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Efficient Heating and Cooling

Heating and cooling are key elements in most buildings, and the technologies or systems used to heat and cool them are numerous and varied. Different climates require different heating and cooling methods. Today, residential heating and cooling systems consist mainly of furnaces or boilers, air and ground heat pumps, air conditioners, and evaporative coolers. Larger commercial buildings use chillers, furnaces and boilers, cooling towers, and pumps for heating and cooling. Energy-efficient heating and cooling systems are important elements in whole building design.

Similar to other energy efficiency technologies, the cost of heating and cooling is widely variable, ranging from low-cost duct sealing projects to multimillion dollar investments for new heating and cooling equipment in large commercial buildings. In residential applications ensuring that ducts are installed correctly in new construction costs less than $250. This can reduce air leakage from 25% to less than 6%. A new air-conditioning system in a typical U.S. home costs $2,000-$4,000. Energy-efficient heating and cooling systems currently cost 10%-20% more than standard units, which adds $200-$800 to the cost of the unit. Many of these technologies, when used properly, pay for themselves in reduced utility bills.

The following section explains how primary heating and cooling technologies work. Energy-efficient heating and cooling of buildings can be aided with automated controls, ventilation, improved duct systems, and advanced technologies. Proper duct sealing for new and older construction is one of the most cost-effective energy efficiency measures available.

Furnaces and Boilers

Fossil-fueled residential furnaces and boilers that have the energy-efficient ENERGY STAR® label use condensing technology that condenses the water vapor produced in the combustion process and captures the heat released from this condensation. Efficient commercial boilers and furnaces maintain stack temperatures high enough to prevent condensation. To improve energy efficiency, commercial equipment uses heat exchangers or economizers to capture the waste exhaust heat and preheat an inlet stream.

Air Conditioners

An air conditioner works much like a refrigerator. Both evaporate a refrigerant such as Freon gas to provide cooling. The refrigerant gas is compressed or pumped through the outdoor condenser coil. The hot gas dissipates heat and gives up its heat to the air that flows over the coil and condenses the refrigerant back to a liquid. The refrigerant runs through an expansion valve. In the process, the liquid refrigerant evaporates in the indoor evaporator coil, pulling heat out of the indoor air and cooling the building. For more information, see A Consumer's Guide to Energy Efficiency and Renewable Energy—Air Conditioning.

The efficiency of central air conditioning units is governed by U.S. law and regulated by DOE. Every air-conditioning unit is assigned an efficiency rating known as its seasonal energy efficiency ratio (SEER), which is the total cooling output (in British thermal units) provided by the unit during its normal annual usage period divided by its total energy input (in watt-hours) during the same period.

Heat Pumps

An air source heat pump is a reversible air conditioner that pulls heat to the indoors from the outside air in the winter, and from the indoor air in the summer. A heat pump uses the same refrigeration cycle. The system consists of a compressor and two coils (one inside and one outside) surrounded by aluminum fins to aid heat transfer. Like a refrigerator, the coils contain a refrigerant that flows through the pipes. In the heating mode, the liquid refrigerant flows through the outside coils and extracts heat from the outside air and the compressor moves it inside as it evaporates into a gas. The indoor coils transfer heat from the refrigerant as it condenses back into a liquid. A reversing valve near the compressor can change the direction of refrigerant flow for cooling in the summer. Heat pumps work best in moderate climates. DOE has more information on heat pumps (PDF 188 KB). Download Adobe Reader.

Geothermal Heat Pumps (GHPs)

Geothermal heat pumps (GHPs) work on the same principle except they use the ground, which is nearly constant at 50°-60°F, as a heat source in winter. GHPs are relatively simple systems: A system of pipes is installed under the ground and into the building. A fluid circulates in the pipes via a pump, which provides a more efficient heat transfer process heat.

Chillers, Cooling Towers and Pumps

A chiller is a refrigeration system that cools water. Air conditioners cool the air directly, but a chiller uses the same refrigerating principles to cool water or other liquids. This chilled liquid can be used to cool a wide range of operations. In commercial-scale air-conditioning systems, this chilled water is pumped to coils in specific areas of the building. The air handling systems for each area regulate the water flow, keeping the air at a desired temperature. The refrigerant is condensed in a cooling tower, similar to the condensing coil on an air conditioner. The refrigerant cycle, the water cycle, and the air cycle se are large-scale fluid handling systems. Consequently, the optimization of pumps, motors, and valves contributes to the overall energy efficiency of the system.

Evaporative Cooling

In low-humidity areas, evaporating water into incoming air provides a natural and energy-efficient means of cooling. Evaporative coolers, also called swamp coolers, rely on this principle. Fresh outside air is pulled through moist pads where it is cooled by water evaporation and circulated through a building by a large blower. As this happens, the temperature of the air can be lowered as much as 30°F. For more information, see evaporative coolers.

Desiccant Cooling

A desiccant material naturally attracts moisture from gases and liquids. The material becomes saturated as moisture and is absorbed or collects on the surface. When heated, the desiccant dries out or regenerates-and can be used again. In a dehumidifier, the desiccant removes moisture from the air, which releases heat and raises the air temperature. Heat recovery units and cooling devices such as evaporative coolers or the cooling coils of a conventional air conditioner then cool the air. In a stand-alone desiccant cooling system, air is first dried, then cooled by a heat exchanger and a set of evaporative coolers. In most systems, a wheel that contains desiccants continuously dehumidifies outside air entering the cooling unit. The desiccant is then regenerated by thermal energy supplied by natural gas, waste heat, or the sun. A desiccant system can also supplement a conventional air-conditioning system. The desiccant removes the humidity load while the evaporator of the air conditioner lowers the air temperature. This can offset part of a building's air-conditioning load and may save money on utility bills. Desiccant cooling technology has the distinct advantage of being free of ozone-depleting chlorofluorocarbons and hydrochlorofluorocarbon refrigerants. Most desiccant cooling systems are intended for large applications such as supermarkets and warehouses. They are also ideal for humid climates.

Combined Heating and Power

Integrated systems for cooling, heating, and power systems, called combined heat and power, may increase energy efficiency by 70%-90% by using thermal energy from power generation equipment for cooling, heating and humidity control systems. Conventional systems require 65% more energy than the integrated systems. These systems are typically located at or near buildings that use power and space conditioning. See Distributed Energy for more information on combined heating and power.

Transpired Solar Collectors

Transpired solar collectors are made of dark, perforated metal, mounted as an exterior cladding on a building's south-facing wall. The sun heats the metal, and the ambient air is heated as a fan pulls it through holes in the metal and into the building's ventilation system. Solar energy absorbed by the facade and transferred to the air flowing through it can preheat the intake air by as much as 30°F. Reduced heating costs pay for the systems in 3-12 years.

Preheated ventilation air is a universal need in cold climates, and is a major energy user. Transpired air collectors are easily incorporated into standard commercial building ventilation systems. They pay for themselves quickly and have substantial environmental and economic benefits with no negative side effects. They substitute renewable energy for fossil fuel consumption. Transpired solar collectors are used at many locations, including apartment buildings, warehouses, large manufacturing plants, loading docks, distribution centers, and airplane maintenance hangars.

The net cost of transpired solar collector systems in new construction is about $6.00-$8.00 per ft², depending on the type of building material displaced by the collector and the extent to which the collector is integrated into the design of the building. Transpired solar collectors may be added to older buildings for about $10 per ft². The collector area depends on building size and fresh air requirements. Transpired collectors are 300 ft² to 100,000 ft².