AFUE furnace efficiency is a measure of how efficient your home’s furnace is at heating your home. AFUE is an acronym for annual fuel utilization efficiency. Newer furnaces have advanced technology components that enable very high AFUE ratings.
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How AFUE is Calculated
AFUE is a ratio of the annual furnace output heat energy to the annual input of fuel energy. The current procedure for determining AFUE ratings takes into account more than the older AFUE calculation. For example, any non-heating season furnace pilot input loss goes into the calculation. This is one of the reasons why furnace manufacturers went away from the standing pilot light. The pilot light consumed fuel every hour of the day but did not contribute any to heating the home. In the case of natural gas furnaces, the AFUE rating does not include the electricity usage to operate furnace components.
In addition, the modern way of determining the AFUE rating of a furnace incorporates more real world usage. The AFUE rating represents the actual season long average efficiency of the furnace. This includes operating transients. And it also takes into account the lower furnace efficiency associated with cold startup.
AFUE Listed on Yellow Energy Guide Label
Furnaces and certain other appliances are required to have an Energy Guide label from the factory. The yellow label identifies the AFUE rating of the furnace. A template of such an Energy Guide label is shown below. The XX.X shown in the box is where the AFUE rating would go. For example, one might show 95.0 for the efficiency rating. The Federal Trade Commission is charged with regulating these Energy Guide labels on new furnaces and other select HVAC equipment. Other select appliances also fall under this Energy Guide labeling requirement.
AFUE vs True Thermal Efficiency of Furnace
A different measure of furnace efficiency is the true thermal efficiency of a furnace. This would be a measure of furnace efficiency at a specific moment in time. For example, assume an AFUE rating of 95% (this is the year long average efficiency). On initial cold furnace startup, the true thermal efficiency would be lower than the 95% AFUE rating. Then once the furnace has been operating a few minutes the true thermal efficiency would increase. Once the furnace is warmed up, the true thermal efficiency would be higher than the 95% AFUE rating.
Homeowners would find the AFUE rating more meaningful as it allows them to compare a new furnace to their old furnace. Or to compare the efficiencies of two different furnaces. A new 95% AFUE furnace is 15% more efficient than a new 80% AFUE furnace, for example.
There are Still Useful Applications for New 80% Furnaces
There are, by the way, useful applications for 80% furnaces today. Installing a new 95% AFUE furnace in a home with an older 80% furnace requires vent piping. PVC combustion air piping connecting the 95% furnace with the outside air is required. In some applications, there is no convenient way to run these PVC pipes to the outside. In these cases, a new 80% furnace may be a convenient option.
80% Furnaces Have Round Metal Flue Pipes
A photo of an 80% furnace in a residential application is shown below. The large round metal piping carries the hot flue gases out of the furnace.
The metal flue normally passes up through the roof of the home and out a metal vent cap. A similar metal vent cap is shown in the photo below.
90% And Higher Efficiency Furnaces Have PVC Pipes For Flue and Combustion Air
The photo below shows the two white PVC pipes connected to the top of the furnace. These PVC pipes carry combustion gases between the furnace and the outside of the home. These PVC pipes are required for high efficiency furnaces.
PVC Pipe Terminations For Furnace Flue and Combustion Air Supply On Outside of House
The two PVC pipes run to the outside of the home or structure as shown in the photo below.
ASHRAE Approved Methods for Determining AFUE Ratings
A governing body with the acronym ASHRAE has developed the approved method for determining the AFUE ratings. There is an established procedure that laboratories follow to obtain AFUE ratings on new furnaces. The ASHRAE standard for AFUE determination is Standard 103. ASHRAE stands for the American Society of Heating, Refrigerating and Air-Conditioning Engineers. ASHRAE is an American professional association seeking to advance heating, ventilation, air conditioning and refrigeration systems design and construction. One can visit the ASHRAE website for more information on their organization.
The ASHRAE standard is designed to follow the DOE codes for furnace efficiency. DOE is an acronym for the U.S. Department of Energy. Further information on the DOE codes can be referenced on the DOE website.
The Way AFUE Was Previously Calculated To Measure Furnace Efficiency
The way AFUE was previously calculated resulted in an over estimate of furnace efficiency. An older 80% AFUE rated furnace has an AFUE rating of up to 70%. And in most cases, they were quite a bit less efficient that that. They were certainly not 80% efficient.
