HVAC Systems: Central Vs

HVAC SYSTEM COMPARISON

HVAC Systems: Central vs. Floor-By-Floor

A look at the advantages and disadvantages of central and floor-by-floor HVAC systems

mong the major decisions The decision to use a floor-by- we were the mechanical engineers an office building design floor or central system in a particu- and contractors. The reader should Ateam (owner, architect, engi- lar building often depends more on note that our conclusions are neers, and contractors) must make the individuals involved than on strongly influenced by local condi- is the selection of the HVAC sys- the relative objective merits of the tions, such as the relatively mild tem. There are many choices of two systems. The owner, developer, and dry climate and the high cost of primary cooling plants (e.g., chilled or architect may have prejudices or labor in our area. They also reflect water, refrigerant direct expansion, perceptions of one or the other sys- our very practical and economical ice storage, packaged, air-cooled, tems based on their past experience design approach to both central and and water-cooled), primary heating with buildings they designed, built, floor-by-floor system designs. plants (e.g., hydronic, steam, elec- or leased. In many cases, a decision There are many variations ot tric, heat pump, and heat recovery), is based on what has been done in both central and floor-by-floor sys- and air distribution systems (e.g., nearby competitive buildings tem concepts. This article focuses central, floor-by-floor, variable rather than what may really be the on what might be considered the volume, constant volume, perime- best choice for the project. Simi- extremes of each concept— the ter reheat, dual duct, and fan-pow- larly, most engineers have a prefer- central system consists of a central ered box). The combinations of ence based on prior experience or fan system with an air economizer system and plant options, each with perceived system merits. and a central chilled-water plant distinct advantages and disadvan- The purpose of this article is to serving multiple floors of the build- tages, are virtually infinite in num- focus on some of the major factors ing; the floor-by-floor system, on ber. that should be considered in select- the other hand, has distributed This article addresses only one of ing the HVAC system for a particu- cooling and air handling equip- the many system design decisions lar project. The factors that affect ment. Each floor of the building is —the choice between a central sys- the initial mechanical costs, related served by separate air handling tem and a floor-by-floor system. We building costs, and lifetime main- equipment packaged with a water- have found this to be among the tenance and energy costs are very cooled direct expansion (DX) cool- most difficult design decisions be- complex. As such, it is difficult to ing plant and a water economizer cause of its impact on overall build- make generalizations or draw con- precooling coil. The only central ing first costs, architectural appear- clusions that are applicable to all equipment is a cooling tower, tower ance, building leasability, and fu- buildings, all designers, or all areas water circulating pumps, and a ture building operating costs. of the country. The primary cost central ventilation outside air fan factors, based on our experience, system. While specifically directed By ROBERT G. LINFORD, PhD, PE, are discussed in general terms, then toward the packaged floor-by-floor and STEVEN T. TAYLOR, PE, specifically as they applied to the approach, many of our comments Lin ford Engineering Co., system selection in two existing and generalizations also apply to other floor-by-floor system vari- Oakland, Calif. high-rise office buildings for which HVAC systems

ations, such as floor-by-floor not the engineer. manufacturers' product lines. chilled-water systems with air or • The equipment room layout is Below about 10,000 sq ft, first tower-water economizers. simple and repeated multiple times, costs for floor-by-floor systems are The factors that affect the HVAC which saves design time, whereas generally higher than for central system costs include both direct cost-effective design of central fan systems due to the large number of and indirect factors, such as engi- system duct risers and penthouse small units. The installed cost of a neering costs, installation costs, ar- layout can be very time consuming. 20-ton packaged unit is more than chitectural impact costs, and costs • Load calculations and corre- half that of a 40-ton unit, even for other construction trades. sponding equipment selection is though the cost of the unit itself less critical with packaged floor- may be about half. Items such as Engineering costs by-floor systems because the mod- controls, pipe and duct connec- In the design of any major build- ular sizes of the floor units build an tions, and start-up costs are more ing project, regardless of whether a additional safety factor into the related to the number of units than central or floor-by-floor system is design. to their size. Control costs are a eventually selected, it is imperative major factor in a floor-by-floor sys- that the engineer be involved early Mechanical installation costs tem unless packaged controls are in the design process. This allows The relative mechanical installa- used. The cost for field-installed, more time for a proper analysis of tion costs of floor-by-floor and cen- direct digital controls (DDC) or the various system choices. The en- tral systems vary from project to pneumatic controls can be almost gineer is also available to provide project, with neither system prov- as much for each unit as for a single the necessary input into the archi- ing to be inherently less expensive central air handling system. There- tectural layout of the core and pent- than the other. The only way to fore, to keep the floor-by-floor sys- house to ensure that the design of really know which system is less tem cost reasonable, the number of the HVAC system can be effectively expensive for a given building is to units must be minimized and unit accommodated. perform a detailed cost estimate of sizes must be as large as possible, Analyzing and evaluating design each design. If this is impractical, a