First, we have to answer a few questions that explain what efficiency is and how it’s measured. Additionally, we will look at how your heating system achieves its 80% or 90% efficiencies.

  • What Does Furnace Efficiency Mean? (AFUE)
    Ever heard of 90% or 80% efficiency on your heating system? Technically referred to as AFUE or Annual Fuel Utilization Efficiency, this is a measurement of how efficient the heating system is at utilizing the fossil fuel consumed. For instance, if the furnace is rated at 90%, there will be 10% waste. The best way to understand the real impact? With a 90% furnace, every dollar has .10 cents of waste and with an 80% furnace, every dollar spent to heat the home has about .20 cents of waste. Doesn’t seem like much does it?

    There are many components to each application of furnaces and heating systems and how they operate about the entire house. For instance, many homes in Boise and Meridian have furnaces located in the garage. Older homes in downtown Boise have a heating system located in the home or basement. Why does this matter?

    Let’s break this down and get technical. Hang with us…

  • 80% Versus 90%+ Operation
    An 80% (or less) efficient furnace has a single heat exchanger which adds heat to your home. That heat exchanger is responsible for capturing as much heat as possible; what it can’t capture is vented outside. This venting system is often 300 degrees or higher. Why? The heat that’s left inside the chambers, or, the heat that the blower can’t capture is exhausted outside. All that heat is going outside!

    With a 90%+ furnace, there are now 2 heat exchangers, a primary and a secondary. The secondary heat exchanger is responsible for capturing any additional heat that is left over from the primary. The vent temperature is now closer to 70-100 degrees. Almost all heat that you are paying for (bless the gas company) is now going straight into your home. See the pic below for a diagram that explains this operation:

  • Back to the topic of “application” and its relevancy
    Combustion requires 3 elements: source of ignition, fuel source, and oxygen. Oxygen is the most important part of proper and efficient combustion. “Stop, Drop, and Roll” really does work! When your heater is running, it requires oxygen to properly operate. If your furnace is 80% or less, it most likely has louvers on the front panels. These louvers in the door allow the furnace to pull air/oxygen into the combustion process. On a higher-rated efficient furnace, there are no louvers located in the front panels, but rather a direct vent that comes into a sealed combustion chamber. Lower furnace efficiencies have a negative impact not just on the environment (shameful plug for Al Gore) but more importantly on your home. When your furnace is running it produces a negative pressure inside your home causing the cold winter temperatures to enter your home through every nook and cranny. Higher efficient furnaces have a direct vent that brings this needed oxygen directly into the chambers instead of through the home. This isn’t as important to note if your heating system is located in the garage. But, most of our older Boise homes in the valley are set up with the furnace located inside the home somewhere. So keep an eye on this critical component of operation when considering a furnace replacement or upgrade.

    Issues like this will eliminate all your efficiency-increasing efforts:

  • ROI or, Return on Investment
    How long will it take to recoup my investment? This is a common question that we often hear from our customers. We could break this answer down in a way that supports investing the additional money to upgrade and we could break down the data to prove that the additional cost isn’t worth it. What we look at is the long-term impact of owning a higher-efficiency furnace. 90% of furnaces require additional maintenance and more frequently. Because 2 heat exchangers are condensing the gas down, there is a lot more condensation produced that needs to be drained. These drain lines often get clogged and restricted, causing the furnace to quit. And we all know that they break at the most opportune time, right? NOT!

    So, the year-to-year cost has an impact on our recommendations. Will better efficiency save you money? Certainly! But there are many factors to consider. The cost difference between 80% and 90% is approximately around $1200. The chart earlier in the article breaks down the savings, as it relates only to efficiency differences.
    If the furnace is located inside your home, as opposed to the garage, the savings are even greater! Our heating experts will be able to help navigate this seemingly daunting task!

  • What About Tax Credits?
    The federal tax credit used to be $1500 for upgrading. Once the feds realized they were running out of money, they drastically reduced the credit to a measly $300. It used to make sense to upgrade to better furnace efficiency because the $1500 credit paid the difference. But with the credit a lot less now, there have to be additional benefits elsewhere to justify the upgrade. We take care of the tax credit paperwork to provide your CPA, or whoever does your taxes. The heating system must be 95% efficiency or higher.


Higher efficiency furnaces will certainly save you money but look at the larger picture and application of your home to determine the best approach. It doesn’t always pay to purchase the better efficiency! Contact us to schedule a visit, we’ll help you navigate all your questions.


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