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Furnace companies everywhere try to sell furnaces saying it is 80% efficient, it is 96% efficient, or it has an Annual Fuel Utilization Efficiency (AFUE) rating of 83%, but what does all this really mean?
The percentage rating is actually the amount of fuel your furnace turns into heat; the remaining percentage (20% on an 80% AFUE furnace) is leaked out the flu and converted into carbon dioxide or greenhouse gas which is emitted into the atmosphere.
All new furnaces come equipped with an Energy Guide Sticker reporting their rating and are within the range of 78% to 99% efficient. The higher the rating the better for our environment and the less fuel your furnace is wasting.
Purchasing a high efficiency furnace compared to a mid efficiency can cost you a lot more in up front costs. It is generally more difficult to install in an older home, and the actual price of the unit is higher than that of a mid-efficiency. These factors generally make it tempting to purchase a lower efficiency. However, there are a number of other factors to keep in mind before making that decision.
A higher efficient furnace wastes a lot less fuel, and fuel costs are rising rapidly. So what you spend in up front costs will be repaid to you in your monthly consumption charges over time. Also there are many incentive programs offered by the government, to buy a high efficiency gas furnace.
So, what makes a high-efficiency furnace so much more expensive yet so much more efficient?
Traditional furnaces have a pilot light which runs all the time, burning fuel even when the furnace is not on. High efficiency furnaces generally use some type of ignition system which allows the pilot to be lit only when needed.
High efficiency models use a second heat exchanger, where a traditional furnace only uses one. How the heat exchanger works is that air is pulled into the furnace through your duct-work and blown through the heat exchanger where it is heated by a burner and blown back into the duct-work. By having a second heat exchanger any air which is not heated the first time will be caught the second time and less air becomes wasted.
Combustion or the mixing of air with gas. High efficiency models have better combustion and better control on how the air and gas are mixed.
Exhaust gases from a high efficiency furnace are condensed into liquid which is cool enough that it can be vented though a plastic PVC pipe to any outside wall, then drained, eliminating the need for a chimney. With a traditional furnace the wasted gas is simply blown out into the air through a chimney. This is where the term condensing unit comes from.
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