There is one thing you always can depend on when it comes to mechanical systems in buildings: They never work perfectly.

No matter how carefully designed and constructed, mechanical climate-control systems cannot provide every space in a building with exactly the right amount of air at the right temperature, humidity and velocity. Furthermore, if two people occupy a space, there will be at least three opinions about what constitutes desirable comfort conditions.

Environmental design is a statistical exercise, because people's body systems vary widely. "Psychometric" response in human beings, as the term implies, is measured primarily through perception. Feelings of comfort are influenced by physiological factors such as body metabolism, body mass, physical activity and fatigue and by psychological factors such as season of the year, level and quality of light, time of day, and size and character of space occupied. Even intellectual and emotional expectations or frame of mind affect sense of comfort.

Statistical data predict ranges of temperature and humidity in which most (but not all) people will experience acceptable (but not necessarily ideal) levels of comfort.

With this in mind, architects and engineers design buildings to maintain indoor temperature and humidity within a limited "comfort" range. This range spans approximately 10 degrees, centered around 70 degrees Fahrenheit. In summer, buildings must accommodate outdoor-indoor temperature differentials up to 25 degrees (100 degrees F outside, 75 F inside), while winter differentials can be 70 degrees (minus 5 degrees F outside and 65 degrees F inside).

Probable maximum temperature differentials in each season, along with insulating properties of walls and roofs, determine maximum rates (BTUs per hour) of heat gain or loss per building. These, in turn, dictate the size and cost of system equipment and components. If outdoor temperatures go beyond those assumed, indoor temperatures cannot be maintained in the comfort range.

Most of today's mechanical HVAC (heating, ventilating and air conditioning) systems include the following basic components:

*For heating, furnaces or boilers burn fossil fuels. Electric heat pumps compress, condense and vaporize a refrigerant gas. Electric resistance elements produce heat directly, though expensively.

*For cooling, gas absorption chillers produce cold refrigerant circulated through heat transfer coils. A heat pump, gaining BTUs on one side of its refrigerant-gas loop while losing them on the other, can be used for air conditioning as well as heating. Heat removed during air conditioning must be dissipated or recycled for other purposes.

*In central-air systems, centrifugal blowers are connected to return air ducts that draw in "old" air from individual spaces throughout buildings, or from a few central locations in smaller buildings. Fresh air also may be ducted into the return side of the system. After passing across heat-transfer coils on the output side of the blower, heated or cooled (and dehumidified) air flows through supply ducts to room registers at or near the perimeters of buildings where most heat is lost or gained.

*Air filters, normally on the return side of blowers, trap particles suspended in the air stream, particularly fibers and dust. Most are effective, but if they are not cleaned or changed periodically, the buildup of trapped particles will impede air flow and seriously reduce the system's operating efficiency.

*On dry winter days, humidifiers can inject water vapor into the warm air stream for the benefit of people and indoor plants. However, without insulating window glass, humidification can lead to interior condensation and frost. In summer, dehumidification occurs when moisture in warm humid air condenses on air conditioning coils.

*Instead of carrying large volumes of tempered air through sizable and lengthy ducts, some systems distribute centrally heated or chilled water through pipes to dispersed air-handling, or "fan-coil," units. A centrifugal blower and heat-exchange coil in each unit supply heated or cooled air to discrete zones or spaces. Individual units can be under windows or above suspended ceilings. When fan-coil units only temper air recirculated within zones or rooms, a central ventilation system introduces fresh air while exhausting stale air.

*Networks of ducts, supported by a structure, must thread their way vertically and horizontally through buildings to carry air to its destination. Most ducts consist of thin, galvanized steel sheets formed into rectangular or round sections. As air volume decreases, duct size decreases accordingly. Main supply and return trunks connecting to air-handling units are the largest, diminishing progressively in cross-sectional areas as they subdivide into separate branches and zones.

*Ducts are wrapped with insulation when exposed to outdoor temperatures and should be lined with insulation for interior noise abatement near air-handling units. Flexible, accordion-like connectors isolate main trunk ducts from machine vibrations, while resilient hangers isolate ducts from building structure.

*At the end of each branch of a supply or return air duct is a register, the interface between the mechanical system and the spaces it serves. Some registers have adjustable dampers to control the volume of air flowing through them, and presumably, to permit "balancing" of the system. Unfortunately, after a few personal adjustments are made, balance may be lost. Also, registers are often easy to spot because of gray smudges on adjacent ceilings, walls, carpeting and drapes.

*Thermostats do one basic thing, despite latter-day refinements. Sensing the temperature in a zone, they tell the unit they control either to turn on or to turn off. Contrary to occasionally popular belief, setting a thermostat to a high temperature will not make a furnace produce warmer air. It only makes it run continuously until the thermostat signals it to stop.

What are the architectural implications of such systems? Clearly, rooms must be provided in buildings for chillers, boilers, furnaces and fans, plus all the sheet-metal ductworks, plenums and pipes. A building may contain one room or several.

Ideally, equipment should be accessible for routine servicing, yet noise, vibration and products of combustion should not reach inhabited spaces. Central mechanical rooms often are in basements, but equipment also may be housed on roofs, potentially affecting a building's silhouette.

The major design challenge, apart from ensuring comfort, is integrating mechanical components and networks with a building's structural skeleton and system of enclosure. Here lies the greatest potential for conflict. There's not a contractor alive who doesn't delight in recounting stories about architects' and engineers' plans that show ducts, columns and beams occupying the same space at the same time.

Coordinating the patterns and geometries of ductwork, piping and structural elements is a three-dimensional design act requiring careful communication among architect, mechanical engineer and structural engineer. Lack of coordination can lead to costly field revisions and change orders, not to mention unplanned bumps, bulkheads, pilasters or columns.

This is why many office buildings designed for interior partitioning flexibility employ ceilings hung below the bottom of each floor's structure. Systems coordination is greatly simplified, because almost everything running horizontally can be routed easily within this utilitarian space sandwich.

Perhaps the increasing cost and complexity of mechanical systems, coupled with the integration challenge, led designers in the 1960s to explore the possibility of exposing and expressing systems architecturally. Why spend all that money to be comfortable, and then hide it? With a bit of imagination, a building's "metabolics" might contribute to its visual order and even say something about the meaning and aesthetics of technology. It was an idea that had to be tried.

NEXT: Expressing technology.