Heating, Ventilating, and Air Conditioning

HVAC (heating, ventilating, and air conditioning) refers to the equipment, distribution network, and terminals that provide either collectively or individually the heating, ventilating, or air-conditioning processes to a building.

HVAC systems provide:

Heating
Cooling
Air handling, ventilation, and air quality

HVAC accounts for 40 to 60 percent of the energy used in U.S. commercial and residential buildings. This represents an opportunity for energy savings using proven technologies and design concepts.

HVAC systems have a significant effect on the health, comfort, and productivity of occupants. Issues like user discomfort, improper ventilation, and poor indoor air quality are linked to HVAC system design and operation and can be improved by better mechanical and ventilation systems. In existing buildings, envelope upgrades are often necessary to maximize comfort and energy efficiency, such as reducing envelope leakage.

The best HVAC design considers all the interrelated building systems while addressing indoor air quality, energy consumption, and environmental benefit. Optimizing the design and benefits requires that the mechanical system designer and the architect address these issues early in the schematic design phase and continually revise subsequent decisions throughout the remaining design process. It is also essential to implement well-thought-out commissioning processes and routine preventative maintenance programs. This is good advice for both new and retrofit applications.

To optimize the selection of efficient, cost-effective mechanical and ventilation systems, perform an energy analysis early in the process, during the schematic design phase. Several design and analysis software programs can provide building simulations on an hourly basis to predict the energy behavior of the building's structure, air conditioning system, and central plant equipment.

Following the whole building design approach will enable a reduction in HVAC requirements for new building construction. This design approach can save money and energy by reducing the size requirements of the HVAC system and its energy demand, while still meeting comfort requirements.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ASHRAE) supplies technical information to engineers and other professionals. In addition, ASHRAE writes standards and guidelines in its field of expertise to guide industry in the delivery of goods and services to the public.

DOE conducts research and development on indoor air quality. Learn more.

Heating

Heating is the transfer of energy to a space or to the air in a space by virtue of a difference in temperature between the source and the space or air. This process may take different forms such as direct radiation to the space, direct heating of the circulated air, or through heating of water that is circulated to the vicinity of the space and used to heat the circulated air.

Heating requirements can be reduced through passive solar design and attention to tightening the building envelope. Use the whole building design approach to integrate the various building systems and components.

Available heating technologies include:

Boilers
Electric-resistance heat
High-efficiency gas-fired rooftop units
Heat pumps
Radiant floor heat

Boilers

Boilers are a type of space-heating equipment consisting of a vessel or tank where heat produced from the combustion of such fuels as natural gas, fuel oil, or coal is used to generate hot water or steam. Many buildings have their own boilers, while other buildings have steam or hot water piped in from a central plant. Commercial boilers are manufactured for high- or low-pressure applications.

Most medium-to-large facilities use boilers to generate hot water or steam for space heating, domestic water heating, food preparation, and industrial processes.

For boilers to run at peak efficiency, operators must attend to boiler staging, water chemistry, pumping and boiler controls, boiler and pipe insulation, fuel-air mixtures, burn-to-load ratio, and stack temperatures.

Note recent trends in boiler systems, which include installing multiple small boiler units, decentralizing systems, and installing direct digital control systems, including temperature reset strategies. Because these systems capture the latent heat of vaporization from combustion water vapor, flue-gas temperatures are low enough to vent the exhaust through polyvinyl chloride (PVC) pipes; PVC resists the corrosive action of flue-gas condensate.

Options to consider when replacing boilers include:

Multiple boiler systems are more efficient than single boilers, especially under part-load conditions
Solar-assisted systems and biomass-fired boilers as alternatives to conventional boiler systems
Cogeneration (combined heat and power), including the use of fuel cells and microturbines as the heat source.

Electric Resistance Heating

Electric resistance heating converts nearly 100 percent of the energy in the electricity to heat. However, most electricity is produced from oil, gas, or coal generators that convert only about 30 percent of the fuel's energy into electricity. Because of electricity's generation and transmission losses, electric heat is often more expensive than heat produced on-site with combustion appliances, such as natural gas, propane, and oil furnaces.