with the optimum unit size in the 40 options can consume a great deal of careful study of the major factors to 60 ton range. valuable engineering time. Often, that impact first cost can often Central system costs follow a the project schedule does not allow make the least expensive option similar trend, with costs per square time for such careful study. Because apparent. foot falling as floor size increases the system selection needs to pre- The major factors and trends we but without the sudden cost impact cede the final architectural core and have noted in many of the projects as floor size falls outside the 15,000 penthouse layouts, there is a great we designed and installed in the to 25,000 sq ft optimal range. First- deal of pressure to finalize this past several years include: cost considerations will, therefore, decision quickly. These engineering • Number of floors. Because of generally favor central systems for costs and time pressures often com- the base cost of the central cooling buildings with typical floor areas pel the engineer to select the system tower and pumps, combined with outside this range. with which he is most familiar. The the high cost for the packaged units, • Space available in ceiling engineer should try to convince the we have found that floor-by-floor cavity for ducts. Ceiling cavity di- owner, developer, or architect that systems are generally more ex- mensions and building structure it is cost effective to pay the added pensive than central systems for will often limit the size of cost-ef- engineering cost required to re- buildings less than 8 to 10 stories. fective floor distribution ducts, search system options carefully and For buildings above 12 to 15 floors, which then limits the size of a single then integrate the system into the mechanical costs begin to favor packaged unit on a floor. Round architectural design. floor-by-floor systems. ducts are less expensive than rect- Engineering costs and risk fac- • Typical floor area and load. angular ducts, but they require tors for designing a packaged DX The cost per square foot of floor- more ceiling clearance for the same water-cooled floor-by-floor system by-floor systems generally de- air quantity. As the available duct are usually lower than those for a creases as the floor area on each space decreases, the required aspect central system for the following floor increases, with the optimum ratio of the duct increases, which reasons: size in the 15,000 to 25,000 sq ft increases its cost. In an extreme • The complex portions of the range. Above 25,000 sq ft, there is case, the difference can be $0.50 per packaged floor-by-floor system are usually inadequate ceiling clear- sq ft or more between round and factory designed. The design of the ance available for a cost-effective extremely shallow rectangular duct refrigeration and air-side systems, duct distribution system radiating distribution systems. cooling and fan capacity controls, from a single unit, and the unit size Often, a shallow ceiling cavity and internal wiring become the re- required may be above 60 nominal problem can be solved most cost sponsibility of the manufacturer, tons, which is the extent of most effectively by distributing air from more than one point on each floor. • Penthouse, shafts, and me- herently include this capability, This inherently favors central sys- chanical room architectural con- dedicated smoke exhaust and tems, for which adding another straints. For a central system, the makeup air systems must be added cooling air supply riser from the penthouse or mechanical floor lay- at a cost typically on the order of central fan system would generally out has a significant impact on sys- $0.50 per sq ft. In addition to the add much less cost than using mul- tem cost. It is easy to spend many added cost, these dedicated systems tiple, low-tonnage floor-by-floor thousands of dollars adapting a require additional space, and they packages. mechanical system into an architec- are seldom as effective at smoke re- • Cost effectiveness of the sys- turally pleasing penthouse that is moval as central systems because of tem design approach. Design phi- inefficient from a mechanical point generally poor makeup air distribu- losophy and design criteria have a of view. Because of the large size tion. In jurisdictions where me- large impact on the cost of the sys- and subsequent high cost of central chanical smoke removal is required, tem. For instance, one design may system components, the designer cost considerations will usually fa- include multiple pieces of equip- must carefully lay out systems to vor the central system. ment for redundancy and standby minimize the space required and capability while another may have the number of duct and pipe fittings only single pieces of equipment for and offsets. The cost difference be- Architectural impact each function (e.g., single boiler, tween a poorly laid out system and a One of the most compelling rea- pump, or chiller). One engineer may clever, economical approach can be sons for selecting a floor-by-floor size a large, variable-volume duct as much as $1.00 per sq ft. HVAC system is the reduced archi- riser for 4500 fpm while another Although the impact of a poor tectural impact compared to a cen- may limit velocities to 1500 fpm. architectural configuration on tral system. While a central system One design may call for a central floor-by-floor system costs is would require at least a very large system surrounded by 4-in. double- smaller, a poorly laid out system penthouse, and often both a pent- wall sheet metal plenums while an- can still add as much as $0.75 per sq house and dedicated intermediate other may use drywall enclosures ft. For example, duct distribution mechanical floors in buildings over where possible. All in all, the differ- costs can be greatly increased if the 20 to 25 stories or so, the floor-by- ence between a very conservative mechanical room is located between floor system requires penthouse design and an overly economical obstructions that force all the space only for a cooling tower, design can be several dollars per supply and return air to leave and pumps, and outside air ventilation square foot. enter the room at one point or if the fans (and boilers, if required for the As