furnace energy

Understanding Furnace Energy and AFUE Ratings

Furnaces are firm favorites among Americans for heating their homes. They do this by heating air, and then sending it in ducts around the house. Decades ago, folk were not overly concerned about their energy efficiency. Nowadays we know wasting fuel is contributing to global warming. Hence interest in AFUE ratings is rising.

Steady On: I Thought Furnaces Had Flues

Yes, they do have chimneys and vents, and we do lose heat through them when it escapes outside. However, an AFUE is something different. The four letters are an abbreviation for Annual Fuel Utilization Efficiency. This is an input-output ratio in a similar vein to miles-per-gallon, except that’s the other way around.

Technicians calculate it by doing tests on furnaces. Then, because demand varies according to the season, they smooth it over an entire year. Hence, the AFUE of a boiler is a measure of the furnace’s efficiency in converting fuel to energy, projected for an entire year. In a word, the higher the AFUE the better it gets.

A Worked Example of What an AFUE Means

Imagine for a moment we are shopping for a furnace and the salesperson shows us one with an 85% AFUE. This means 85% of the fuel that goes in becomes heat that warms our homes. The remaining 15% go up the chimney or does not materialize. Note this does not include heat leaking from the ducting. We are measuring only the furnace itself.

So What’s Better, Electricity or Gas Fuel

In engineering terms, electricity is the winner because there is no chimney for the heat to escape. In fact, most all-electric furnace AFUE’s are between 95 and 100%. However, there is a catch. Electricity is becoming expensive in San Diego, and much of it comes from power stations that pollute. If you want electric heating, rather consider a heat pump.

How Do I Know If My Furnace is Energy-Efficient

If you have an old furnace, your AFUE is likely in the 55 to 75% range. Unless it is electric, this means you are pumping carbon into the atmosphere besides wasting your money! Here is a short summary of your choices when it comes to gas furnaces:

  • Live with your current low energy system with an AFUE in the 55 to 75% range. This likely has a pilot light that stays on, has a heavy heat exchanger, and uses natural airflow to ‘stoke the fire’.
  • Retrofit a mid-range AFUE furnace with electronic ignition, and a score of 80 to 83%. The greater efficiency is thanks to an exhaust fan, a smaller flue pipe, and a more compact design.
  • Cut across to state of the art heating efficiency enjoying a 90 to 98% AFUE rating. This comes courtesy of a sealed combustion chamber, plus a second heat exchanger that harvests flue gases to create more heat.

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