Electric resistance heat can be supplied by centralized forced-air furnaces or by zonal heaters, both of which can be composed of a variety of heater types. Zonal heaters distribute electric resistance heat more efficiently than electric furnaces because room temperatures are set according to occupancy. In addition, zonal heaters have no ducts that can lose heat before it reaches the room. However, electric furnaces can accommodate central cooling easier than zonal electric heating, because the air conditioner can share the furnace's ducts.

Electric resistance heat can be provided by electric baseboard heaters, electric wall heaters, electric radiant heat, electric space heaters, electric furnaces, or electric thermal storage systems.

High-Efficiency Gas-Fired Rooftop Units

In a packaged central unit, the evaporator, condenser, and compressor are all located in one cabinet, which usually is placed on a roof. Air supply and return ducts come from indoors through the building's exterior wall or roof to connect with the packaged air conditioner, which is usually located outdoors. Packaged air conditioners often include electric heating coils or a natural gas furnace. This combination of air conditioner and central heater eliminates the need for a separate furnace indoors.

The majority of current commercial gas-fired rooftop HVAC units are single-speed noncondensing units with combustion efficiencies in the range of 78 to 82 percent. Newer high efficiency units using condensing heat exchangers or pulse combustion can boost this efficiency to 89 to 97 percent. Another method of increasing energy efficiency is modulating the burner and combustion air flows. This modulating approach provides greater control over temperature and eliminates much of the cycling losses, resulting in higher seasonal efficiencies. There are currently several manufacturers producing high efficiency units, with modulating units being more common.

Heat Pumps

Heat pumps function by moving (or pumping) heat from one place to another. Like a standard air conditioner, a heat pump takes heat from inside a building and dumps it outside. The difference is that a heat pump can be reversed to take heat from a heat source outside and pump it inside. Heat pumps use electricity to operate pumps that alternately evaporate and condense a refrigerant fluid to move that heat. In the heating mode, heat pumps are far more "efficient" at converting electricity into usable heat because the electricity is used to move heat, not to generate it.

The most common type of heat pump—an air-source heat pump—uses outside air as the heat source during the heating season and the heat sink during the air conditioning season. Ground-source and water-source heat pumps work the same way, except that the heat source/sink is the ground, groundwater, or a body of surface water, such as a lake. (For simplicity, water-source heat pumps are often lumped with ground-source heat pumps, as is the case here.)

Ground-source heat pumps are also known as geothermal heat pumps. The efficiency or coefficient of performance (COP) of ground-source heat pumps is significantly higher than that of air-source heat pumps because the heat source is warmer during the heating season and the heat sink is cooler during the cooling season. COP is a ratio calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btus per hour), by the total electrical input (watts) x 3.412.

Ground-source heat pumps are environmentally attractive because they deliver so much heat or cooling energy per unit of electricity consumed. The COP is usually 3 or higher. The best ground-source heat pumps are more efficient than high-efficiency gas combustion, even when the source efficiency of the electricity is taken into account.

Ground source heat pumps are generally most appropriate for residential and small commercial buildings, such as small-town post offices. In residential and small (skin-dominated) commercial buildings, ground-source heat pumps make the most sense in mixed climates with significant heating and cooling loads because the high-cost heat pump replaces both the heating and air conditioning system.

Because ground-source heat pumps are expensive to install in residential and small commercial buildings, it sometimes makes better economic sense to invest in energy efficiency measures that significantly reduce heating and cooling loads, then install less expensive heating and cooling equipment. The savings in equipment may be able to pay for most of the envelope.

If a ground-source heat pump is to be used, plan the site work and project scheduling carefully so that the ground loop can be installed with minimum site disturbance or in an area that will be covered by a parking lot or driveway.

