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Wednesday, 13 November 2013



8 Key Factors That Affect The Selection Of A HVAC System


Introduction



The goal of the heating, ventilating, and air conditioning (HVAC) system is to create and maintain a comfortable environment within a building. A comfortable environment, however, is broader than just temperature and humidity. Comfort requirements that are typically impacted by the HVAC system include dry-bulb temperature, humidity, air movement, fresh air, cleanliness of the air, noise levels etc. Some HVAC systems address these comfort requirements better than others. In addition, there are other factors that affect comfort but are not directly related to the HVAC system. Examples include adequate lighting, and proper furniture and work surfaces.

Of all the decisions made by owners, architects, engineers, and contractors during the design and construction phases of a building, the selection of the HVAC system tends to be a decision that is often revisited throughout the life of the building. The building may be an architectural wonder, with state-of-the-art lighting, easy access, fast elevators, lots of parking, the finest furnishings, and superior energy efficiency, but if there are comfort problems, all of the positives seem to go unnoticed. When it comes to the selection of the appropriate HVAC system for a given building, there are some factors that can affect the decision. In the next paragraphs eight of the most-common factors that influence the selection of the HVAC system will be analyzed:
  1. Preference of the building owner
  2. Available construction budget
  3. Size and shape of the building
  4. Function of the building
  5. Architectural limitations
  6. Life-cycle cost
  7. Ease of operation and maintenance
  8. Time available for construction


1. Preference of the building owner



Preference Of The Building Owner

The individual (or company/organization) that decides to build, expand, or renovate a building may actually own and occupy the facility, or may be a contractor whose business is to lease and/or sell the building. Some owners and contractors have preferences toward certain HVAC systems, possibly based on past experience. It is important to note that the motivations affecting building decisions often differ between an owner and a contractor. This doesn't mean that what is important to an owner is not important to a contractor, but rather that each has different priorities when approaching a building project.

If an owner will occupy the building, life-cycle cost, maintenance cost, system reliability, and a productive work environment may be emphasized in the decision-making process. The selection of the HVAC system becomes more personal when the owner has to work or live in the building. A contractor typically has two motivations. First is the financial performance of the project. Second is the ability to attract and retain tenants. These concerns are related because the financial success of a project depends on the contractor's ability to market the building to prospective tenants, who are often the only source of operating income. Some contractors may sell the property quickly, either upon completion of construction or within one to three years. For this reason, first cost, building marketability, ability to bill individual tenants for energy use, and flexible work space may be most important to them.



2. Available construction budget



Available Construction Budget

The available budget for purchasing and installing the HVAC system may be imposed on the design team by the owner or contractor, or it may be calculated with the aid of the design team. If the owner or the contractor has predetermined what money is available to construct the building, then the design team is challenged to provide an HVAC system that meets the requirements of the building with the available money. This is not always an easy thing to do! Often, some requirements are sacrificed along the way because the stated requirements do not match the available budget to meet those requirements.



3. Size and shape of the building



Size And Shape Of The Building

The building size and shape can quickly narrow down the available HVAC system choices. High-rise buildings are not often well-suited for packaged direct expansion (DX) rooftop equipment because of the long distances that the air must be transported. In split DX systems, the allowable distance between the components of the refrigeration loop is limited to ensure reliable operation. Chilled-water systems, however, are ideal for applications where the refrigeration equipment is centrally located within a building, or among a campus of buildings, and the cooling loads are remote.

The desired location of HVAC equipment within the building can also impact the selection. If the owner or contractor does not want equipment located outdoors, it can be located in basements, in penthouses, or in equipment room(s) for each floor. If there is limited space inside the building, the HVAC system may be located on the roof, in a separate building alongside the main building, or even at a remote location.



4. Function of the building



Function Of The Building

This can be the answer to the question: How will the building be used? Several comfort requirements were referred in introduction section. These requirements may differ from building to building, or may be prioritized differently due to differing building functions. For example, desired background-noise levels are much different in a classroom than they are in a manufacturing assembly area. Humidity-control requirements are much different in a supermarket than they are in an office building. If the comfort requirements for the project are considered in advance and planned for, then an HVAC system can be selected and designed to meet those requirements. When one or more of these requirements are unknown or ignored, the finger pointing typically begins.