design/build engineers and unit is not centrally located on the heating system). Since it seems contractors, we try to design sys- floor. most architects dislike the large tems that are between these ex- • Local code requirements for louvered skin characteristic of cen- tremes, balancing high system smoke control and removal. Many tral system mechanical floors, the quality (excellent comfort and flex- local building departments and fire minimal architectural impact of the ibility, low maintenance and energy districts require mechanical smoke floor-by-floor system can be one of costs) with first-cost considerations control and removal systems. While its strongest selling points. and budget constraints. We gener- the model code adopted by most The typical floor-by-floor system ally spend a great deal of design local jurisdictions offers alterna- mechanical room serving the typi- time laying out systems to minimize tives such as break-out windows, cal 15,000 to 25,000 sq ft office floor material costs and the number of one major jurisdiction in the San is about 18 by 12 ft, the former duct and pipe fittings. Sound traps Francisco Bay area disallows these dimension often being determined in ductwork are used only where options, thereby mandating me- by the adjacent stair or elevator shown by analysis to be necessary. chanical smoke control systems, core width. If the central minimum Pipe and duct systems are designed and many other building de- outside air ventilation system to minimize costs within noise, per- partments strongly encourage their serves no more than about 10 floors, formance, and serviceability con- use. the ventilation air shaft may often straints. Equipment capacities are This requirement can have a ma- be taken out of a corner of the room. matched to the calculated loads jor impact on system selection. The Otherwise, more area must be taken without large safety factors. (The central system with an air econo- from the floor. reader should be aware that much mizer essentially has built-in smoke Walls enclosing floor-by-floor of the cost data and comparisons removal and control capability be- fan rooms, unless adjacent to non- herein are a function of our compa- cause of its ability to supply 100 critical areas such as toilet rooms or ny's design philosophy; conclusions percent outside air and exhaust an stairwells, are generally required to may change radically for designers equivalent amount of smoke. Be- be acoustically treated in some with substantially different design cause the floor-by-floor system manner. Requirements vary de- approaches.) with water economizer does not in- pending on desired noise criteria HVAC systems

levels, but on most of our projects, central system shafts will exceed by-floor systems also generally have walls have been drywall construc- that required by floor-by-floor sys- factory-packaged controls, which tion with staggered or 6-in. studs tem mechanical rooms for buildings reduces control wiring costs. with batt insulation, several layers greater than 8 or 10 stories, based Plumbing costs for floor-by-floor of gypsum board, and an exposed on a typical floor area of 15,000 to systems will generally be higher acoustical lining on the interior 25,000 sq ft. For buildings smaller than those for central systems due face. Where mechanical room doors than 8 stories, the size of the central to the larger number of floor drains open to tenant areas, double solid - system penthouse becomes a less and condensate drains. However, core doors are usually provided, one significant factor architecturally the impact is generally small and acoustically rated opening into the compared to the increasing burden should not be a key factor in system room with the outside door selected of the floor-by-floor system me- selection. to match architectural finishes. chanical room space required. For While the space required by the that reason, as well as the increased Energy costs floor-by-floor system mechanical mechanical cost of a floor-by-floor Proponents of the floor-by-floor room is certainly not usable by ten- system for small buildings, central system often cite the reduced fan ants, ANSI Standard Z65.1 systems are generally favored for energy required (ostensibly due to (prepared by Building Owners and buildings less than 8 stories in the lack of central duct riser) as one Managers Association Inter- height. of the system's major assets. In fact, national) designates it as rentable For high-rise buildings over in most medium-velocity VAV ap- space. However, at least in the cur- about 15 stories, overall building plications, there should be little rent "office glut," many sophis- first costs will almost always favor difference in fan energy use be- ticated tenants will negotiate a the floor-by-floor system. While the tween the two systems. lower overall rental rate to com- cost of building acoustically treated The pressure drop in the duct pensate for such nonusable space. fan rooms is not insignificant, the riser is relatively small for a me- Shafts required for central sys- cost of the penthouse and/or dedi- dium-velocity system. Friction tems are considered nonrentable cated mechanical floors to house rates in large duct risers are usually space. Our rule of thumb for shaft central system equipment is almost relatively low (0.15 to 0.25 in. WG per area required by a central, single- always significantly higher. This 100 ft) due to velocity limitations. duct VAV system is about 1.3 sq ft cost difference generally grows with For a central system serving 15 sto- per 1000 sq ft of total conditioned the number of floors. Where a cen- ries, the additional pressure drop of area served by the fan system. This tral system may require a dedicated the riser and riser tap will be on the includes space for a medium veloc- mechanical floor for air handling order of 0.5 to 0.6 in. WG at design ity duct riser as well as for air return equipment every 15 or 30 stories, conditions. The added pressure back up the shaft. This is the shaft the floor-by-floor system will usu- drop on the return side is generally area required on the first few floors ally only require a single penthouse negligible due to very low velocities adjacent to the fan room; the shaft for the cooling tower, pumps, and in the return