Ground-source heat pumps are generally classified according to the type of loop used to exchange heat with the heat source/sink. Most common are closed-loop horizontal and closed-loop vertical systems (see illustration). Using a body of water as the heat source/sink is very effective, but seldom available as an option. Open-loop systems are less common than closed-loop systems due to performance problems (if detritus gets into the heat pump) and risk of contaminating the water source or, in the case of well water, inadequately recharging the aquifer.

Ground-source heat pumps are complex. Basically, water or a nontoxic antifreeze-water mix is circulated through buried polyethylene or polybutylene piping. This water is then pumped through one of two heat exchangers in the heat pump. When used in the heating mode, this circulating water is pumped through the cold heat exchanger, where its heat is absorbed by evaporation of the refrigerant. The refrigerant is then pumped to the warm heat exchanger, where the refrigerant is condensed, releasing heat in the process. This sequence is reversed for operation in the cooling mode.

Direct-exchange ground-source heat pumps use copper ground-loop coils that are charged with refrigerant. This ground loop thus serves as one of the two heat exchangers in the heat pump. The overall efficiency is higher because one of the two separate heat exchangers is eliminated, but the risk of releasing the refrigerant into the environment is greater. Direct-exchange systems have a small market share.

Radiant Floor Heat

There are three types of radiant floor heat: radiant air floors (air is the heat-carrying medium); electric radiant floors; and hot water (hydronic) radiant floors. All three types can be further subdivided by the type of installation: those that make use of the large thermal mass of a concrete slab floor or lightweight concrete over a wooden subfloor (these are called "wet" installations); and those in which the installer "sandwiches" the radiant floor tubing between two layers of plywood or attaches the tubing under the finished floor or subfloor (dry installations).

Electric radiant floors are usually only cost-effective if your electric utility company offers time-of-use rates. Time-of-use rates allow you to "charge" the concrete floor with heat during off-peak hours (approximately 9 p.m. to 6 a.m.). If the floor's thermal mass is large enough, the heat stored in it will keep the building comfortable for eight to ten hours, without any further electrical input. This saves a considerable number of energy dollars compared to heating at peak electric rates during the day.

Hydronic (liquid) systems are the most popular and cost-effective systems for heating-dominated climates. They have been in extensive use in Europe for decades. Hydronic radiant floor systems pump heated water from a boiler through tubing laid in a pattern underneath the floor. The temperature in each room is controlled by regulating the flow of hot water through each tubing loop. This is done by a system of zoning valves or pumps and thermostats.

Wet installations are the oldest form of modern radiant floor systems. In a "wet" installation, the tubing is embedded in the concrete foundation slab, or in a lightweight concrete slab on top of a subfloor, or over a previously poured slab. If the new floor is not on solid earth, additional floor support may be necessary because of the added weight. You should consult a professional engineer to determine the floor's carrying capacity.

However, due to recent innovations in floor technology, "dry" floors have been gaining a lot of popularity over wet floors. Much of this is because a dry floor is faster and less expensive to build. There are several ways to make a dry radiant floor. Some "dry" installations involve suspending the tubing underneath the subfloor between the joists. This method usually requires drilling through the floor joists in order to install the tubing. Reflective insulation must also be installed under the tubes to direct the heat upward. Tubing may also be installed from above the floor, between two layers of subfloor. In these instances, the tubes are often in aluminum diffusers that spread the water's heat across the floor in order to heat the floor more evenly. The tubing and heat diffusers are secured between furring strips (sleepers), which carry the weight of the new subfloor and finished floor surface.

At least one company has improved on this idea by making a plywood subfloor material manufactured with tubing grooves and aluminum heat diffuser plates built into them. The manufacturer claims that this product makes a radiant floor system (for new construction) considerably less expensive to install and faster to react to room temperature changes. Such products also allow for the use of half as much tubing since the heat transfer of the floor is greatly improved over more traditional dry or wet floors.

Cooling

Cooling is the transfer of energy by virtue of a difference in temperature between the cooling source and the space or air. In the usual cooling process, air is circulated over a surface maintained at a low temperature. The surface may be in the space to be cooled or at some remote location from it, the air being ducted to and from the space. Usually water or a volatile refrigerant is the cooling medium.

