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

Among the major decisions an office building design team (owner, architect, engi-

neers, and contractors) must make is the selection of the HVAC sys-tem. There are many choices of primary cooling plants (e.g., chilled water, refrigerant direct expansion, ice storage, packaged, air-cooled, and water-cooled), primary heating plants (e.g., hydronic, steam, elec-tric, heat pump, and heat recovery), and air distribution systems (e.g., central, floor-by-floor, variable volume, constant volume, perime-ter reheat, dual duct, and fan-pow-ered box). The combinations of system and plant options, each with distinct advantages and disadvan-tages, are virtually infinite in num-ber.

This article addresses only one of the many system design decisions —the choice between a central sys-tem and a floor-by-floor system. We have found this to be among the most difficult design decisions be-cause of its impact on overall build-ing first costs, architectural appear-ance, building leasability, and fu-ture building operating costs.

By ROBERT G. LINFORD, PhD, PE, and STEVEN T. TAYLOR, PE, Lin ford Engineering Co., Oakland, Calif.

The decision to use a floor-by-floor or central system in a particu-lar building often depends more on the individuals involved than on the relative objective merits of the two systems. The owner, developer, or architect may have prejudices or perceptions of one or the other sys-tems based on their past experience with buildings they designed, built, or leased. In many cases, a decision is based on what has been done in nearby compet i t ive bui ld ings rather than what may really be the best choice for the project. Simi-larly, most engineers have a prefer-ence based on prior experience or perceived system merits.

The purpose of this article is to focus on some of the major factors that should be considered in select-ing the HVAC system for a particu-lar project. The factors that affect the initial mechanical costs, related building costs, and lifetime main-tenance and energy costs are very complex. As such, it is difficult to make generalizations or draw con-clusions that are applicable to all buildings, all designers, or all areas of the country. The primary cost factors, based on our experience, are discussed in general terms, then specifically as they applied to the system selection in two existing high-rise office buildings for which

we were the mechanical engineers and contractors. The reader should note t ha t our conclusions are strongly influenced by local condi-tions, such as the relatively mild and dry climate and the high cost of labor in our area. They also reflect our very practical and economical design approach to both central and floor-by-floor system designs.

There are many variations ot both central and floor-by-floor sys-tem concepts. This article focuses on what might be considered the extremes of each concept— the central system consists of a central fan system with an air economizer and a central chilled-water plant serving multiple floors of the build-ing; the floor-by-floor system, on the other hand, has distributed cooling and air handling equip-ment. Each floor of the building is served by separate air handling equipment packaged with a water-cooled direct expansion (DX) cool-ing plant and a water economizer precooling coil. The only central equipment is a cooling tower, tower water circulating pumps, and a central ventilation outside air fan system. While specifically directed toward the packaged floor-by-floor approach, many of our comments and generalizations also apply to other floor-by-floor system vari-

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a t i ons , such as f l oo r -by - f loo r chilled-water systems with air or tower-water economizers.

The factors that affect the HVAC system costs include both direct and indirect factors, such as engi-neering costs, installation costs, ar-chitectural impact costs, and costs for other construction trades.

Engineer ing costs In the design of any major build-

ing project, regardless of whether a central or floor-by-floor system is eventually selected, it is imperative that the engineer be involved early in the design process. This allows more time for a proper analysis of the various system choices. The en-gineer is also available to provide the necessary input into the archi-tectural layout of the core and pent-house to ensure tha t the design of the HVAC system can be effectively accommodated.

Analyzing and evaluating design options can consume a great deal of valuable engineering time. Often, the project schedule does not allow time for such careful study. Because the system selection needs to pre-cede the final architectural core and penthouse layouts, there is a great deal of pressure to finalize this decision quickly. These engineering costs and time pressures often com-pel the engineer to select the system with which he is most familiar. The engineer should try to convince the owner, developer, or architect that it is cost effective to pay the added engineering cost required to re-search system options carefully and then integrate the system into the architectural design.

Engineering costs and risk fac-tors for designing a packaged DX water-cooled floor-by-floor system are usually lower than those for a central system for the following reasons:

• The complex portions of the packaged floor-by-floor system are factory designed. The design of the refrigeration and air-side systems, cooling and fan capacity controls, and internal wiring become the re-sponsibility of the manufacturer,

not the engineer. • The equipment room layout is

simple and repeated multiple times, which saves design time, whereas cost-effective design of central fan system duct risers and penthouse layout can be very time consuming.

