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:
- Preference of the building owner
- Available construction budget
- Size and shape of the building
- Function of the building
- Architectural limitations
- Life-cycle cost
- Ease of operation and maintenance
- Time available for construction
1. Preference of the building owner
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
3. 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
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
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
7. Ease of operation and maintenance
8. Time available for construction
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.
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
Control Loops Used In HVAC Applications
5 Common Inefficiencies That Affect HVAC System’s Efficiency