shaft. Unfortunately, is generally stepped back as floors boilers. many designers overestimate the drop off the system to maximize pressure drop of plenum return rentable space. Impaet on other trades systems and install large, inefficient One flexibility offered by central The impact on electrical costs of return fans that operate mostly at system duct shafts is that they may central versus floor-by-floor sys- low load. We prefer to use relief fans be strangely shaped. Floor-by-floor tems varies from project to project. in lieu of return fans. They are usu- mechanical rooms must generally The central system has fewer pieces ally less expensive and offer signifi- be rectangular to accommodate the of equipment to power and will al- cant energy savings since they oper- shape of the packaged air handler. most always have a lower total ate only for economizer relief, while Since the central system shaft area connected load due to building di- the more efficient supply fans pick is primarily used for return air, the versity, higher-efficiency cooling up the return air pressure drop shaft may be tucked in among other equipment, and fewer equipment when the economizer is locked out core elements resulting in L- selection "round-off" errors. in warm weather. shaped, trapezoidal, and even (Floor-by-floor system equipment But the floor-by-floor system curved shaft footprints. The return must be selected from an equip- usually has compensating pressure and supply portions of the shaft ment line with a limited selection of drops as well as lower fan effi- may also be separated if required by cooling capacities.) However, the ciencies. In most buildings, the the core configuration, although central equipment is most often floor-by-floor air handler discharge this usually results in a slight in- located high up in the building, arrangement is less than ideal due crease in space required. requiring longer runs of larger bus to space constraints, forcing the use Based on the rules of thumb sug- duct, while floor-by-floor system of a discharge plenum or sharply gested above, the area required by loads drop off at each floor. Floor- turning duct elbows and adding a Table 2—Comparison of equipment alternatives and costs for a build- Table 1—Comparison of equipment alternatives and costs for a building located in Foster City, Calif. (Project 2). ing located in San Francisco, Calif. (Project 1). Gross area 450,000 sq ft Gross area 550,000 sq ft Number of floors 17 office, 3 office/retail Number of floors 42 total, 33 of offices Area per office floor 20.000 sq ft (average) Area per office floor 14,000 sq ft (average) Type of structure Steel Type of structure Steel Floor-to-floor height 13 ft 6 in. with 9 ft ceilings Floor-to-floor height 13 ft 6 in. with 8 ft 9 in. ceilings Design conditions Design conditions Summer 84 F DB, 65 F WB Summer 79 F DB, &3 F WB Winter 35 F _ _ Winter 38 F Installed HVAC system Installed HVAC system Cooling system Cooling system Type 788 funs DX packages, 40 and 50 tons Type 900 tons—centrifugal water chillers Location Floors 4-21. each floor Location 33rd floor Heating system Heating system Type Central warm air VAV, gas boilers Type Hot water-gas-fired boilers Location Penthouse Location Penthouse (42nd floor) Fan system Fan system Type Forward-curved centrifugal, no return fans Type Airfoil centrifugal, supply and return Location Each floor Location 33rd floor, 1st basement Fan volume control Discharge dampers, (FC tans) Fan volume control Scroll dampers, 100 percent shut-off Type of economizer Water-side, closed circuit Type of economizer 100 percent outside air, dry bulb lockout Smallest operable unit One floor Control system Combination DDC and pneumatic Control system Combination electronic, D(DC, and pneumatic Smallest operable unit One floor Auxiliary cooling Extra condenser water loop capacity Auxiliary cooling 250 tons closed-circuit condenser water Interior zone control Shut-off VAV Interior zone control Shut-off VAV Exterior zone control Dual-duct VAV Exterior zone control VAV with hot water reheat HVAC costs HVAC costs Core $3.67 per sq ft Core $4.30 per sq ft Tenant $2.65 per sq ft (average) Tenant $3.10 per sq ft (average) Maintenance/repair costs .. . $0.08 per sq ft per yr, $0.68 per sq ft present worth Maintenance/repair costs .. . $0.06 per sq ft per yr, S0.50 per sq ft present worth HVAC energy costs $0.31 per sq ft per yr, $2.50 per sq ft present worth HVAC energy costs $0.26 per sq ft per yr. $2.10 per sq ft present worth Total present worth $9.50 per sq ft (12 pcrcent, 30 yr) Total present worth $10.00 per sq ft (12 percent, 30 yr) Alternate system proposal Alternate system proposal Cooling system Cooling system Type 720 tons—centrifugal water chillers Type 1000 ton DX packages, 30 and 40 tons Location Penthouse (21st floor) Location Each floor Heating system Heating system Type Central warm air VAV, gas boilers Type Hot water-gas-fired boilers Location Penthouse Location Penthouse (42nd floor) Fan system Fan system Type Airfoil centrifugal supply, propeller relief Type Forward-curved centrifugal, no return fans Location Penthouse Location Each fioor Fan volume control Variable-speed drive Fan volume control Inlet vanes Type of economizer 100 percent outside air, dry bulb lockout Type of economizer Water-side, open-circuit Control system Combination DDC and pneumatic Control system Combination electronic, DDC and pneumatic Smallest operable unit One floor Smallest operable unit One floor Auxiliary cooling Closed circuit HX on tower Auxiliary cooling Extra condenser water loop capacity Interior zone control Shut-off VAV Interior zone control Shut-off VAV Exterior zone control Dual-duct VAV Exterior zone control VAV with hot water reheat HVAC costs HVAC costs Core $4.01 persqft Core $4.32 per sq ft Tenant $2.65 per sq ft (average) Tenant $3.10 per sq ft (average) Maintenance/repair costs .. . $0.06 per sq ft per yr, $0.52 per sq ft present worth Maintenance/repair costs . . . $0.09 per sq ft per yr, $0.73 per sq ft present worth HVAC energy costs $0.24 per sq ft per yr, $1.93 per sq ft present worth HVAC energy costs $0.35 per sq ft per yr, $2.82 per sq ft present worth Added penthouse cost $0.28 persqft ($125,000) Total present worth ...... $10.97 per sq ft_(12 percent, 30 yr) Total present worth $9.39 persq ft (12 percent, 30 yr)_

