The Differences Among Furnace Efficiencies

The Differences Among Furnace Efficiencies
By Clint Kennon, CEO of Kennon Heating & Air Conditioning

Furnace efficiency is also known as AFUE, which stands for annual fuel utilization efficiency. This refers to the amount of fuel consumed by the furnace and changed into heat for your home. It is comparable to gas mileage for cars—the higher the AFUE, the higher the “gas mileage.” Many other variables can affect efficiency, such as leaking ductwork, gas pressure and elevation. Proper installation by a licensed contractor is so important—installing a furnace incorrectly can cause loss of efficiency.
The most common way efficiency is referenced in the HVAC industry is by percentage—the percentage of gas that is converted into heat and delivered to the living space. So, if you have an 80% furnace, then 80% of the gas is converted to heat; the rest is sent out the flue as waste. Electrical consumption among furnaces is low—comparable to a ceiling fan. Electrical consumption is not a factor for gas furnaces since most furnaces use about the same amount.
Typical home furnaces can be put into three main efficiency categories: 60%, 80% and 90%. The 90% group can include furnaces with 90%, 92%, 95% and 98% efficiencies. Federal regulations dictate the minimum efficiencies that can be installed in your area. Georgia heating and air conditioning companies are allowed to install a minimum efficiency of 80%, but your local heating and air conditioning company can tell you what is available for your region.
The 60% (the least efficient) group is known as natural draft furnaces, which are generally over 20 years old. Luckily, they are no longer manufactured. The 60% furnace is a dinosaur, and they’re quickly being replaced with higher-efficiency furnaces. These furnaces have a standing pilot light that constantly burns, using up gas every day of the year. This is a problem with the 60% furnace—not only do you have gas burning all of the time, you also worry about the pilot light going out. If you currently own one of these furnaces, you could greatly benefit from upgrading to a high-efficiency furnace. You could save as much as 37% on your gas consumption by replacing a 60% furnace!
The most common furnace in homes is the 80% furnace, a mid-efficiency furnace. This furnace can often be identified by the type of flue pipe and by the presence of an induced draft motor, which expels the exhaust fumes from the furnace. The 80% furnace has a metal flue pipe instead of a PVC flue pipe because the heat loss through the exhaust is about 20%, so the flue gets very hot to the touch.
The 90% plus is the last furnace group. They’re also called condensing furnaces because they expel water. They shouldn’t be confused with geothermal units or water source heat pumps, which are entirely different units. The water comes from the condensation of the gas fumes in the secondary heat exchanger. Unlike other furnaces, condensing furnaces have two heat exchangers. The first heat exchanger absorbs heat from the gas exhaust, and the exhaust goes through a second heat exchanger where it is cooled again. The hot gas exhaust cools down so much that condensate is released. The water is generally diverted to a pump or drain outside the home. These furnaces are the most efficient, and some only waste a few percentage points of heat. A correctly installed 95% furnace will turn 95% of the gas put into the furnace into heat and will waste only 5%, maximizing your heating energy savings.
With all of the variables and options today, the most important part of selecting a furnace is the care and attention that is put into the installation of the unit. To ensure you get the most out of a furnace, be sure to use a licensed and insured heating and air conditioning contractor. Consumers can check with the licensing board to verify a contractor’s license status. The heating and air contractor can show you a current certificate of liability to prove they are insured.

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