Types of Heating Systems
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A variety of technologies are available for heating your house. In addition to heat pumps, many homes use the following approaches:
- Furnaces and Boilers: This is by far the most common way to heat a home.
- Wood and Pellet-Fuel Heating: These systems provide a way to heat your home using biomass or waste sources.
- Electric Resistance Heating: This is among the most expensive ways to heat a home.
- Active Solar Heating: This system uses the sun to heat either air or liquid and can serve as a supplemental heat source.
- Radiant Heating: Radiant heating can draw on a number of energy sources, including electricity, boilers, solar energy, and wood and pellet-fuel heating.
- Small Space Heaters: These heaters are less efficient than central heating systems but can save energy when used appropriately. While forced-air heating systems rely on the same type of ducts used by heat pumps and air conditioners, water and steam heat systems use radiators that only deliver heat.
Furnaces and Boilers
Most U.S. homes are heated with either furnaces or boilers. Furnaces heat air and distribute the heated air through the house using ducts. Boilers heat water and provide either hot water or steam for heating. Steam is distributed via pipes to steam radiators, and hot water can be distributed via baseboard radiators or radiant floor systems or can heat air via a coil. Steam boilers operate at a higher temperature than hot water boilers and are inherently less efficient, but high-efficiency versions of all types of furnaces and boilers are currently available.
Understanding the Efficiency Rating of Furnaces and Boilers
A central furnace or boiler's efficiency is measured by annual fuel utilization efficiency (AFUE). The Federal Trade Commission requires new furnaces or boilers to display their AFUE so consumers can compare the heating efficiencies of various models. AFUE is a measure of how efficient the appliance is in converting the energy in its fuel to heat over the course of a typical year.
Specifically, AFUE is the ratio of annual heat output of the furnace or boiler compared to the total annual fossil fuel energy consumed by a furnace or boiler. An AFUE of 90% means that 90% of the energy in the fuel becomes heat for the home and the other 10% escapes up the chimney and elsewhere. AFUE doesn't include the heat losses of the duct system or piping, which can be as much as 35% of the energy for output of the furnace when ducts are located in the attic, garage, or other partially conditioned or unconditioned space.
An all-electric furnace or boiler has no flue loss through a chimney. The AFUE rating for an all-electric furnace or boiler is between 95% and 100%. The lower values are for units installed outdoors because they have greater jacket heat loss. However, despite their high efficiency, the higher cost of electricity in most parts of the country makes all-electric furnaces or boilers an uneconomic choice. If you are interested in electric heating, consider installing a heat pump system.
The minimum allowed AFUE rating for a non-condensing fossil-fueled, warm-air furnace is currently 78% for all types of furnaces except for those designed and manufactured specifically for use in mobile homes, for which the minimum AFUE is 75%. In the future, minimum AFUE requirements for furnaces will be based on the type of fuel consumed (i.e., gas, oil, or electricity), whether the unit is non-weatherized (i.e., intended for installation indoors) or weatherized (i.e., intended for installation outdoors), and whether the unit is designed specifically for use in a mobile home.