"system effect" pressure drop typi- for factory-installed discharge tem itself is less than that of the cally on the order of 0.3 in. WG, but plenums. The water economizer central fan system. The typical as high as 1.0 in. WG, depending on precooling coil adds another 0.25 in. floor-by-floor air handler will in- discharge arrangement, according WG pressure drop. Finally, the effi- clude a forward-curved fan with to one manufacturer's catalog data ciency of the floor-by-floor fan sys- inlet vanes and a relatively small HVAC systems

Case studies 3 Project 1 is a 42-story structure in San Francisco (Thble 1). The building i consists of, from bottom to top, 3 floors of underground parking" and j mechanical space, 32 floors of offices, 1 floor for mechanical equipment, 7 I floors of condominiums, and 2 floors for elevator machinery, boilers, cooling | towers, and life-safety fans. | The installed HVAC system includes central centrifugal chillers and | central VAV fan systems on the 33rd floor and in the basement. Zone control ] is variable volume with hot water reheat at perimeter zones. A floor-by-I'loor | packaged water-cooled system was studied during the design phase, j The selection of a central system over the iloor-by-lloor system for this | building was related to higher perceived quality of the central chilled-water system, potential noise on the office floors with the lloor-by-floor system, | and the system types installed in competing buildings. The origiual esti- '] mated costs were similar for the two systems, but life-cycle cost consid- ! erations favored the central system. | The floor-by-floor system would have allowed recovery of most of the 33rd i floor for use as rentable office space, and the fan room area on each floor is | technically rentable space. But the fan rooms did not fit well architecturally j with other core elements, ultimately reducing core efficiency and increasing core costs. The problems with smoke control, ventilation, noise, and competitive similar buildings were considered more important than the small increase in rentable space in the slow rental market of the time. It is our opinion that the primary reason the owner finally chose the central system was the fact that most of the other top-quality buildings in San Francisco have central systems. (As an interesting aside, the owner made a similar decision regarding air distribution system: a VAV hot water reheat, system was selected over a less expensive and more energy-efficient double- duct VAV system primarily because no other major high rise in San Fran- cisco at that time had a double-duct system. The owner did not want to have to explain his system choice to prospective tenants.) By using the code-required smoke dampers to shut off air to unoccupied floors, the central system off-hour energy efficiency was competitive with that of the floor-by-floor design. Connecting the risers from the basement and the 33rd floor fan rooms with a 25 percent capacity duct allows any one of the four supply fans in the building to serve the entire building during off- hour periods. The chillers were not equipped with hot gas bypass, which has caused some minor problems in excessive chiller cycling during weekend off- hour operation; night and early morning partial-occupancy operation has not been a problem because the cool climate allows the economizer to handle the entire cooling load without the chillers.

(15 to 20 hp) motor, with an overall the floor-by-floor system. The pri- option.) static efficiency on the order of 45 to mary reason is the superior part Another reason for the better 50 percent (including inlet vane and load performance of the variable- part-load performance of the cen- belt drive and motor losses). The speed drive compared to the for- tral fan system is building diversity central fan system, consisting of an ward-curved fan with inlet vanes. between floors. For instance, some airfoil centrifugal fan with an AC (Variable-speed drives may also be floors may be near full load while frequency variable-speed drive and used on floor-by-floor fan systems, others may be only lightly loaded. It large high-efficiency motor, will but they are not as cost effective as is more efficient to operate a single have an overall efficiency of about when they are applied to large cen- fan at, say, 75 percent than to oper- 50 to 60 percent (including drive tral systems. We have found their ate two fans with one at 50 percent and motor losses). use in the floor-by-floor application and the other at 100 percent. This is Even with a somewhat higher to be uncommon at this time, but due to fan laws (the power required full-load fan power requirement, this may change as the cost of the is proportional to the cube of the the central VAV fan system may drives continues to fall and drives velocity) and to reduced fan system have lower annual energy costs than are included as a factory-installed efficiency at low loads. Project 2 is a 20 story building in Foster City, Calif., about 15 miles south of San Francisco (Table 2) The weather in Foster City is very mild but considerably warmer than San Francisco. The lower three floors ot the building are for office or retail tenants, while the upper floors are offices. The installed HVAC system serving the oflice areas is a dual-fan/dual-duct VAV system with floor- by-floor packaged water-cooled units with water- side economizers and a central roof-mounted heating supply air system. An alternative central fan system with centrifugal chillers was studied during the design phase. Cost and architectural factors favored the floor-b}-floor system lor this project. UBC 1807 smoke removal requirements could be met by modifying the central ducted VAV heating system for mechanical smoke removal. The include humidification capability. floor area and height of this building made it ideal lor the floor-by-floor The water economizer is essen- system from an initial-cost, standpoint. The typical floor area was within the tially an indirect evaporative optimum range for packaged units. cooler. Water is circulated through Mechanical costs were higher for the central system since it is generally not a cooling tower where it is evapo- efficient, to serve as many