Cooling systems are often oversized; equipment like lights and computers put off less heat now than in the past. Simulating the whole building's energy use during design can help properly size the cooling equipment.

Growing use of cooling is contributing to high demand for power in the summertime months. To meet this demand, more power plants are being built. Minimizing cooling loads not only reduces your bills, it also reduces the need for more power plants.

Cooling technologies for buildings include:

Absorption cooling
High-efficiency gas-fired rooftop units
Chillers
Dessicant dehumidification
Evaporative cooling

Absorption Cooling

On the surface, the idea of using an open flame or steam to generate cooling might appear contradictory, but the idea is actually very elegant, and it has been around for quite a while. The first patent for absorption cooling was issued in 1859 and the first system built in 1860. Absorption cooling is more common today than most people realize. Large, high-efficiency, double-effect absorption chillers using water as the refrigerant dominate the Japanese commercial air conditioning market. While less common in the U.S., interest in absorption cooling is growing, largely as a result of deregulation in the electric power industry. The technology is even finding widespread use in hotels that use small built-in absorption refrigerators (because of their virtually silent operation) and for refrigerators in recreational vehicles (because they do not require electricity).

Absorption cooling is most frequently used to air condition large commercial buildings. Because there are no simplifying rules of thumb to help determine when absorption chillers should be used, a life-cycle cost analysis should be performed on a case-by-case basis to determine whether this is an appropriate technology.

Absorption chillers may make sense in the following situations:

Electric demand charges are high
Electricity use rates are high
Summertime natural gas prices are favorable
Utility and manufacturer rebates exist

Absorption chillers can be teamed with electric chillers in "hybrid" central plants to provide cooling at the lowest energy costs. In this case, the absorption chillers are used during the summer to avoid high electric demand charges, and the electric chillers are used during the winter when they are more economical. Because absorption chillers can make use of waste heat, they can essentially provide free cooling in certain facilities.

Absorption cooling systems can most easily be incorporated into new construction, though they can also be used as replacements for conventional electric chillers. A good time to consider absorption cooling is when an old electric chiller is due for replacement.

An absorption cooling cycle is similar to a vapor-compression cycle in that it relies on the same three basic principles:

When a liquid is heated it boils (vaporizes) and when a gas is cooled it condenses
Lowering the pressure above a liquid reduces its boiling point
Heat flows from warmer to cooler surfaces.

Instead of mechanically compressing a gas (as occurs with a vapor-compression refrigeration cycle), absorption cooling relies on a thermochemical "compressor." Two different fluids are used, a refrigerant and an absorbent, that have high "affinity" for each other (one dissolves easily in the other). The refrigerant (usually water) can change phase easily between liquid and vapor and circulates through the system. Heat from natural gas combustion or a waste-heat source drives the process. The high affinity of the refrigerant for the absorbent (usually lithium bromide or ammonia) causes the refrigerant to boil at a lower temperature and pressure than it normally would and transfers heat from one place to another.

Absorption chillers can be direct-fired or indirect-fired, and they can be single-effect or double-effect (explanation of these differences is beyond the scope of this discussion). Double-effect absorption cycles capture some internal heat to provide part of the energy required in the generator or "desorber" to create the high-pressure refrigerant vapor. Using the heat of absorption reduces the steam or natural gas requirements and boosts system efficiency.

Absorption cooling equipment on the market ranges in capacity from less than 10 tons to over 1,500 tons (35 to 5,300 kilowatts per ton [kW/ton]). Coefficients of performance (COP) range from about 0.7 to 1.2, and electricity use ranges from 0.004 to 0.04 kW/ton of cooling. Though an electric pump is usually used (the principal exceptions being the small hotel and RV refrigerators), pump energy requirements are relatively small because pumping a liquid to the high-side pressure requires much less electricity than does compressing a gas to the same pressure.

