The Difference between High Efficiency (condensing) and Standard (non-condensing) furnaces

Furnaces2Standard efficiency gas furnaces are generally rated at or near 80% A.F.U.E. which stands for “Annual Fuel Utilization Efficiency”. This means that 80% of the heat produced through combustion is used to heat your home while the remaining 20% is lost with the combustion products through the flue. The “flue” is the vent that escorts the combustion products out of your home. Those combustion products include steam (water), carbon dioxide, carbon monoxide, and various other gases. The flues for the standard efficiency gas furnaces are typically one of two types. They are “Mason” or “B-Vent”. Mason flues are brick or masonry block chimneys that vent the combustion products to the roof. B-Vent flues are double-walled metal pipe that also vent the combustion products to the roof. Both “Mason” and “B-Vent” flues may be “common vented”. Which means they provide venting for more than one gas appliance. Typically, you will find them “common vented”.


Furnaces1High efficiency gas furnaces, also known as “Condensing Furnaces”, generally fall between 90% and 97% A.F.U.E. For example, if your furnace is rated at 95% A.F.U.E., then 95% of heat produced through the combustion process is used for heating your home, while only 5% is lost through the flue. High efficiency condensing gas furnaces differ from the standard efficiency models in several ways. The primary difference is the manner in which they are vented. Since the heat lost through the flue of a high efficiency condensing furnace is a lot less than the heat lost through the flue on a standard efficiency furnace, the steam is much cooler on the high efficiency condensing furnaces. So much cooler that the steam condenses into liquid water. This is a big problem for venting into masonry or b-vent flues. The solution is to vent the flue gases through water-tight, sealed pvc piping. This requires a dedicated pvc flue and does not allow for a “common vent” option. The pvc exhaust and intake (when necessary) are usually sidewall vented as shown in the picture above. The high efficiency gas furnaces also require a condensate drain to remove the water produced by the furnace.


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