The relative cost of the three alternatives outlined in this chapter will vary from project to project and with the geographic area within which the building is located. Moreover, the first cost analysis must include not only the mechanical system but also the electrical system and the general construction cost of the different types of spaces that are provided. There is only one way in which an accurate comparison can be made and that is to provide a developed set of schematic plans in sufficient detail to allow a cost estimate to be completed by either the contracting team retained for the project or a professional estimating service. These cost estimates should also reflect the altered total space required for each of the three considered alternative schemes. A matrix that details the comparative points of first cost comparison is provided in Table 5-1. The intent of this matrix is merely to outline points of comparison by trade; it is only through a complete cost estimate that the true comparative costs can be determined.
The capital costs for the three alternative solutions will be affected by the local con- struction trades and their abilities, for example, to handle the more complicated piping, sheet metal, and control systems that will be necessary in Alternative 1 with its large central mechanical equipment rooms. Both Alternatives 2 and 3 use package air-han- dling units in local fan rooms, which are typically repeated on each floor of a high-rise building. The installed simplicity of Alternative 3 is even greater than Alternative 2 in that there is no central refrigeration plant with a need for somewhat complex piping. Moreover, in Alternative 3, the internal wiring and control of the air-conditioning sup- ply systems is the responsibility of the unit manufacturer, with the temperature control design responsibility of the HVAC engineer being limited to the interface between the unit and the building management system.
Project experience with large buildings, i.e., those with 20 or more stories and areas
of 400,000 ft2 (37,000 m2) to 500,000 ft2 (46,000 m2), in the Northeast United States
has indicated that the mechanical and electrical system costs are approximately equal
Chapter 5—Central Mechanical Equipment Room vs. Floor-by-Floor Fan Rooms | 49
for all three schemes. When the cost of the additional mechanical equipment room area and volume of Alternative 1 and to a lesser degree Alternative 2 is considered, Alterna- tive 3 may have a lower first cost than either of the other alternatives. The difference, however, is not significant and consideration of other points of comparisons may result in a decision to use either Alternative 1 or Alternative 2.
On smaller buildings with fewer floors, the package DX solution of Alternative 3 will almost always be the least costly. Generally, the smaller the building and the fewer the number of floors in a building, the greater the cost advantage of Alternative 3 will become.
It must be apparent that first cost, while always important, is not the only consider- ation. As is discussed in chapter 1, the nature of the owner, specific needs, and the owner’s perception of the market must all be reviewed to allow the owner to select the alternative that will best satisfy the owner’s judgment as to which alternative best ful- fills the owner’s understanding of the project requirements.
Corporate headquarters, for example, will usually favor either chilled water alterna- tive and could well lean toward Alternative 1 with central mechanical equipment rooms, since this will result in simpler maintenance, more flexible operation, and potentially longer equipment life after the building is completed. There is a further advantage in a corporate headquarters with Alternative 1 in that maintenance personnel will not need access to every floor for normal maintenance, which could be a significant advantage from the perspective of office space security.
Developer buildings, particularly those being constructed with the expectation of single floor leases, may well favor Alternative 3, which minimizes if not eliminates overtime operation by the owner’s staff and eliminates the allocation of operating costs that is necessary in Alternative 1 and, to a lesser degree, in Alternative 2. This issue of the allocation of operating costs is discussed in more detail later in this chapter.
Table 5-1. First Cost Considerations
Alternative 1 Central Fan Systems Central Chilled Water
Alternative 2 Floor-by-Floor Fan Systems
Central Chilled Water
Alternative 3 Floor-by-Floor DX Systems
Central Cooling Tower ISSUE—HVAC IMPACT ON COST
• Fewer units, field erected. • More complex and expen-
sive duct systems. • More complex field-
installed controls.
• Central chilled water plant.
• More units, factory-fabri- cated and assembled. • Simpler ductwork. • Field-installed control sys-
tem.
• Central chilled water plant.
• More units, factory-fabri- cated and assembled. • Simpler ductwork.
• Factory-installed control sys- tem.
• No central chilled water plant; cooling tower only. ISSUE—BUILDING MANAGEMENT SYSTEM COSTS
• Complex controls and inter- face with BMS and smoke control system.
• Controls are relatively simple but field installed. Interface with BMS and smoke control system less complex.
• Unit controls provided by manufacturer. Interface with BMS and smoke control sys- tem simple.
ISSUE—ELECTRICAL IMPACT ON COST • Electrical loads concen-
trated in central location. • Probably lowest electrical
cost.
• Minor cost premium for distributed fan motors. • Probably higher electrical
cost than Alternative 1.
• Additional cost for electrical distribution to local DX units.
• Highest electrical cost. ISSUE—GENERAL CONSTRUCTION COST
• Additional gross space needed.
• No separate outside air or smoke exhaust shaft.
• Additional cost of sound treatment of local floor-by- floor fan room.
• Need separate outdoor air and smoke exhaust shaft.
• Additional cost of sound treatment of local floor-by- floor fan room.
• Need separate outdoor air and smoke exhaust shaft.