as 20 stories from a single fan room. The architec- ratively cooled, then circulated tural impact was also more significant—the central system duct shafts were through cooling coils to cool supply larger than the area required by floor-by-floor equipment rooms, and the air indirectly. Although water econ- penthouse space required for fan rooms significantly impacted building omizers can substantially reduce architectural appearance and first costs. cooling compressor energy, their The decision to use a floor-by-floor system primarily resulted from the overall efficiency is almost always following factors: o The original design scheme included floor-by-floor fan rooms, and the less than that of air-side econo- architectural changes required to accommodate a central system were mizers in the Bay area climate. This substantial. is primarily due to the parasitic o First costs were more important to the owner than annual energy and energy use of the cooling tower fans maintenance costs. and pumps, which operate almost o The owners like'floor-by-floor and felt it was easier to lease because of a continuously at full load whenever a perceived tenant preference for systems with individual floor units for the system is on, unlike the air econ- efficient off-hour operation. omizer, which provides truly "free" Although the system works well, there have been occasional packaged unit cooling during cool and cold failures. There is no redundancy on the individual floors; when a unit or its weather. Furthermore, because of control system fails, cooling capability is totally lost on the floor until the unit the heat exchange inefficiencies of is repaired. The units do have some redundancy with multiple compressors. Repair costs at Metro Center have not been as high as predicted by the the cooling tower and cooling coil, maintenance costs estimate. But in other of our buildings, the repair costs the water economizer will not gen- have been higher than predicted. erally provide as large a fraction of the cooling load as an air economizer, except in very dry, mild weather, which seldom occurs in this area due to the influence of the The ability of an HVAC system most HVAC designs use either air- bay. to take advantage of the San Fran- or water-side economizers to reduce For instance, at 55 F outside air cisco Bay area's very mild climate is cooling energy use. dry bulb temperature, an air econo- probably the single most significant In this climate, the most efficient mizer would provide the entire factor in determining overall energy type of economizer is the air-side cooling load for a VAV system with performance. The dry bulb tem- economizer. In some colder cli- a fixed 55 F supply temperature. perature around the Bay in a typi- mates, energy used for humid- Assuming a coincident wet bulb cal year is between 40 and 70 F ification systems can substantially temperature of 50 F (typical of the during more than 80 percent of a offset air economizer savings. But area), for the water economizer to typical building's operating hours. the weather is seldom cold in this be equally effective, the combined (This figure rises to about 92 per- area, and comfort problems due to tower leaving water temperature cent for San Francisco, the "air- very low relative humidity caused approach to wet bulb and cooling conditioned" city.) With such mild by cooling with outdoor air are vir- coil leaving air temperature ap- weather, heat recovery systems are tually nonexistent. Very few com- proach to tower water temperature generally not cost effective, and fort HVAC systems in this area would have to be 5 F or less. This is HVAC systems

generally not possible even with an outside air. This reduces the like- study buildings, with both floor- oversized tower and coils. (With lihood of tight building syndrome, a by-floor DX and central systems both air- and water-side econo- building malady caused by in- simulated using the DOE-2.1C mizers, we have found that it is creased concentrations of indoor air program along with a custom plant generally cost effective to reset contaminants that is becoming in- program used to model the water- supply air temperatures upwards in creasingly more common in today's side economizer (which the DOE these mild weather conditions to energy-efficient buildings. The program cannot do). The central prolong the time during which the causes of TBS are not generally system outperforms the floor-by- economizer can meet 100 percent of known, but the cure almost always floor system primarily due to the the load, as well as to reduce zonal involves increasing outdoor air superior performance of the air-side reheat losses, despite the resulting ventilation levels. This increase is economizer. increase in fan energy. A side bene- essentially built into systems with fit is an increase in air circulation air-side economizers. In addition, Off-hour energy costs rates, which can be uncomfortably the efficiency of air filters is typi- In the past, central systems were low with VAV systems at part load.) cally higher for central systems be- very inefficient at serving a par- The energy performance of the cause space constraints usually tially occupied building. If only a water economizer is very dependent preclude the use of bag filters on few floors were occupied, say during on system component design and floor-by-floor systems. (To be fair, late hours or over the weekend, the sizing and on how well the water it should be noted that we have yet system would have to condition the side of the system is maintained. to have a "sick" building even entire building. Floor-by-floor sys- The tower should be oversized com- among those with code-minimum tems are inherently more efficient pared to normal air conditioning outside air supplies and pleated fil- in this regard since they may serve duty to provide as close an ap- ters, but the added outside air in- each floor independently. proach to wet bulb temperatures as take capability of the air econo- To improve the efficiency of off- economically possible. The tower mizer and the high-efficiency fil- hour central system operation, vir- should be the type using propeller tering typical of central systems tually all of our recent central sys- fans (either draw-through or blow- certainly minimizes