A gas furnace is responsible for keeping your home warm. There are three main choices when looking for a new forced air furnace. Each of these types of gas furnaces work in different ways and have their own advantages. The most energy efficient systems are the modern condensing furnaces. So the best way to choose a particular furnace would be to base it on how efficient it can heat up your house.

Gas Furnace Efficiency

The efficiency of these furnaces is measured using the AFUE or the Annual Fuel Utilization Efficiency, which is also used to rate the efficiency of boilers and heaters. The AFUE rating is the ratio of the amount of fuel that is transformed into heat to the amount of fuel that goes into the furnace. In basic terms... the amount of heat you get for dollar you spend on heat! For example: An 80% AFUE Furnace will return .80 cents worth of heat for every dollar you spend on natural gas. A 95% AFUE furnace, will return .95 cents for each dollar, etc.

Types of Gas Furnaces

The three types of gas furnaces are the condensing furnace, induced draft furnace, and conventional warm air furnaces. These furnaces burn natural gas in order to produce heat. The heat is then transferred to the various parts of the house using a distribution system of ducts and fans. More air can be pulled from the cool air coming from the house to the furnace to be reheated or used to expel the flue gases through an exhaust.

The conventional warm air furnace uses natural draft, which is air pulled through the gap at the flue and at the front of the furnace. A better way of getting air into the furnace is by using a fan, which the induced draft furnace uses making it more efficient than the conventional warm air furnace. Some of the natural or induced draft is mixed with the exhaust gases and then expelled through a chimney. The problem with using a chimney is that it also expels heat along with the flue gases.

The condensing furnace is specifically designed to make use of the escaping heat by condensing the flue gases with an additional heat exchanger. This provides extra heat for the house making it more efficient than the other two types of furnaces with a 97 percent AUFE rating. Because this type of furnace condenses the escaping heated gases, it does not need a chimney. The flue gases have a relatively low temperature, which makes them easy to release outside the house through a small PVC plastic pipe.

The conventional warm air furnace is the least efficient among the three gas furnaces. Its efficiency drops way below the minimum AFUE rating of 78 percent as it ages. Old conventional warm air furnaces have only 55 percent AFUE rating. Another reason why a conventional warm air furnace is not as efficient as the induced draft furnace is because it uses a standing pilot. Both the induced draft furnace and condensed furnace use an electronic ignition system such as hot surface ignition and intermittent pilot that can be electronically controlled, and is cheaper, and more dependable. The induced draft furnace has 20 percent higher AFUE than the conventional warm air furnace.

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Filed under: Gas & Oil Furnaces

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