How do High Efficiency Furnaces Work?

Do you want to better understand how high efficiency furnaces work, and what the main differences are between high efficiency furnaces and traditional furnaces?

You’re in the right place!

In this proHVACinfo guide, you’ll learn:

  • What a high efficiency furnace is
  • What the advantages of using a high efficiency furnace are
  • How a high efficiency furnace works; and
  • The main differences between high efficiency furnaces and traditional furnaces
How do High Efficiency Furnaces Work

What is a High Efficiency Furnace?

The first thing to understand about efficiency when it comes to furnaces is a number called the annual fuel utilization efficiency rating (“AFUE”).

The AFUE tells you about the energy efficiency of your furnace. Typically expressed as a percent, the AFUE tells you how much of the energy going into the furnace is converted to actual heat used in your home.

For example, if a furnace has an AFUE rating of 90%, that would mean that 90% of the energy going into the unit is output as heat.

In 2015, the federal government established a minimum AFUE rating for residential furnaces of 80%, but if you have an older system, your AFUE could be down in the mid-sixties.

To qualify as a “high efficiency” furnace (and earn an EnergyStar certification), the AFUE must be at least 90% (in southern US states) or 95% (in northern US states). The most efficient furnaces have AFUEs up to 98.5%!

What are the Advantages of Using a High Efficiency Furnace?

Less Waste (and better for the environment)

Like we said above, the AFUE tells you how much input into your furnace is converted into useful output, with 80% being the federal minimum.

What about the other 20%? In short, that energy is wasted. High efficiency furnaces can reduce your furnace’s waste by nearly 20 percentage points!

Cheaper to Run

Of course, less energy waste means a smaller utility bill for you!

Let’s put the above example of an 80%-AFUE furnace in terms of dollars. For every $1 that appears on your utility bill due to your furnace, you’re basically throwing away 20 cents.

Add that up over a year (or over the life of your furnace) and you can see the financial benefit of a high efficiency furnace. If you have an older furnace (with a lower AFUE), the savings will be even higher!

Below is a table from that breaks down how a 98% furnace could save around $2,000 as compared to an 80% furnace.


Of course, if you’re considering switching from a traditional furnace to high-efficiency, you’ll need to factor in the upfront cost of the new furnace when determining whether a high-efficiency furnace makes sense for you financially.

Better comfort throughout your home

The newer technology in high-efficiency furnaces have features to help ensure a consistent temperature throughout day in the home, and include features like humidity control.

Longer useful life

If you have a traditional furnace, chances are you’re used to hearing the furnace turn on and off as the temperature in your home rises and falls.

That constant switching between on and off really puts a damper on the life of your furnace!

High-efficiency furnaces are designed to minimize that constant cycling between on and off, which maximizes the length of your furnace.

Minimizing the on / off cycling has the added benefit of reducing the noise coming from your furnace.

How do High Efficiency Furnaces work?

Broadly speaking, high efficiency furnaces work the same way traditional furnaces work.

That is, the furnace draws in cold air from the surrounding environment (known as “combustion air”), heats the air (with the help of the furnace burners) inside the combustion chamber, and circulates the air into the home with the blower.

The focus of this section will be how high efficiency furnaces differ from traditional furnaces.

In other words: what makes high efficiency furnaces “high efficiency”?

Second Heat Exchanger

Remember above, where we said that in traditional furnaces, 20% – 35% of the energy going into your furnace is wasted?

Most of that waste happens in the heat exchanger, where heat from the burners is transferred to the air before being circulated throughout your home.

With a single heat exchanger, much of the heat that could be transferred to air and used inside your home is vented outside as exhaust gas.

Many high efficiency furnaces have a second heat exchanger. The heat from the exhaust goes through the process a second time. This has the effect of capturing much of the remaining heat for use in your home.

Sealed Combustion Chambers

Traditional furnaces draw in cold air from the surrounding environment into the combustion chamber for it to be heated and circulated throughout your home.

Because these furnaces are simply taking air from the immediate environment, there’s plenty of opportunity for heat loss as the burners work.

Many high efficiency furnaces have a sealed combustion chamber. That is, they don’t get their air from simply taking in cold air around the furnace. Instead, they draw air directly from a pipe that leads to the house exterior.

This more targeted air suction reduces the possibility for heat loss.

Multi-stage burners

In traditional furnaces, the burners only have one setting.

That is to say, the burners are either on (and running at 100% capacity) or they’re off.

That means there’s a lot of potential for waste when you don’t really need the burners running at 100% capacity.

High-efficiency burners have burners that can run at a range of settings, so you don’t run the same risk of waste.

proHVACinfo | Did You Know (Burners)

Variable Speed Blowers

Similar to the burners, most traditional furnaces have blowers that run at a single speed, which creates another opportunity for waste in the furnace.

High efficiency furnaces can run the blowers at different speed levels, depending on how much heat is being called for by the thermostat, ensuring you’re never using more energy than what’s needed.

Final thoughts on High Efficiency Furnaces

High efficiency furnaces are furnaces with an AFUE rating of 90% or above.

They can offer several advantages over traditional furnaces, including lower utility bills, cutting down on waste, and providing a quieter, more comfortable environment in your home (which can itself make the investment worth it!).

Of course, you’ll have to consider your personal needs and budget, as high efficiency furnaces do tend to carry a higher price tag than traditional models.

However, if you’re in the market for a new furnace, we strongly recommend you consider a high-efficiency option!

Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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Stephen Marks

Stephen Marks

Stephen is an HVAC and home-repair enthusiast. He's here to answer any of your questions about HVAC!

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