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High Efficiency Furnace - What is your AFUE?


Trane S9V2 96% high efficiency furnace with ultra-efficient Vortica™ II variable speed blower for consistent flow of warm, comfortable air with quiet operation.

No matter how well you have maintained your furnace with annual tune ups and clean air filters, every furnace has a life expectancy. When it is time to replace, it is helpful to have a basic understanding of what a forced air high efficiency furnace and how Department of Energy regulations could influence your furnace choices.

So first, what is a high efficiency furnace? Like almost everything else in our world, furnaces have improved over the years. The furnace you may have bought in the 1970s, or even 1990s, is a different furnace then what you are likely to buy today. One of the main differences is that furnace efficiency has increased greatly over the years, thanks to the development of the condensing furnace. As mentioned in a prior blog post, furnace efficiency is rated by AFUE – Annual Fuel Use Efficiency. A 100% AFUE furnace would convert all the BTUs in the gas fuel into heat output with no waste. For example, a 90% AFUE furnace puts out 90 BTUs of heat for every 100 BTUs of gas consumed. Any furnace that is at least 89% AFUE is considered high efficiency. With such a furnace, you get more heat output from the fuel you consume which saves you money. A conventional 80% efficiency furnace (also called a non-condensing furnace) has been pretty much the standard in the industry since the 1980s.

Why should you even care what a condensing furnace is? The reason is because the Department of Energy sets standards for minimum furnace efficiency, starting back in 1975. These standards are updated periodically. In 1987, the National Appliance Energy Conservation Act set a minimum of 78% Annual Fuel Utilization Efficiency (AFUE), which was increased to 80% AFUE in 2007, effective 2015. (This 2% increase was a bit odd given that there were virtually no furnaces on the market below 80% anyway!) In September 2016, the DOE proposed a new standard that will require a minimum 92% AFUE efficiency for all except small furnaces less than 55,000 BTUs. This would become law 5 years after the rule is finalized. It is interesting to note that the setting of standards has been fraught with argument and lawsuits over fairness to different climates, disagreements over the validity of the process used to decide the standards, and concerns that replacement furnace costs will be difficult for some to afford. It is unclear if or when the controversial 92% AFUE rule will be accepted.

What technology made the improved efficiency possible? Basically, the addition of a secondary heat exchanger is the key. Recall how a conventional non-condensing furnace works: Basically, the fuel combusts inside the heat exchanger, heating up the metal coils or loops inside the heat exchanger. The fan, or blower motor, blows air across the heat exchanger, which heats (most of the water vapor is burned off) that are the by products of combustion are contained and vented out of the house through metal flues. Wouldn’t it be great if the heat in those vented gases could be extracted and used to provide additional heat to the residence instead of just ending up outside?

That’s exactly what the condensing furnace does – it uses the latent heat in the very hot exhaust gases to heat a secondary heat exchanger before venting those gases. Air warmed by the primary heat exchanger iis further warmed as it passes over the secondary heat exchanger. As the gases cool in the secondary heat exchanger, the water vapor produced by the combustion process (basically, hydrogen atoms in the fuel are released to combine with oxygen in the air, creating water) condenses to liquid water as it cools. The water created in the secondary heat exchanger drains into a plastic condensate collector box, and then into a trap and out into a house plumbing system. The cooled gases are vented outside through a PVC pipe. Note that a PVC pipe is used instead of metal both because the gases are cool, and because the gases they carry are corrosive (which is also why the collector box is plastic). There is still some water vapor in the gases that will condense as the gases continue to cool, and this water from the vent pipe must also be drained, either into the collector box or directly into the trap.

It is possible that your next furnace will be a high efficiency furnace, either because you choose a more energy efficient model, or because government regulations mandate such. The installation requires a few more steps than that of a conventional furnace, so it is best to use a licensed HVAC installer who pulls a permit. Beware of anyone who promises to “install to code” without pulling a permit. Because code is always changing, installers who don’t pull permits can easily become out of step with those changes. Differences between installing a standard efficiency vs. high efficiency furnace include changes to your existing venting, and installation of a condensate line from the furnace condensate trap into the plumbing. A new maintenance issue will be keeping the condensate drain system free of clogs, including all internal and external hoses and condensate trap. Because a clogged drain line could cause the furnace to shut down, it is important to have annual service performed on these furnaces.

Please call KJ Thomas Mechanical to talk with Karl about furnace options if you are considering a new furnace. Estimates are always free. And if you haven’t had your furnace maintenance done this year, there’s still time to do that. Call us at 303-435-8141 or contact us.

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Expert Service from People You Can Trust
6175 Trevarton Dr. Longmont, CO 80503
Not Open to Walk-in Traffic
PHONE:   303-435-8141
HOURS:   Monday - Friday: 8AM to 5PM
Expert Service from People You Can Trust
6175 Trevarton Dr. Longmont, CO 80503
Not Open to Walk-in Traffic
HOURS:   Monday - Friday: 8AM to 5PM
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