The problem-solving process requires some criteria that can be applied in describing and evaluating alternatives. In the selection of HVAC systems, the following criteria are used—consciously or unconsciously—because only rarely is the problem-solving process formally applied.

1. Requirements of comfort or process. These requirements include temperature, always; humidity, ventilation, and pressurization, sometimes; and zoning for better control, if needed. In theory at least, the comfort requirement should have a high priority.

In practice, this criterion is sometimes subordinated to first cost or to the desires of someone in authority. This is happening less often as building occupants become more sophisticated in their expectations.

Process requirements are more difficult and require a thorough inquiry by the HVAC designer into the process and its needs. Until the process is fully understood, the designer cannot provide an adequate HVAC system. Most often, different parts of the process have different temperature, humidity, pressure, and cleanliness requirements; the most extreme of these can penalize the entire HVAC system.

2. Energy conservation. This is usually a code requirement and not optional. State and local building codes almost invariably include requirements constraining the use of new, nonrenewable energy. Nonrenewable refers primarily to fossil-fuel sources.

Renewable sources include solar power, wind, water, geothermal, waste processing, heat reclaim, and the like. The strictest codes prohibit any form of reheat (except from reclaimed or renewable sources) unless humidity control is essential.

This restriction eliminates such popular systems as terminal reheat, two-deck multizone, multizone, and constant volume dual-duct systems, although the two-fan dual-duct system is still possible and the three-duct multizone system is acceptable.

Most HVAC systems for process environments have opportunities for heat reclaim and other ingenious ways of conserving energy. Off-peak thermal storage systems are becoming popular for energy cost savings, although these systems may actually consume more energy than conventional systems. Thermal storage is a variation on the age-old practice of cutting and storing ice from the lake in winter, for later use in the summer.

3. First cost and life-cycle cost. The first cost reflects only the initial price, installed and ready to operate. The first cost ignores such factors as expected life, ease of maintenance, and even, to some extent, efficiency, although most energy codes require some minimum efficiency rating.

The life-cycle cost includes all cost factors (first cost, operation, maintenance, replacement, and estimated energy use) and can be used to evaluate the total cost of the system over a period of years. A common method of comparing the life-cycle costs of two or more systems is to convert all costs to present-worth values.

Typically, first cost governs in buildings being built for speculation or short-term investment. Life-cycle costs are most often used by institutional builders—schools, hospitals, government—and owners who expect to occupy the building for an indefinite extended period. Life-cycle cost analysis requires the assumption of an interest, or discount, rate and may also include anticipated inflation.

4. Desires of owner, architect, or design office. Very often, someone in authority lays down guidelines which must be followed by the designer. This is particularly true for institutional owners and major retailers. Here the designer’s job is to follow the criteria of the employer or the client unless it is obvious that some requirements are unsuitable in an unusual environment.

Examples of such environmental conditions are extremely high or low outside-air humidity, high altitude (which affects the AHU and air-cooled condenser capacity), and contaminated outside air (which may require special filtration and treatment).

5. Space limitations. Architects can influence the HVAC system selection by the space they make available in a new building. In retrofit situations, designers must work with existing space. Sometimes in existing buildings it is necessary to take additional space to provide a suitable HVAC system.

For example, in adding air conditioning to a school, it is often necessary to convert a classroom to an equipment room. Rooftop systems are another alternative where space is limited, if the building structure will support such systems.

In new buildings, if space is too restricted, it is desirable to discuss the implications of the space limitations in terms of equipment efficiency and maintainability with the architect. There are ways of providing a functional HVAC system in very little space, such as individual room units and rooftop units, but these systems often have a high life-cycle cost.

6. Maintainability. This criterion includes equipment quality (the mean time between failures is commonly used); ease of maintenance (are high-maintenance items readily accessible in the unit?); and accessibility (Is the unit readily accessible? Is there adequate space around it for removing and replacing items?).

Rooftop units may be readily accessible if an inside stair and a roof penthouse exist; but if an outside ladder must be climbed, the adjective readily must be deleted. Many equipment rooms are easy to get to but are too small for adequate access or maintenance. This criterion is critical in the lifecycle cost analysis and in the long-term satisfaction of the building owner and occupants.

7. Central plant versus distributed systems. Central plants may include only a chilled water source, both heating and chilled water, an intermediate temperature water supply for individual room heat pumps, or even a large, central air-handling system. Many buildings have no central plant.

This decision is, in part, influenced by previously cited criteria and is itself a factor in the life-cycle cost analysis. In general, central plant equipment has a longer life than packaged equipment and can be operated more efficiently.

The disadvantages include the cost of pumping and piping or, for the central AHU, longer duct systems and more fan horsepower. There is no simple answer to this choice. Each building must be evaluated separately.

8. Simplicity and controllability. Although listed last, this is the most important criterion in terms of how the system will really work. There is an accepted truism that operators will soon reduce the HVAC system and controls to their level of understanding.

This is not to criticize the operator, who may have had little or no instruction about the system. It is simply a fact of life. The designer who wants or needs to use a complex system must provide for adequate training—and retraining—for operators. The best rule is: Never add an unnecessary complication to the system or its controls.

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