High efficiency furnace (C) Carson Dunlop Associates What is AFUE
Definition & Calculation of Annual Fuel Utilization Efficiency

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This article gives a definition of AFUE and explains AFUE (Annual Fuel Utilization Efficiency) or heating system efficiency ratings and numbers.

The articles at this website describe the basic components of a home heating system, how to find the rated heating capacity of an heating system by examining various data tags and components, how to recognize common heating system operating or safety defects, and how to save money on home heating costs.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

How is heating boiler efficiency or economy measured? What does boiler AFUE mean?

How is heating boiler efficiency or economy measured? What does boiler AFUE mean?

High efficiency furnace (C) Carson Dunlop AssociatesIn short, the AFUE (Annual Fuel Utilization Efficiency) tells you,

for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat.

Here's a very rough example: if the AFUE number for your heater is 80%, that means that for every dollar you spend on heat, twenty cents "goes up the chimney" or is lost in various other ways, and eighty cents of each heating dollar shows up in your occupied space as heat.

[Click to enlarge any image]

Each model of heating boiler or furnace is assigned an AFUE number. Our page top sketch of a high efficiency furnace is provided courtesy of Carson Dunlop Associates.

At left we illustrate a high efficiency gas furnace - expected to have a high AFUE number. Sketch courtesy of Carson Dunlop Associates.

Also see this sketch of a high efficiency condensing furnace and this high efficiency pulse combustion heating system.

AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better.

If your boiler or furnace or steam boiler has an AFUE rating of 90, that means that for every dollar you spend on fuel, 90 cents worth of heat is delivered into your building. The remaining 10 cents is lost in inefficiency such as heat that escapes up the chimney along with the products of combustion.

How is the Efficiency of a Gas or Oil Fired Boiler Actually Measured?

Audel guide to oil burner efficiency (C) Daniel Friedman - AudelThe combustion efficiency of an oil or gas fired heater is in effect a measurement of how effectively the heating appliance is converting the chemical energy of its fuel (oil or gas boilers or oil, gas or electric furnaces) into heat. Combustion air, burner adjustments, temperatures and other factors in the heater's design determine the actual efficiency of its burner.

The output of these heating appliances will be in the form of heated air (furnaces), heated water (hydronic or "hot water" boilers), or steam (steam boilers).

Details of the test procedure used to determine AFUE for a heating appliance are specified in the U.S. by ASHRAE Standard 103 cited below.

You'll notice in ASHRAE 103 that the standard writers took care to note that the "factory" AFUE cannot be assumed to be the actual AFUE for a specific installation in the field.

But efficiency is usually measured as well in the field at installed heating appliances as a final step in the annual cleaning and tune-up of the heater. In our opinion this is likelty to be a more accurate measurement than the factory specified AFUE as it is measuring the actual heater in-situ, even though those results cannot be extended to other installations whose site conditions will be different.

A technician actually determines the combustion efficiency of the heater by using instruments to measure both the operating temperature (measured in the exhaust gases in the flue) and the carbon dioxide level (CO2) in the exhaust gas exiting the heater in the breech - an area in the flue vent connector between the surface of the the heater and the draft regulator (if one is present there). The heater has to have been on or operating long enough to have reached full operating temperature or to be in "steady state" condition.

Oil burner smoke test equipment, Bachrach Kit (C) Daniel Friedman at OIL BURNER CO2 TEST you can see an example of this procedure for oil burner fired heating appliances.

Watch out: do not make heating appliance measurements before the heater has been on long enough to have reached steady state; that's often about 5 minutes, and it's easy enough to determine by simply monitoring the flue temperature.

When the flue temperature stops increasing the heater is at steady state. This is critical because combustion will be incomplete and thus inefficient when the heating appliance and its combustion chamber have not reached full operating temperature.

Given the CO2 measurement the technician simply looks up the corresponding fuel efficiency number (a percentage) in a table for the fuel being used (oil or gas).