If the building will have multiple tenants, can the HVAC system accommodate differing requirements between tenants? Is the interior layout expected to change in the future? How many zones should the building have? Do occupants require after-hours use of the HVAC system, and who is paying the energy bill—the tenants or the building owner? The answers to these questions may determine whether the system uses individual HVAC units or a central system with a building automation system that can track energy usage by tenant.



5. Architectural limitations



Architectural Limitations

The thousands of different components that make up a building must all fit together in a coordinated way. Many buildings are designed to make an architectural statement. It may be hard to make that statement with a cooling tower on the front lawn or packaged DX units in every window. However, there are creative ways to conceal equipment for aesthetic reasons. The floor-to-floor height is generally squeezed as tight as possible to reduce construction costs, or in the case of taller buildings, to get as many floors in the building as possible. This results in limited space in the ceiling plenum. This can be particularly challenging for a central air-handling system, and may result in the use of a system, such as chilled-water terminal units or water-source heat pumps, where the equipment is located closer to each zone.

Sometimes, building trades can influence the type of HVAC system installed. In some geographical regions, sheet-metal trades prefer to install "dry" systems, that is, systems with central equipment rooms that duct supply air throughout the building. In other regions, plumbing trades prefer to install "wet" systems with piping that runs throughout the building.



6. Life-cycle cost



Life-Cycle Cost

Decisions made solely, or primarily, on installed (first) cost often ignore such factors as energy use, maintenance requirements, or expected life. Life-cycle cost includes the total cost of owning and operating the HVAC system over a given period of years. This includes installed cost, energy cost, maintenance cost, replacement cost, and any other known and expected costs. As could be expected, some HVAC systems use more energy than others. Software tools are available to help the HVAC system designer analyze various HVAC systems based on life-cycle cost. In addition, many state and local building codes include requirements for energy efficiency. Some requirements relate to the efficiency of various components, such as packaged DX rooftop units or water chillers, and some requirements relate to the design and control of the entire system. Some of these requirements even prohibit the use of certain types of HVAC systems for certain applications.



7. Ease of operation and maintenance



Ease Of Operation And Maintenance

Will there be building personnel on site to operate and maintain the HVAC system? What level of training is required to operate the system? If the building will not have someone onsite to operate or maintain the HVAC system, this will impact the system choice. Most types of HVAC systems have some level of automatic control. The use of communicating building automation systems (BAS) has made this less of a concern, because the operator can be located off site and still diagnose the cause of a problem, just as if he or she were inside the building. Moreover, some local or state codes require an on-site building operator for certain types of systems. Some of these requirements are based on the capacity of the HVAC equipment. For certain building types, this type of code requirement may cause the selection of a system that uses several smaller pieces of equipment rather than a few large pieces of equipment.



8. Time available for construction



Time Available For Construction

The speed with which a building must be built or renovated, and when the HVAC equipment must be installed during that process, can also influence the selection of the HVAC system. Some types of HVAC equipment are made-to-order and have lead times for manufacturing. For this reason, equipment that is in stock may be selected for a fast-track building project. Packaged equipment with factory-installed controls is often selected because it can typically be installed and commissioned very quickly.

Analyzing and evaluating the many system choices can consume a great deal of engineering time. For this reason, it is imperative that the HVAC system design engineer become involved early in the design process. Often, the project schedule does not allow sufficient time for the design team to properly evaluate HVAC system alternatives. There is a great deal of pressure to finalize the system choice quickly, and design decisions are often made because "that’s what we did last time." The owner, contractor, or architect is frequently better served by the added engineering costs required to analyze system options carefully, and then to integrate the HVAC system into architectural design.



References



The post was based on material found on the following ASHRAE handbooks:
2008 - Fundamentals of HVAC Systems
2011 - HVAC Applications
2012 - HVAC Systems And Equipment



Read also



Control Loops Used In HVAC Applications
Human Comfort & HVAC System Operation 
5 Common Inefficiencies That Affect HVAC System’s Efficiency 

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Mechanical Engineer (Ph.D. cand.), M.Sc. Cranfield University, Dipl.-Ing. Aristotle University, Thessaloniki - Greece.
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