Central Heating and Cooling


Furnaces and



Rule of thumb: furnaces heat air, which is distributed through ducts. Boilers heat water, which is distributed via radiators, fin tube baseboards, radiant pipes or coils.


Today's gas-fired furnaces and boilers may be fueled by either natural gas or propane (with a few modifications to account for each fuel's differing properties). Oil-fired furnaces and boilers offer the possibility of using renewable biodiesel fuels blended into their heating oil. Electric furnaces and boilers are very efficient but electricity is expensive, making these units costly to run.


There are also furnaces and boilers designed as central heating units which burn biomass fuels, i.e., corn, pellets, or wood.

Heat Pumps

Heat pumps can provide a home with both heating and cooling.  By using electricity to move heat from a cool space into a warm, heat pumps make the cool space cooler and the warm space warmer.  As Energy Savers puts it, "during the heating season, heat pumps move heat from the cool outdoors into your warm house; during the cooling season, heat pumps move heat from your cool house into the warm outdoors. Because they move heat rather than generate heat, heat pumps can provide up to 4 times the amount of energy they consume."  Consumers who heat with electricity have found that they can save a lot of money by adding a heat pump to use their heated air more effectively.

There are several varieties of heat pumps on the market.  Air-source heat pumps, the most common type, have been used most efficiently in moderate to warm climates, but new models are claiming to work well in cold climates.   The ductless, "mini-split", popular in Europe and Asiai, is expensive, but small. easily installed and very flexible.  It can be used to heat and cool a new room addition, to provide ductless air conditioning and/or additional heat in a home with an unducted (i.e., boiler) system, or as part of a "zoned" heating and cooling system. The dual-fuel heat pump system combines a heat pump with a gas furnace, which adds more warmth in really cold weather.  Electicity powers the system in more moderate cold and in hot weather.  Geothermal heat pumps don't move the outside air temperature, but instead use a loop of underground piping to tap the relatively constant temperature of the earth, then feed it through a "ground heat exchanger" into a ducted, air source heat pump for distribution.  Such a system is expensive to install, but once in place is very efficient in all climates, easy to maintain, long-lasting and fuel-free.  An interesting system now in development, the absorption heat pump, or GAX, is discussed in "Go Deeper."


Central Air Conditioning

A central air conditioner is typically bought and installed at the same time as a forced air furnace, so for maxiumum efficiency at minimum cost, the unit is subject to the same sizing criteria as the furnace. In addition ductwork must be adequate for both heating and cooling functions. Air conditioners run on electric power, so the more efficient the unit (the higher the SEER rating), the lower the cost of using it.

Go Deeper ...
More About Heat Pumps
Reference Corner

The Seasonal Energy Efficiency Rating, or SEER, measures the efficiency of residential central air conditioning systems, and by extension, the cooling efficiency of heat pumps.

The Energy Efficiency Ratio (EER) measures cooling efficiency, or the ratio of total cooling capacity to electrical energy input. 

One Btu (British thermal unit) equals the amount of heat energy needed to raise the temperature of one pound of water by one degree Fahrenheit at a constant pressure on one atmosphere.   It describes the energy content (heat value) of fuels, as well as the power of heating and cooling systems.

Annual Fuel Utilization Efficiency (AFUE) measures how effectively a heating system turns its fuel into heat.  The higher the percentage, the more efficient the system.

The Heating Seasonal Performance Factor, or HSPF, measures the heating efficiency of heat pumps.

The Coefficient of Performance (COP) measures heating efficiency, or the ratio of total heating capacity to electrical energy input.

Winter Home Fuels
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