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How to Compare Furnaces

When buying or replacing a heating system consider two main components: the furnace, which burns the fuel, and the distribution system, which circulates the heat.

Within the infrastructure of most houses, warm air travels through a system of ducts. The heat runs through heat exchangers within the ducts. The ducts also confined the fuel and keep toxic gases out of living areas. Electric powered fans circulate the air and push it through vents in the floors or walls. A basement or below-ground furnace uses gravity to exchange air. Cold air stays low, goes into the vents, and circulates through the ducts. The warm furnace air rises back through the vents to maintain a comfortable temperature.

Less common, but still in use in some older buildings, is the hydronic heat distribution system. You may know it as "radiant" heat. Pipes in the walls or floors circulate hot water or other liquid. The relevant surfaces become warm, thus raising the internal temperature. In this case the "furnace" is a boiler built to heat water.

Residential Heating Options

  • Heat Pumps: These move air and exchange cold air for hot in winter and hot for cold in summer. They use warmth from the air, ground, or a water source. Heat exchange coils carry refrigerant, which evaporates in one coil and condenses into liquid form in the other. The process releases heat that is used to produce electricity.
  • Natural Gas: Currently the most popular type of furnace fuel in the US, natural gas burns clean, remains reasonably inexpensive (although this may change at any time), and is low maintenance. Gas burns in a combustion chamber and the resulting hot air circulates through the heat distribution system.
  • Oil: Long the most widely used furnace, oil furnaces are giving way to gas in many cases. Oil is not clean-burning, but still prevails where natural gas isn't available. Because the fuel is dirty, these furnaces require maintenance at least annually. Thanks to the advent of bio-degradable diesel fuels, their efficiency is improving.
  • Electric: This is mainly used in areas where electricity is low-cost. Environmentalists like the fact that it is clean but raise the concern that it is an over-burdened resource.
  • Wood Pellet or Corn: These sources are popular with those who want to rely less on fossil fuels. These units work in basically the same way as oil and gas furnaces, but they burn renewable resources instead. They heat efficiently and inexpensively.

Furnace Efficiency Ratings

There are two main rating systems for furnaces. Both offer essentially the same information indicating how well each one converts fuel to energy in the span of one year. The two ratings are the Annual Fuel Utilization Efficiency (AFUE) and the Seasonal Fuel Efficiency (SFE). To find the best system for you, work with your contractor or a heating expert and talk to other homeowners.

Some important considerations are:

  • Climate
  • Size and style of your home
  • Whether you're building a new home or replacing a furnace in an existing home
  • Fuel and distribution preferences, if you have them
  • Cost
  • Environmental concerns
Most furnaces measure their heat output in British thermal units (BTU), watts (W), or kilowatts (kW).

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