• Load calculations and corre-sponding equipment selection is less critical with packaged floor-by-floor systems because the mod-ular sizes of the floor units build an additional safety factor into the design.

Mechanica l instal lat ion costs The relative mechanical installa-

tion costs of floor-by-floor and cen-tral systems vary from project to project, with neither system prov-ing to be inherently less expensive than the other. The only way to really know which system is less expensive for a given building is to perform a detailed cost estimate of each design. If this is impractical, a careful study of the major factors t h a t impact first cost can often make the least expensive option apparent.

The major factors and trends we have noted in many of the projects we designed and installed in the past several years include:

• Number of floors. Because of the base cost of the central cooling tower and pumps, combined with the high cost for the packaged units, we have found tha t floor-by-floor systems are generally more ex-pensive than central systems for buildings less than 8 to 10 stories. For buildings above 12 to 15 floors, mechanical costs begin to favor floor-by-floor systems.

• Typical floor area and load. The cost per square foot of floor-by-f loor sys tems general ly de-creases as the floor area on each floor increases, with the optimum size in the 15,000 to 25,000 sq f t range. Above 25,000 sq ft, there is usually inadequate ceiling clear-ance available for a cost-effective duct distribution system radiating from a single unit, and the unit size required may be above 60 nominal tons, which is the extent of most

manufacturers' product lines. Below about 10,000 sq ft, first

costs for floor-by-floor systems are generally higher than for central systems due to the large number of small units. The installed cost of a 20-ton packaged unit is more than half t ha t of a 40-ton unit , even though the cost of the unit itself may be about half. Items such as controls, pipe and duct connec-tions, and start-up costs are more related to the number of units than to their size. Control costs are a major factor in a floor-by-floor sys-tem unless packaged controls are used. The cost for field-installed, direct digital controls (DDC) or pneumatic controls can be almost as much for each unit as for a single central air handling system. There-fore, to keep the floor-by-floor sys-tem cost reasonable, the number of units must be minimized and unit sizes must be as large as possible, with the optimum unit size in the 40 to 60 ton range.

Central system costs follow a similar trend, with costs per square foot falling as floor size increases but without the sudden cost impact as floor size falls outside the 15,000 to 25,000 sq f t optimal range. First-cost considerations will, therefore, generally favor central systems for buildings with typical floor areas outside this range.

• Space available in ceiling cavity for ducts. Ceiling cavity di-mensions and building structure will often limit the size of cost-ef-fective floor dis t r ibut ion ducts , which then limits the size of a single packaged unit on a floor. Round ducts are less expensive than rect-angular ducts, but they require more ceiling clearance for the same air quantity. As the available duct space decreases, the required aspect ratio of the duct increases, which increases its cost. In an extreme case, the difference can be $0.50 per sq f t or more between round and extremely shallow rectangular duct distribution systems.

Often, a shallow ceiling cavity problem can be solved most cost effectively by distributing air from

more than one point on each floor. This inherently favors central sys-tems, for which adding another cooling air supply riser from the central fan system would generally add much less cost than using mul-tiple, low-tonnage floor-by-floor packages.

• Cost effectiveness of the sys-tem design approach. Design phi-losophy and design criteria have a large impact on the cost of the sys-tem. For instance, one design may include multiple pieces of equip-ment for redundancy and standby capability while another may have only single pieces of equipment for each function (e.g., single boiler, pump, or chiller). One engineer may size a large, variable-volume duct riser for 4500 fpm while another may limit velocities to 1500 fpm. One design may call for a central system surrounded by 4-in. double-wall sheet metal plenums while an-other may use drywall enclosures where possible. All in all, the differ-ence between a very conservative design and an overly economical design can be several dollars per square foot.

As design/build engineers and contractors, we try to design sys-tems tha t are between these ex-t remes , ba lanc ing high sys tem quality (excellent comfort and flex-ibility, low maintenance and energy costs) with first-cost considerations and budget constraints. We gener-ally spend a great deal of design time laying out systems to minimize material costs and the number of duct and pipe fittings. Sound traps in ductwork are used only where shown by analysis to be necessary. Pipe and duct systems are designed to minimize costs within noise, per-formance, and serviceability con-straints. Equipment capacities are matched to the calculated loads without large safety factors. (The reader should be aware tha t much of the cost data and comparisons herein are a function of our compa-ny's design philosophy; conclusions may change radically for designers with substantially different design approaches.)