the likelihood tem designs have included the abil- through) rather than centrifugal of air quality problems and in- ity to shut off air supply to unoccu- blowers, unless absolutely pre- creases our design liability comfort pied areas or floors. (This capabil- cluded by acoustical concerns. level.) ity is required under the latest draft Propeller fans generally require A factor often overlooked in cal- of ASHRAE Standard 90.IP.) In half the horsepower of blower fans culations of floor-by-floor system most of our designs, this is accom- in tower applications due to the low energy usage is the use of hot gas plished by using smoke dampers at static pressures required. bypass, found as a standard feature supply duct shaft penetrations at Coil selection is also critical. In- of most units, to ensure frost-free, each floor. In some jurisdictions, creased economizer coil heat-trans- stable, low-load operation. This such dampers are required for life- fer effectiveness, which improves control essentially causes the unit's safety purposes, and we have been economizer cooling performance, energy demand to be constant at allowed to use them for off-hour must be balanced against increased the lowest step of unloading, even floor isolation, provided the fire- air-side pressure drop, which in- when much lower coil loads are ex- men's control of the dampers takes creases parasitic fan energy. Since perienced. For this reason, units precedence over any automatic an open-circuit tower is generally with more steps of unloading will be controls. For those buildings where used for highest cooling efficiency, more efficient at low loads. In- smoke dampers are not required, it is generally advisable to use me- credibly, some air conditioning the cost to convert shaft fire damp- chanically cleanable economizer units found on the market have ers (which are almost always re- coils for ease in maintenance, and large compressors with only one or quired in buildings over two stories) an automatic, well-maintained wa- two steps of unloading, resulting in to combination fire/smoke dampers ter treatment system is essential. extremely inefficient part-load (without end switches and fire- Besides reduced energy costs, operation. On the other extreme, men's override controls, which air-side economizers offer an addi- recently introduced units using should not be required in this case) tional advantage over water-side scroll compressors instead of recip- is typically very small. An alterna- economizers: increased outside air rocating compressors are able to tive approach for these buildings is intake. During more than 85 per- operate at low loads without the use to use normally closed VAV boxes cent of the building's operating of hot gas bypass, resulting in al- and control air EP switches to shut hours in the typical Bay area cli- most linear part-load performance, off air to unoccupied areas. New mate, the air economizer controls degraded only by cycling losses. VAV boxes with DDC controls al- will cause introduction of more low this type of off-hour control in Tables 1 and 2 summarize pre- software without additional hard- than code-minimum amounts of dicted energy use of the two case ware expenditures. damper for parallel fan systems. the office floors operate after hours The central system with off-hour The problem of chiller low-load each weeknight from 6 to 10 PM and isolation capability must be able to stability can often be solved by us- on Saturdays from 9 AM to 2 PM to operate stably at the very low loads ing multiple chillers staged as re- account approximately for system that might occur when only one quired by the load, provided the off-hour efficiency. floor or area is being served. Of minimum stable-load capability of primary concern are the fan system, the single chiller is on the order of Maintenance costs which may be forced to operate in expected off-hour loads. Where The central system has several unstable or surge portions of its lower loads can be expected for long large-expense maintenance items. characteristic fan curve, and the periods of time, hot gas bypass The chillers require both regular central water chillers, which may should be installed (typically on maintenance and a more extensive short cycle or shut off on safety only one compressor to reduce first regular inspection or teardown. devices if operated for long periods costs). Repair or replacement of one of the at low loads. Central systems with off-hour major pieces of equipment, such as Fan stability is a function of fan isolation capability, especially fans or chillers, due to a total failure type as well as the VAV duct pres- those with variable speed fan can be extremely expensive. sure control device. If large airfoil drives, can be very efficient at serv- The higher-efficiency bag filters centrifugal fans are used, inlet ing even a single floor at a time. It is we typically use on central systems vanes should not be used for pres- often difficult to convince owners are much more expensive than the sure control. Vanes leak signifi- and tenants of this, however—they pleated filters used in the floor-by- cantly at low loads, causing over- cannot imagine how the huge floor systems. However, they need pressurization of (and sometimes supply fans could not always re- to be replaced less frequently and, significant damage to) duct sys- quire huge amounts of electricity. as some filter manufacturers claim, tems. In addition, at low loads the In fact, the central system can be may actually cost less to maintain fan will almost certainly be forced more efficient than the floor-by- when labor costs are included. to operate in an unstable region of floor system in the off-hour mode. The floor-by-floor system main- its fan curve, often causing an an- This is partly due to the superior tenance and repair costs center noying pulsating rumble and vibra- performance of the air economizer upon the packaged water-cooled tion. For airfoil centrifugal fans, we but is also due to the energy con- units (as well as the cooling tower, have successfully used variable sumed by floor-by-floor system which is common to both central speed drives as well as variable cooling tower fans and pumps. and floor-by-floor systems). Com- scroll dampers, both of which have While they may