Details of this procedure and charts describing key relationships like the important effects of both inlet water temperature and return water temperature on the efficiency of a condensing boiler, were given by Durkin (2006).

Below in our citations the Cold Climate Housing Research Center has published an excellent article reviewing AFUE and how it is obtained and used for buildings in a cold climate.

Research & Standards on AFUE Definition, Measurement Standards, & Heating System Efficiency Measurements

What is the Reliability & Accuracy of Heating System Efficiency Ratings - AFUE?

Alternative heating sources (C) Carson Dunlop AssociatesAFUE is not the whole story of heating cost efficiency. A high-efficiency heating system that has not been cleaned and serviced may be running poorly and wasting money.

In fact, an 85% AFUE heating boiler that has not been cleaned might be running at an efficiency much lower, perhaps 65%.

Furthermore, if your building is drafty or poorly insulated, you may be delivering heat at high efficiency but losing it from the building much faster than necessary.

Really? Well while AFUE is important in comparing two heating appliances that use the same fuel, the cost of yoru fuel and the general efficiency range of your fuel choice (oil, gas, propane, wood, electric) has a greater effect on othe actual annual cost to heat your building than the heater's AFUE rating, and

These articles can help with a more complete approach to saving money on heat:

Current AFUE Requirements for Heating Equipment in Canada & the U.S. ?

Oil burner moved up to fire through upper door of an old coal fired cast iron boiler on Grand Avenue in Poughkeepsie NY (C) DanieL Friedman

Typical AFUE Ratings for Heater Types & Various Fuels

Typical AFUE Percentages by Fuel, Heater Type

Furnace or Boiler Type
Typical AFUE Rating
No. 2 Heating Oil Pre-1970 cast-iron (boiler, probably with older type oil burner) 60% (4)
Retention head burner, probably high speed motor 70–78%
Mid efficiency 83–89%
Electricity Central or baseboard 100%
Geothermal heat pump see COP Coefficient of Performance
Air-source heat pump see HSPF Heating System Performance Factor
Natural Gas Standard efficiency 78–84%
Condensing 90–97%
Propane Standard efficiency 79–85%
Condensing 88–95%
Wood (firewood) Conventional 45–55%
Advanced 55–65%
State-of-the-Art 75–90%


  1. Adapted from Wikipedia, "Annual fuel utilization efficiency", retrieved 2017/05/23, original source: , Wikipedia provided background information about some AFUE topics discussed at this website provided. Because Wikipedia and other website entries can be amended in real time, we cite the retrieval date of such citations and we do not assert that the information found there is always authoritative.
  2. These Wikipedia general AFUE numbers do not distinguish between heat media (boiler vs furnace or water, steam, air) and do not reflect the particular AFUE that will be measured at a specific residential building's heating furnace or boiler.
  3. See HEATING COST FUEL & BTU COST TABLES where we include actual fuel costs per BTU or other heating units.
  4. The author [DF] has been able to raise the efficiency of a pre-1970 cast iron heating boiler from 60% up to 76% by installing a new high-speed oil burner and adjusting the burner mount location - photo shown here.

    We replaced an old 1725 rpm burner with a high speed Beckett 3450 rpm oil burner and we moved the oil burner up to fire through the upper door of a cast iron heating boiler that had originally been coal fired and that was installed in this Poughkeepsie NY home in 1900.

    Details are at OIL BURNERS
  5. A Guide to Residential Wood Heating [PDF] (2008 ed.), ISBN 978-0-660-19848-4, Canada Mortgage and Housing Corporation, ed. (2008) retrieved 2017/05/23, original source
  6. Residential Earth Energy Systems, Buyer's Guide [PDF], Canada Mortgage and Housing Corporation, ed. (2002) ISBN 0-662-30980-4 retrieved 2017/05/23, original source

Other Heating System Performance Measurements & Standards

Also see our definition of HSPF, SEER, and other measurements found at SEER RATINGS & OTHER DEFINITIONS [live linke just below] or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


Continue reading at APPLIANCE EFFICIENCY RATINGS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.






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