficiency heating systems (80-83 AFUE)

High-efficiency heating systems (90 to 98.5 AFUE)

Of course if you want to take the guesswork out of the equation and get the most accurate reading of the efficiency rating of your furnace in your home, we can do that for you – contact us to learn more.

Current minimum efficiency requirements for warm air furnaces

Minimum requirements for furnace AFUE ratings are set by the federal government. Here are the current minimum furnace efficiency requirements for warm air furnaces, courtesy of the DOE:


Equipment TypeSize Category (Input)Subcategory or Rating ConditionMinimum Efficiency
Warm-air furnace, gas fired< 225,000 Btu/hMaximum capacity≥ 95.0% (U.S. North/Canada)
≥ 90.0% (U.S. South)
≥ 225,000 Btu/h*Maximum capacity80% Et
Warm-air furnace, oil fired< 225,000 Btu/hMaximum capacity≥ 85% AFUE
≥ 225,000 Btu/h*Maximum capacity81% Et
* Furnaces with input equal to or greater than 225,000 Btu/h are not covered by federal purchasing requirements. Minimum efficiency presented is consistent with ASRHAE 90.1-2013.

These minimum efficiency requirements are significantly higher than they were even 10-20 years ago – a big part of the reason why upgrading to a high efficiency propane- or oil-fired furnace is often one of the smartest moves you can make for your home comfort.

Want to know EXACTLY what the efficiency of your current furnace is? We can tell you! Contact us to schedule an appointment and we’ll measure the performance of your furnace in your home. We can then break down how much investing in a new high efficiency furnace could save you so you can make the best decision possible for your home heating system.