When you think of a furnace, your memories may drift back to long ago days at your grandparents’ house, kept cozy and warm by an old rattletrap furnace that dated back to the late Jurassic period. However, those old school gas furnaces have been supplanted by newer systems that are much more efficient when it comes to gas consumption and heat generation. The City of Calgary’s site reports that replacing an old furnace with a newer, more efficient one can cut heating bills by as much as 35 per cent.
Do you know how efficient your furnace is? To answer that question, you first have to know what AFUE means. AFUE (pronounced “a few”), or Annual Fuel Utilization Efficiency, is a benchmark for determining how efficient a furnace is at turning natural gas into usable heat energy. Every type of fuel, from firewood to uranium, has a certain amount of potential energy that it contains for a given amount—sort of like how you can predict how far you’ll be able to drive on a tank of gasoline. This measure is referred to as “specific energy.”
The fuel used by your furnace, natural gas, has a specific energy of 55.5 megajoules per kilogram. In order to measure furnace efficiency, the amount of specific energy contained in the natural gas burned by a furnace is compared to the amount of heat produced by the furnace—as measured by how much a certain volume of air is heated by the furnace. If a furnace burns one kilo of natural gas (containing 55.5 MJ of energy), and produces an amount of heat equal to 54.9 megajoules of energy, then that furnace has an AFUE of about 98.9 per cent. The other 1.1 per cent of the gas’s energy was wasted in one way or another.
So now, that takes us back to the question, “How efficient is your furnace?” According to the Utilities Consumer Advocate, there are three types of gas-driven furnace in Alberta, each with its own expected efficiency.
Standard Efficiency Furnace
This is Flintstones era furnace that your grandparents had. Featuring an always-on pilot light, a single heat exchanger (this is what transfers heat from the hot but nasty-to-breathe exhaust gases to the air in your home), passive heat circulation, and the use of indoor air as a source of oxygen, these waste a LOT of energy. Standard furnaces in good condition have AFUE ratings that range from 50 per cent to 79 per cent. While these are still quite common, you actually can’t purchase standard furnaces anymore.
Mid-Efficiency Furnaces
You can think of these as the Hondas or Toyotas of the furnace floor room. They’re nothing fancy, but they’re still a big step up from the dinosaurs of yesterday. Mid-efficiency furnaces typically have a single heat exchanger and draw air from indoors, just like their older brothers, but also feature electronic ignition, and powered vent fans. The electronic ignition means you aren’t constantly wasting a trickle of gas, while the fan cuts down on the heat that goes up the flue. Together, these features yield an improved efficiency of 80 to 85 percent AFUE.
High-Efficiency Furnaces
Now, this is the gooood stuff. These are the Ferraris of gas furnaces. Modern high efficiency furnaces differ from their more pedestrian brethren in one key way: they use two heat exchangers. After the hot gases produced from the burning natural gas passes through the main heat exchanger of the furnace, it is collected by a secondary exchanger, which retains these gases until water vapour condenses out of it, releasing additional heat. The water compensation is drained away, and the exhaust gases are diverted into a plastic pipe through internal walls (which absorb those last bits of heat), until they are exhausted out of the side of the home or building.
High-efficiency furnaces are designed to wring every last tiny bit of heat from natural gas combustion. As a result, they feature AFUE ratings from 90 percent to as high as 97 per cent. If you do the math, this means that high-efficiency furnaces can produce anywhere from 14 to 94 per cent more heat than a standard furnace with the same amount of gas. Obviously, this means you can save a LOT of money, even in a short amount of time.
Now, it’s time for you to take a look at how you’re heating your home and answer the question: How efficient is your furnace? You may discover that it’s time to start shopping around for a newer, more cost effective model.
