Q&A Steady state, Thermal and A.F.U.E. efficiency ratings

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New Member
Staff member
Nov 27, 2012

Would you please define the three most common efficiency rating systems Steady State, Thermal Efficiency and A.F.U.E. In your opinion which rating system gives the consumer the most realistic information for comparison purposes. I am sure you have seen Steady State ratings of 84% with an A.F.U.E. rating of 70% on some models with another model claiming a Thermal efficiency of only 75% but having an A.F.U.E. rating of 73%. Also is their a difference in the rating systems between Canadian and U.S. manufacturers.


OK, lets' talk about a gas furnace for example:

Steady State: You started up the furnace, brought it up to proper operating temperature and took a measurement of the efficiency. This is often a few points higher than the AFUE, since it does not take standby (when furnace is off) efficiencies into account.

AFUE: Annual Fuel Utilization Efficiency: This takes the standby heat loss (heated air going up the chimney when furnace is off) and other factors into account. This is a more accurate measure of figuring yearly costs and efficiencies.

Thermal: Really means nothing. Efficiency is often defined as thermal efficiency times the heat transfer efficiency. A bonfire might have a high thermal efficiency, but is terrible at transferring heat into your home. Most modern wood and gas stoves have very high thermal efficiencies. So, in the case of the furnace, this efficiency is a measure of how much of the "fuel" in the gas is consumed...with no thought as to how much goes into your home.

This is not a scientific explaination, but is my understanding of the terms.
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