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AFUE:- How Furnace Efficiency Is Measured ?




When choosing a new heating system for your home, one of the most important considerations is how efficient the furnace or heat pump will be. The efficiency of a gas furnace is measured by the Annual Fuel Utilization Efficiency (AFUE), a rating that takes into consideration losses from pilot lights, start-up, and stopping. The minimum AFUE for most furnaces is now 78%, with efficiencies ranging up to 97% for furnaces with condensing heat exchangers. The AFUE does not consider the unit’s electricity use for fans and blowers, which can easily exceed $50 annually. An AFUE rating of 78% means that for every $1.00 worth of fuel used by the unit, approximately $.78 worth of usable heat is produced. The remaining $.22 worth of energy is lost as waste heat and exhaust up the flue. 


Efficiency is highest if the furnace operates for longer periods. Oversized units run intermittently and have reduced operating efficiencies.Furnaces with AFUEs of 78% to 87% include components such as electronic ignitions, efficient heat exchangers, better intake air controls, and induced draft blowers to exhaust combustion products. Models with efficiencies over 90%, commonly called condensing furnaces, include special secondary heat exchangers that actually cool flue gases until they partially condense, so that heat losses up the exhaust pipe are virtually eliminated.


A drain line must be connected to the flue to catch condensate. One advantage of the cooler exhaust gas is that the flue can be made of plastic pipe rather than metal and can be vented horizontally through a side wall. There are a variety of condensing furnaces available. Some rely primarily on the secondary heat exchanger to increase efficiency, while others, such as the pulse furnace, have revamped the entire combustion process.
A pulse furnace achieves efficiencies over 90% using a spark plug to explode gases, sending a shock wave out an exhaust tailpipe. The wave creates suction to draw in more gas through one-way flapper valves, and the process repeats. Once such a furnace warms up, the spark plug is not needed because the heat of combustion will ignite the next batch of gas. The biggest problem is noise, so make sure the furnace is supplied with a good muffler, and do not install the exhaust pipe where any noise will be annoying.


AFUE and selecting a furnace :-

Having a high-fuel efficiency furnace can means large energy savings. However, AFUE only measures the direct production of heat, and does not account for heat loss from other sources, such as from ducts and home insulation leaks. Therefore, although AFUE is an important factor in picking a furnace, it should not be the only factor. A high-efficiency furnace may save money with efficient heat, but that heating may not actually be effective. This is why you should have professional installers assist you in picking a furnace so that you get a model that will deliver you the combination of effective heating and efficient energy use that will work the best for you.

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