High-efficiency, double-effect absorption chillers are more expensive than electric-driven chillers. They require larger heat exchangers because of higher heat rejection loads; this translates directly into higher costs. Non-energy operating and maintenance costs for electric and absorption chillers are comparable. Significant developments in controls and operating practice have led the current generation of double-effect absorption chillers to be praised by end-users for their low maintenance requirements.

The potential of absorption cooling systems to use waste heat can greatly improve their economics. Indirect-fired chillers use steam or hot water as their primary energy source, and they lend themselves to integration with on-site power generation or heat recovery from incinerators, industrial furnaces, or manufacturing equipment. Indirect-fired, double-effect absorption chillers require steam at around 370 degrees F and 115 psig (190 degrees C and 900 kPa), while the less efficient (but also less expensive) single-effect chillers require hot water or steam at only 167-270 degrees F (75-132 degrees C). Triple-effect chillers are also available.

High-Efficiency Gas-Fired Rooftop Units

Chillers

Chillers are a type of cooling equipment that produces chilled water to cool air. The chilled water is distributed throughout the building by pipes. The two major categories of chillers are water-cooled and air-cooled. Water-cooled chillers use water to transport away the heat rejected in their condensers. The water, called condenser water, is cooled in a cooling tower. Air-cooled chillers have condensers that are cooled with ambient air.

In large facilities, the equipment used to produce chilled water for HVAC systems can account for up to 35 percent of a facility's electrical energy use. If replacement is determined to be the most cost-effective option, there are some excellent new chillers on the market.

The most efficient chillers currently available operate at full-load efficiencies of 0.50 kilowatts per ton (kW/ton) or better, a savings of 0.15 to 0.30 kW/ton over much existing equipment. When considering chiller types and specific products, part-load efficiencies must also be compared. If existing chiller equipment is to be kept, there are a number of measures that can be carried out to improve performance.

Consider chiller replacement when existing equipment is more than ten years old and the life-cycle cost analysis confirms that replacement is worthwhile. First-cost, energy performance, and maintenance costs are the major components of life-cycle costing, but refrigerants may also be a factor.

An excellent time to consider chiller replacement is when lighting retrofits, glazing replacement, or other modifications are being done to the building that will reduce cooling loads. Conversely, when a chiller is being replaced, consider whether such energy improvements should be carried out; in some situations those energy improvements can be essentially done for free because they will be paid for from savings achieved in downsizing the chiller. Be aware that there can be lead times of six months or more for delivery of new chillers.

Electric chillers use a vapor compression refrigerant cycle to transfer heat. The basic components of an electric chiller include an electric motor, refrigerant compressor, condenser, evaporator, expansion device, and controls. Electric chiller classification is based on the type of compressor used: common types include centrifugal, screw, and reciprocating. The scroll compressor is another type frequently used for smaller applications of 20 to 60 tons. Hydraulic compressors are a fifth type (still under development).

Both the heat rejection system and building distribution loop can use water or air as the working fluid. Wet condensers usually incorporate one or several cooling towers. Evaporative condensers can be used in certain (generally dry) climates. Air-cooled condensers incorporate one or more fans to cool refrigerant coils and are common on smaller, packaged rooftop units. Air-cooled condensers may also be located remotely from the chillers.

Overall HVAC system efficiency should be considered when altering chiller settings. The complex interrelationships of chiller system components can make it difficult for operators to understand the effects of their actions on all components of the systems. For example, one way to improve chiller efficiency is to decrease the condensing water temperature. However, this requires additional cooling tower operation that may actually increase total operating costs if taken to an extreme. In humid climates, increasing the chilled water temperature to save energy may unacceptably reduce the effective removal of humidity if the coil size is not also adjusted.

Desiccant Dehumidification

Desiccants are materials that attract and hold moisture, and desiccant air-conditioning systems provide a method of drying air before it enters a conditioned space. With the high levels of fresh air required for building ventilation, removing moisture has become increasingly important. Desiccant dehumidification systems are growing in popularity because of their ability to remove moisture from outdoor ventilation air while allowing conventional air conditioning systems to deal primarily with control temperature.