• Penthouse, shafts, and me-chanical room architectural con-straints. For a central system, the penthouse or mechanical floor lay-out has a significant impact on sys-tem cost. It is easy to spend many thousands of dollars adapting a mechanical system into an architec-turally pleasing penthouse that is inefficient from a mechanical point of view. Because of the large size and subsequent high cost of central system components, the designer must carefully lay out systems to minimize the space required and the number of duct and pipe fittings and offsets. The cost difference be-tween a poorly laid out system and a clever, economical approach can be as much as $1.00 per sq ft.

Although the impact of a poor a r c h i t e c t u r a l con f igu ra t i on on f l o o r - b y - f l o o r s y s t e m cos t s is smaller, a poorly laid out system can still add as much as $0.75 per sq ft. For example, duct distribution costs can be greatly increased if the mechanical room is located between obs t ruc t ions t h a t force all the supply and return air to leave and enter the room at one point or if the unit is not centrally located on the floor.

• Local code requirements for smoke control and removal. Many local building departments and fire districts require mechanical smoke control and removal systems. While the model code adopted by most local jurisdictions offers alterna-tives such as break-out windows, one major jurisdiction in the San Francisco Bay area disallows these options, thereby mandat ing me-chanical smoke control systems, a n d m a n y o t h e r b u i l d i n g de-partments strongly encourage their use.

This requirement can have a ma-jor impact on system selection. The central system with an air econo-mizer essentially has built-in smoke removal and control capability be-cause of its ability to supply 100 percent outside air and exhaust an equivalent amount of smoke. Be-cause the floor-by-floor system with water economizer does not in-

herently include this capability, d e d i c a t e d smoke e x h a u s t and makeup air systems must be added at a cost typically on the order of $0.50 per sq ft. In addition to the added cost, these dedicated systems require additional space, and they are seldom as effective at smoke re-moval as central systems because of generally poor makeup air distribu-tion. In jurisdictions where me-chanical smoke removal is required, cost considerations will usually fa-vor the central system.

Architectural impact One of the most compelling rea-

sons for selecting a floor-by-floor HVAC system is the reduced archi-tectural impact compared to a cen-tral system. While a central system would require at least a very large penthouse, and often both a pent-house and dedicated intermediate mechanical floors in buildings over 20 to 25 stories or so, the floor-by-floor system requires penthouse space only for a cooling tower, pumps, and outside air ventilation fans (and boilers, if required for the heating system). Since it seems most architects dislike the large louvered skin characteristic of cen-tral system mechanical floors, the minimal architectural impact of the floor-by-floor system can be one of its strongest selling points.

The typical floor-by-floor system mechanical room serving the typi-cal 15,000 to 25,000 sq f t office floor is about 18 by 12 ft, the former dimension often being determined by the adjacent stair or elevator core width. If the central minimum outs ide air ven t i l a t ion sys tem serves no more than about 10 floors, the ventilation air shaft may often be taken out of a corner of the room. Otherwise, more area must be taken from the floor.

Walls enclosing floor-by-floor fan rooms, unless adjacent to non-critical areas such as toilet rooms or stairwells, are generally required to be acoustically t rea ted in some manner. Requirements vary de-pending on desired noise criteria

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levels, but on most of our projects, walls have been drywall construc-tion with staggered or 6-in. studs with batt insulation, several layers of gypsum board, and an exposed acoustical lining on the interior face. Where mechanical room doors open to tenant areas, double solid -core doors are usually provided, one acoustically rated opening into the room with the outside door selected to match architectural finishes.

While the space required by the floor-by-floor system mechanical room is certainly not usable by ten-a n t s , A N S I S t a n d a r d Z65.1 (prepared by Building Owners and Manage r s Assoc ia t ion I n t e r -national) designates it as rentable space. However, at least in the cur-rent "office glut," many sophis-ticated tenants will negotiate a lower overall rental rate to com-pensate for such nonusable space.

Shafts required for central sys-tems are considered nonrentable space. Our rule of thumb for shaft area required by a central, single-duct VAV system is about 1.3 sq f t per 1000 sq ft of total conditioned area served by the fan system. This includes space for a medium veloc-ity duct riser as well as for air return back up the shaft. This is the shaft area required on the first few floors adjacent to the fan room; the shaft is generally stepped back as floors drop off the system to maximize rentable space.

One flexibility offered by central system duct shafts is that they may be strangely shaped. Floor-by-floor mechanical rooms must generally be rectangular to accommodate the shape of the packaged air handler. Since the central system shaft area is primarily used for return air, the shaft may be tucked in among other core e lements resul t ing in L-shaped, t rapezoidal , and even curved shaft footprints. The return and supply portions of the shaft may also be separated if required by the core configuration, although this usually results in a slight in-crease in space required.