comprise only 3 to 5 pressor failures and control failures proved to allow stable low-load percent of the total connected cool- are all too common in our experi- operation. ing equipment full-load electrical ence. On some of our more recent Variable speed drives offer the demand, the pumps are generally projects, we have started using additional advantages of very en- constant volume, which means screw and scroll compressor floor- ergy-efficient and incredibly quiet constant energy demand, and the by-floor units and are optimistic part-load operation. Although the tower fans run frequently to main- that they will prove to be more fan technically operates in surge at tain cold water for economizer oper- reliable than traditional recip- low loads, we have found it difficult ation. When serving only one floor rocating compressor units. Despite to detect in practice. (Our primary of, say, a 20-story building, pumps the increased likelihood of equip- fan supplier believes that this sta- and tower fans may use more energy ment failure with floor-by-floor bility will occur provided the fan than the floor air handler and com- equipment, the impact of a failure is static pressure control set point is pressor. generally less severe than that for a less than about 1.5 in. WG.) The cost On projects where a significant central system failure because only of variable speed drives has fallen to amount of off-hour operation is ex- one floor would be affected. This the point that they are easily life- pected, it may be cost effective to problem with central systems can cycle cost justified (less than 2 yr install an automatic shut-off valve be mitigated by using multiple or payback periods) versus inlet vanes at each floor-by-floor air condi- redundant primary equipment, for large fan systems (50 hp and tioning unit, interlocked to shut off such as multiple fans, chillers, and greater) and may be so for smaller flow to the unit when the unit is off. pumps. Despite the higher risk of system's as well. Pumps may then "ride the pump catastrophic failure of primary Variable scroll dampers do not curve" (i.e., run wild), or they may central system equipment, the provide the very efficient part-load be staged, variable speed driven, or overall life-cycle maintenance and operation of variable speed drives, a combination of the two to reduce replacement costs of the central but they do offer the advantage of energy usage. system are usually less than those of 100 percent fan shut-off, which may The energy cost estimates in Ta- the floor-by-floor system. serve as an effective backdraft bles 1 and 2 assume 10 percent of The maintenance/repair costs (or HVAC systems

guaranteed maintenance costs) building. But this perception ig- shown in Tables 1 and 2 are based nores the fact that large central on our experiences with current system equipment is generally in- projects. Obviously, it is difficult to herently more reliable, making predict the exact repair costs or the failure less likely. A properly de- time when a piece of equipment signed central system will also have must be replaced. The annual costs redundant equipment to minimize shown are those expected for the the impact of equipment failure. first few years of service; repair ex- This redundancy is impractical penses will increase as the system with floor-by-floor systems, for ages and repairs and replacement which equipment failure generally become more frequent. The costs means the entire loss of cooling for a assume maintenance by an in- given floor. house engineering staff, with major repairs and inspections by an out- Conclusions side service company. As with many design options, the choice between a floor-by-floor and Subjective factors a central system requires weighing Owner and general market per- advantages and disadvantages of ceptions of the relative merits of each system with the requirements floor-by-floor and central systems of the specific project. The system can be more significant in system choice can have a major impact on selection than the results of rig- the building's appearance, first orous design and cost studies. A costs, operating costs, and leas- chilled-water system is generally ability. perceived to be of significantly While each project is different, higher quality and reliability than a we have identified the following direct expansion system. We have trends for high-rise offices in our seen cases where major prospective area: tenants did not want to rent space • The overall building costs of a in a building because of potential central system will be less than reliability and noise problems those of a floor-by-floor system for associated with packaged DX re- buildings 8 to 10 stories and less. frigeration systems. (The improved Above about 15 stories, cost con- reliability and lower noise of the siderations generally favor floor- new screw and scroll compressors by-floor systems. may eventually change this percep- • The ideal floor size for a floor- tion, however.) by-floor system is between 15,000 A floor-by-floor system is gener- and 25,000 sq ft. Outside of this ally perceived to be easier and more range, a central system will proba- efficient to use on weekends and bly be less expensive. after hours—a key feature in mar- • Central systems are usually keting the building to increasingly less expensive to operate and main- energy-conscious tenants. But this tain than are floor-by-floor sys- perception may not be correct if the tems. central system includes the capa- • Central systems with off-hour bility for off-hour isolation of floors floor isolation capability should be and is properly designed for low- as efficient to operate as floor-by- load operation. However, reality floor systems for partial occupancy does not change the perception, and operation. this perception is often a factor in • The choice between a central the decision to use a floor-by-floor and a floor-by-floor system more system. often than not is based on subjec- The floor-by-floor system is also tive factors, such as biases of the perceived to reduce exposure to design team or perceived tenant equipment failure because failure of preferences, rather than on the re- a single component is unlikely to sults of rigorous design and cost affect more than one floor of the studies.