When it comes time to replace your home’s furnace, you have essentially two options when it comes to efficiency – “standard” or 80% efficiency and “high efficiency” of 95% or more. The energy savings sounds tempting, but is replacing your current standard furnace with a high-efficiency unit the best choice? We’ll explain the differences and answer that question here.
As in all HVAC installations, every situation is different. The “high efficiency” unit may not be the ideal equipment for your home – and it depends on your goals for saving energy dollars, the location of your furnace, and other attributes of your house.
What Are the Differences Between 80% and 95% Furnaces?
The standard used to measure furnace efficiency is Annual Fuel Utilization Efficiency (AFUE). A furnace’s AFUE rating is the percentage of fuel it can convert into usable heat, ranging from around 30 percent to 100. As an example, a furnace with an AFUE rating of 85 means that 85% of its fuel is converted to usable energy to heat the home. The remaining 85% is expended through the exhaust.
At 80% AFUE, a standard furnace loses 20% of its energy through exhaust. High efficiency furnaces, or more than 90% AFUE, simply use that energy more efficiently. That’s suggests a high-efficiency furnace would be the right choice every time. Not so fast, my friend!
What’s the issue? An 80% standard furnace draws air from within the house to generate heat from natural gas, also known as “open combustion.” On the other hand, a 95% high-efficiency furnace uses “sealed combustion” which draws air in from the outdoors, usually through a dedicated PVC pipe.
What difference does this make? Why does it even matter where the air comes from?
Cost + Furnace Location
Replacing a standard efficiency furnace that’s installed in a crawl space or vented attic probably won’t result in much of an advantage if installing a 95% unit. The standard furnace needs air for its combustion; in a vented space, the combustion air is already available. Installing a 95% furnace will require creating a source for the air to get to the furnace. Installation will be more expensive, and energy savings will probably not offset that installation cost.
Safety + Carbon Monoxide
A furnace produces carbon monoxide as a by-product of combustion. When installed in a vented area (like a basement), it doesn’t cause much of a safety risk, when installed and maintained properly. If your furnace is located in a sealed attic or an enclosed crawl space, open combustion has a great deal of potential to cause safety concerns. In this situation, you should install a 95% furnace with a concentric air vent that provides intake and exhaust functions. In an unvented environment, a high-efficiency furnace is definitely the safer choice.
Location of your furnace is so important. It’s impossible to ignore the safety risk of an 80% open combustion furnace in a sealed environment, and equally difficult to justify the additional expense of a 95% furnace in a vented space.
Furnace Efficiency is Influenced by Other Factors as Well
Besides AFUE, here are two other factors that can influence the choice of a new furnace. It’s important to understand:
Stages of Heat – One stage, two stage or modulating
Airflow Speed – On/Off or Variable Speed
- Single-stage furnaces are binary: completely on or completely off. When a single-state furnace runs, it blows hot air at its full capacity.
- Two-stage furnaces have two different speeds: “slow” or about 65% of the furnace capacity, and full blast.
The slow speed is usually sufficient for heating the house most of the time, doesn’t need as much energy, and is much quieter than a single-stage furnace running at full bore.
However, when the temperature drops quite a bit, the second stage kicks in to run the furnace at full blast to keep the home at the desired temperature.
Again, what difference does this make? Well, since both 80% and 95% furnaces can be purchased as single-stage or two-stage units, it turns out that an 80%, two-stage furnace actually runs more efficiently than a single-stage, 95% furnace.
Some higher-priced furnaces include a fully modulating gas valve. It works similarly to a two-stage unit; but instead of just two stages, it has multiple speeds to exactly match the home’s heating needs at any given instant.
Another feature that gives a great advantage is variable speed airflow. Units with variable speed airflow better control the amount of hot air that enters the house at any time, versus a standard unit with only “full blast” mode. This offers much more efficiency than a standard single-stage furnace.
So, depending on your situation, AFUE might be the least important factor for selecting the right furnace for your home.
What’s the Bottom Line for a New Furnace?
Our decades of experience tell us that every single furnace situation is unique, and doesn’t always fits the convenient guidelines in this article. Here are our bottom line suggestions for the best furnace choice for your home.
Existing furnace is 80% in a vented space
Stick with the standard 80% efficiency unit. If you decide you want to spend a little more and gain energy efficiency, a two-stage unit with variable airflow will save over a 95% furnace.
Existing furnace in an enclosed space
A high-efficiency 95% furnace makes the most sense here, for both safety and efficiency reasons. You can add a two-stage or fully modulating unit with variable airflow to be even more efficient.
Your current furnace exhausts into a chimney
Installing a high-efficiency 95% furnace with its own exhaust vent is a better solution, considering the chimney liner can get compromised over time. You’ll want to avoid this situation that could allow CO exhaust gases to escape into the house. If you do decide to go with an 80% model, do yourself a favor and install a new chimney liner for safety.
Your main concern is energy efficiency
If you have the budget, a 95% furnace with variable airflow and fully modulation will give you the highest energy efficiency. Your return on investment won’t be the highest, and it will take longer to recover your cost with lower energy costs, but you will be saving energy.
No matter what unit you decide to install to replace your current furnace, you’ll probably improve energy efficiency. With the technology gains we’ve seen in the past few years, unless you’re replacing an existing 80% single-stage furnace with an identical unit, you’ll save energy and have a more efficient unit.
So, it’s not necessary to go for the highest-priced, highest-rated AFUE furnace to get the optimal energy savings for your home. We suggest discussing your furnace replacement with one of our trained and certified HVAC consultants to help you determine the perfect match for your budget and energy saving needs.
How We Can Help
The Dor-Mar team offers top-rated replacement heating and cooling equipment from Rheem & Coleman, two of the top HVAC manufacturers in the USA. And, as a “Top Team Contractor” we offer a wide array of routine maintenance services for your heating and cooling system, allowing your family to breathe fresh, safe, clean air year-round, and save energy in the process.
Schedule an appointment for a free, in-home estimate for a new heating system by using our online contact form, or call one of our seven neighborhood offices listed below. We pride ourselves on our customer-focused service, and our reviews show it.
Dor-Mar…Your Climate Hero!