What this means is that for a 70% efficient older furnace, 70% of the fuel’s energy goes to heating the home. The other 30% goes right out the furnace flue. Newer furnace designs are commonly in the 95% AFUE range, with some models even more efficient. For furnaces in the northern US, an Energy Star Rating requires the furnace to be at least 95% efficient. Visit the Energy Star website for more information on these EPA qualified furnaces and appliances.
Why Newer Furnaces Achieve Very High Efficiencies
Newer furnace technologies and designs have allowed these very high furnace efficiencies. The following features make newer furnaces more efficient.
Electronic Ignition Systems
Electronic Ignition Systems: Furnace designers eliminated the older design for using a standing pilot light to ignite the furnace burners. The standing pilot was replaced with an electronic ignition system. The electronic circuitry lights the burners with either a direct spark igniter or a hot surface igniter. There is a noticeable efficiency gain with not having that pilot light run all the time.
The image below is of a standing furnace pilot light.
Sealed Furnace Combustion Chamber
Sealed Furnace Combustion Chambers: If your home has a 20 year old furnace, it likely pulls room air to support combustion. Air from your home, which you have already paid to heat with the furnace, is used for combustion. This air then goes right out the furnace flue. The home is then at a slightly lower pressure than the outside. So outside air is pulled inside through any cracks or crevices around window frames and doors. This makes the house seem drafty as well. Newer high efficiency furnace designs pull outside air through PVC pipes to the furnace. The combustion gasses are also exhausted through PVC pipes outside. This design improves overall efficiency and makes the home more comfortable. There are less temperature fluctuations in the home. This design is also safer for home occupants.
The image below (which we have seen earlier) also shows the sealed combustion chamber in this older 90% furnace. Looking closely, the rectangular metal box near the center of the picture shows the sealed combustion chamber. There is a round observation window with a black rubber gasket surrounding the observation window.
Secondary Furnace Heat Exchanger
Secondary Heat Exchanger and Inducers: Newer high efficiency furnaces have a secondary heat exchanger that pulls that last bit of heat out of the furnace. This last bit of heat is then used to heat the home instead of going out the furnace flue. An inducer (combustion air blower) is used to help raise the efficiency of the furnace.
Advanced Furnace Heat Exchanger Design
Advanced Heat Exchanger Design: The burner flame is directed into the furnace heat exchanger. Hot combustion gasses pass through the internal passages of the heat exchanger. The furnace blower directs air flow over the hot heat exchanger thereby warming the air. This warm air is then directed throughout the home via ventilation ducting. The advanced heat exchanger design uses exotic metal designs that transfer heat better. In this way, more of the heat from combustion goes into heating your home. Less of the heat from combustion goes out the flue.
Two Stage Furnace Design
Two-Stage Furnace Designs: Older furnaces have single stage burner designs. The blower in the furnace also operated at a single speed for heating. Early in the heating season, the heat load is lower. The furnace kicks on with its full heating capacity however. The furnace likely doesn’t run that long to satisfy the thermostat call for heating. This leads to some temperature swings in the home. Newer two stage furnaces have a lower heat capacity available. This lower stage runs when heat load is lower. A more uniform home temperature profile is the result. And the furnace efficiency improves as well. The second stage is available for the colder periods.
Variable Speed Furnace Blower
Variable Speed Blower: The use of variable speed blower fans in the furnace allows a more comfortable home. Air flow is lower during first stage heating. And a higher air speed is used when the second stage heat is called for. The variable speed blower creates a more uniform temperature range in the home. Variable speed blowers also move air to upstairs rooms better than standard blowers do.
Programmable Thermostats: While not directly associated with the new higher efficiency furnaces, a programmable thermostat raises efficiency. These thermostats have programs that change the temperature setting. When people are away from the home the set point is adjusted to use less energy. Programmable thermostats can also improve home comfort levels. The thermostat can be set to adjust the home to a more comfortable temperature just before occupants get home or wake up.
When upgrade to a higher efficiency furnace, we will advise you on the best furnace for your home application. Call Accurate Heating & Cooling today at 402-238-2425 to schedule an appointment. We professionally install residential and light commercial heating equipment in the greater Omaha metro area. We have an A+ rating with the Better Business Bureau. Check out our Testimonial page to see what our customers say about us! In this blog, we will describe How Air Conditioners Dehumidify.
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