Desiccant dehumidification is a new approach to space conditioning that offers solutions for many of the current economic, environmental, and regulatory issues being faced by facility managers. Indoor air quality is improved through higher ventilation rates, and achieving those fresh air make-up rates becomes more feasible with desiccant systems. At "low load conditions" outdoor air used for ventilation and recirculated air from the building have to be dehumidified more than they have to be cooled.

Properly integrated desiccant dehumidification systems have become cost-effective additions to many building HVAC systems because of:

Their ability to recover energy from conditioned air that is normally exhausted from buildings.
The lower cost of dehumidification when low-sensible load, high-latent load conditions are met.
The greater comfort achieved with dehumidified air.
The promotion of gas cooling for summer air conditioning by utilities in the form of preferential gas cooling rates.
High electric utility demand charges, which encourage a shift away from conventional, electrically driven air conditioning (which requires a heavy daytime loading).

Desiccant systems offer significant potential for energy savings (0.1 to 0.4 quads nationwide). They also inhibit microbiological growth by maintaining lower humidity levels. Better control of humidity prevents moisture, mildew, and rot damage to building materials.

Desiccant dehumidification is particularly attractive in applications where building exhaust air is readily available for an energy-recovery ventilator (ERV, or "passive" desiccant system) or where a source of waste heat from other building operations is available to regenerate an "active" desiccant system.

To dehumidify air streams, desiccant materials are impregnated into a lightweight honeycomb or corrugated matrix that is formed into a wheel. This wheel is rotated through a supply or process air stream on one side that is dried by the desiccant before being routed into the building. The wheel continues to rotate through a reactivation or regeneration air stream on the other side that dries out the desiccant and carries the moisture out of the building. The desiccant can be reactivated with air that is either hotter or drier than the process air.

"Passive" desiccant wheels, which are used in total ERVs and enthalpy exchangers, use dry air that is usually building exhaust air for regeneration. Passive desiccant wheels require additional fan power only to move the air and the energy contained in the exhaust air stream. However, passive desiccants cannot remove as much moisture from incoming ventilation air as active desiccant systems and are ultimately limited in sensible and latent capacity by the temperature and dryness of exhaust air leaving the building.

"Active" desiccant wheels use heated air and require a thermal energy source for regeneration. The advantage of active desiccant wheels is that they dry the supply air continuously—to any desired humidity level—in all weather, regardless of the moisture content of the building's exhaust air. They can be regenerated with humidity control to any desired level.

Evaporative Cooling

Direct evaporative coolers (also known as swamp coolers) have been used for many years in hot, arid parts of the country. These systems are typically roof-mounted. In direct evaporative cooling systems, cooling is provided as hot, dry outside air is blown through an evaporative media that is kept moist.

Indirect evaporative coolers can work in climates where moist air is not wanted in the building, though efficiency is lower.

On larger buildings in hot, dry climates, the benefits of evaporative cooling can be achieved through roof-spray technology. A modified spray-irrigation system can be used on the roof to drop daytime roof-surface temperatures from 135-160 degrees F to 85-90 degrees F (57-71 degrees C to 29-32 degrees C). With a typical (poorly insulated) roof system, this can reduce interior temperatures significantly.

A newer, more innovative use of evaporative cooling is night-sky radiant cooling. This approach works in climates with large diurnal temperature swings and generally clear nights (such as in the Southwest). Water is sprayed onto a low-slope roof surface at night, and the water is cooled through a combination of evaporation and radiation. This process typically cools the water to 5-10 degrees F (2.7-5.5 degrees C) below the night air temperature.

The water drains to a tank in the basement or circulates through tubing embedded in a concrete floor slab. Daytime cooling is accomplished either by circulating cooled water from the tank or through passive means from the concrete slab.

