Furnaces, Boilers, and Heat Efficiency: A Guide to AFUE Ratings

energy guideThe measure of a boiler or central furnace’s efficiency is determined by what’s known as “Annual Fuel Utilization Efficiency”, or an AFUE for short. An AFUE, as the name implies, looks at how well boilers and furnace models use fuel to heat a home on a yearly basis.

Currently, manufacturers of either boilers or furnaces are required by the Federal Trade Commission to display their AFUE. This move towards transparency allows prospective buyers to know what they can expect in terms of heating efficiency as they seek to make informed purchasing decisions.

A Little More About AFUE Ratings

AFUE is a ratio that’s determined by weighing the total heat produced by the appliance against the total amount of fossil fuel energy it consumes each year. An AFUE rating of 90% tells an individual that approximately 90% of the fossil fuel energy used will be converted into heat for home while the remaining 10% is displaced or sent elsewhere.

Despite the convenience of efficiency being condensed into a single number, it’s worth noting that AFUE isn’t a percentage that factors in losses associated with pipes or the duct system, although these can account for up to 35% of the energy. This is especially true when ducts are situated in unconditioned areas such as garages and attics.

What About Electric Furnaces and Boilers?

A major strength associated with boilers or furnaces that run purely on electricity is that there are no heat losses via chimneys and the like. Typically, with all-electric appliances of this nature, the AFUE rating is in the range of 95% to 100%. At the lower end of this range are outdoor units which are associated with larger amounts of heat loss. Another concern is the expense of electricity in certain cities, which in turn makes all-electric boilers and furnaces impractical. Those who are intrigued by electric heating may be better-served with heat pump technology, or the new tankless hot water on demand systems.

What’s the Minimum AFUE Rating?

The lowest allowable AFUE rating allowed for a home furnace is 78% unless the purchase is for a mobile home in which case the threshold is 75%. It’s expected that in the future AFUE minimums for furnaces will be decided on the basis of a number of factors such as indoor or outdoor installation, fuel usage, and whether the furnace is designed for use in a mobile home.

In the case of a boiler, AFUE minimums depend on both the means of heating and the fuel used. For example, a gas-based water boiler the AFUE minimum is 82% while a water boiler that’s oil-fired has a number of 84%. Meanwhile, an oil-based steam boiler has a minimum AFUE rating of 82%. Further conditions include hot water boilers needing the ability to automatically adjust the water temperature while a gas-fired boiler isn’t allowed to burn constantly.

Condensing v Non-Condensing

The difference a condensing boiler or furnace makes is in the way that it takes water vapor and uses it to provide heat. AFUE ratings for this type of appliance are often significantly higher than other types. These furnaces and boilers are often more costly than their non-condensing counterparts, but they pay-off over time and are often more economical in places with harsh winters.

The efficiency of a system can also be discovered by examining a unit’s equipment features.

Older and less efficient:

  • Constant-burning pilot
  • AFUE of 56%-70%
  • Use of a natural draft with combustion gasses
  • Heat exchange

Mid-Efficiency Heating:

  • Ignition uses electricity
  • Flue pipes in smaller size
  • AFUE ratings of 80%-83%
  • A more precise exhaust fan has a greater degree of control over the combustion process
  • Smaller unit size and reduced weight leading to more efficient cycles

Higher-Efficiency Heating:

  • Combustion is a sealed process
  • AFUE ratings in the range of 90%-98.5%
  • A second heat exchanger allows the system to further condense flue gasses

For more information on this subject visit the appropriate government page on heat and energy for furnaces and boilers.