Air Handling, Ventilation, and Air Quality

On an annual basis, continuously operating air distribution fans can consume more electricity than chillers or boilers, which run only intermittently. High-efficiency air distribution systems can substantially reduce fan power required by an HVAC system, resulting in dramatic energy savings. Because fan power increases at the square of air speed, delivering a large mass of air at low velocity is a far more efficient design strategy than pushing air through small ducts at high velocity. Supplying only as much air as is needed to condition or ventilate a space through the use of variable-air-volume systems is more efficient than supplying a constant volume of air at all times.

The largest gains in efficiency for air distribution systems are realized in the system design phase during new construction or major retrofits. Modifications to air distribution systems are difficult to make in existing buildings, except during a major renovation.

Design options for improving air distribution efficiency include:

Variable-air-volume (VAV) systems
Low-pressure-drop ducting design
Low-face-velocity air handlers
Fan sizing and variable-frequency-drive (VFD) fan motors
Displacement ventilation systems.

The following checklist will help optimize air distribution.

Deliver only the air needed—Deliver only the volume of air needed for conditioning the actual load. Variable-air-volume (VAV) systems offer superior energy performance compared with constant-volume systems with dual ducts or terminal reheat that use backward-inclined or airfoil fans. VAV systems are standard design practice, yet even greater efficiency gains can be made through careful selection of equipment and system design. Use local VAV diffusers for individual temperature control. Temperatures across a multiroom zone in a VAV system can vary widely, causing individuals further from the thermostat and VAV box location to be uncomfortable. Local ceiling diffusers ducted from the VAV box to individual rooms can modulate the amount of conditioned air delivered to a space, eliminating the inefficient practice of overheating or overcooling spaces to ensure the comfort of all occupants. VAV diffusers require low duct static pressures—0.25 inches of water column (62 Pa) or less—and thus save on fan energy.
Increase duct size—Increase duct size to reduce duct pressure drop and fan speed. Eliminate resistance in the duct system by improving the aerodynamics of the flow paths and avoiding sharp turns in duct routing. Increasing the size of ducting where possible allows reductions in air velocity, which in turn permit reductions in fan speed and yield substantial energy savings. Small increases in duct diameter can yield large pressure drop and fan energy savings, because the pressure drop in ducts is proportional to the inverse of duct diameter to the fifth power.
Use low-face-velocity air handlers—Specify low-face-velocity air handlers to reduce air velocity across coils. Oversizing the air handler increases the cross-sectional area of the airflow, allowing the delivery of the same required airflow at a slower air speed for only a relatively small loss of floor space. The pressure drop across the coils decreases with the square of the air speed, allowing the use of a smaller fan and smaller VFD, thus reducing the first-costs of those components. Air traveling at a lower velocity remains in contact with cooling coils longer, allowing warmer chilled water temperatures. This can yield substantial compounded savings through downsizing of the chilled water plant (as long as all air-handling units in a facility are sized with these design strategies in mind).
Do not oversize fans—Size fans correctly and install VFDs on fan motors. Replace oversized fans with units that match the load. Electronically control the fan motor's speed and torque to continually match fan speed with changing building-load conditions. Electronic control of the fan speed and airflow can replace inefficient mechanical controls, such as inlet vanes or outlet dampers.
Eliminate ducting—Use the displacement method for special facility types. Displacement ventilation systems can largely eliminate the need for ducting by supplying air through a floor plenum and using a ceiling plenum or ceiling ducts as the return. With raised (access) floors providing the air delivery characteristics, the conditioned supply air does not have to be chilled as much, resulting in additional energy savings. Recent research has shown that a raised floor plenum must be carefully and thoroughly sealed to prevent air leakage or most or all savings will be lost.

There is a newer technology of placing ventilation equipment underneath the floor rather than in the ceiling area. Although this technique, called Under Floor Air Distribution (UFAD), has been used for many years in Europe, it is just beginning to be used in the U.S. and the National Center for Energy Management and Building Technologies (NCEMBT) is developing tests to gain greater knowledge of possible advantages or problems with UFAD systems. If you want to learn more about this technology and what NCEMBT is doing in its research efforts, visit their Web site.

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