Based on the rules of thumb sug-gested above, the area required by

central system shafts will exceed that required by floor-by-floor sys-tem mechanical rooms for buildings greater than 8 or 10 stories, based on a typical floor area of 15,000 to 25,000 sq ft. For buildings smaller than 8 stories, the size of the central system penthouse becomes a less significant factor architecturally compared to the increasing burden of the floor-by-floor system me-chanical room space required. For that reason, as well as the increased mechanical cost of a floor-by-floor system for small buildings, central systems are generally favored for buildings less than 8 stories in height.

For high-rise buildings over about 15 stories, overall building first costs will almost always favor the floor-by-floor system. While the cost of building acoustically treated fan rooms is not insignificant, the cost of the penthouse and/or dedi-cated mechanical floors to house central system equipment is almost always significantly higher. This cost difference generally grows with the number of floors. Where a cen-tral system may require a dedicated mechanical floor for air handling equipment every 15 or 30 stories, the floor-by-floor system will usu-ally only require a single penthouse for the cooling tower, pumps, and boilers.

Impaet on other trades The impact on electrical costs of

central versus floor-by-floor sys-tems varies from project to project. The central system has fewer pieces of equipment to power and will al-most always have a lower total connected load due to building di-versity, higher-efficiency cooling equipment, and fewer equipment se lec t ion " r o u n d - o f f " e r ro r s . (Floor-by-floor system equipment must be selected from an equip-ment line with a limited selection of cooling capacities.) However, the central equipment is most often located high up in the building, requiring longer runs of larger bus duct, while floor-by-floor system loads drop off at each floor. Floor-

by-floor systems also generally have factory-packaged controls, which reduces control wiring costs.

Plumbing costs for floor-by-floor systems will generally be higher than those for central systems due to the larger number of floor drains and condensate drains. However, the impact is generally small and should not be a key factor in system selection.

Energy costs Proponents of the floor-by-floor

system often cite the reduced fan energy required (ostensibly due to the lack of central duct riser) as one of the system's major assets. In fact, in most medium-velocity VAV ap-plications, there should be little difference in fan energy use be-tween the two systems.

The pressure drop in the duct riser is relatively small for a me-dium-velocity system. Friction rates in large duct risers are usually relatively low (0.15 to 0.25 in. WG per 100 ft) due to velocity limitations. For a central system serving 15 sto-ries, the additional pressure drop of the riser and riser tap will be on the order of 0.5 to 0.6 in. WG at design conditions. The added pressure drop on the return side is generally negligible due to very low velocities in the return shaft. Unfortunately, many designers overestimate the pressure drop of plenum return systems and install large, inefficient return fans that operate mostly at low load. We prefer to use relief fans in lieu of return fans. They are usu-ally less expensive and offer signifi-cant energy savings since they oper-ate only for economizer relief, while the more efficient supply fans pick up the return air pressure drop when the economizer is locked out in warm weather.

But the floor-by-floor system usually has compensating pressure drops as well as lower fan effi-ciencies. In most buildings, the floor-by-floor air handler discharge arrangement is less than ideal due to space constraints, forcing the use of a discharge plenum or sharply turning duct elbows and adding a

Table 1—Comparison of equipment alternatives and costs for a build-ing located in San Francisco, Calif. (Project 1).

G r o s s a r e a 5 5 0 , 0 0 0 s q f t N u m b e r of f l o o r s 4 2 t o t a l , 3 3 of o f f i c e s Area p e r o f f i c e f l oo r 1 4 , 0 0 0 sq f t ( a v e r a g e ) T y p e of s t r u c t u r e S t e e l F loo r - to - f l oo r h e i g h t 1 3 f t 6 in. wi th 8 f t 9 in. c e i l i ngs Des ign c o n d i t i o n s

S u m m e r 7 9 F DB, &3 F WB W i n t e r 3 8 F

Instal led HVAC s y s t e m Cool ing s y s t e m

T y p e 9 0 0 t o n s — c e n t r i f u g a l w a t e r ch i l l e r s L o c a t i o n 3 3 r d f loor

H e a t i n g s y s t e m T y p e Hot w a t e r - g a s - f i r e d b o i l e r s L o c a t i o n P e n t h o u s e ( 4 2 n d f l o o r )

Fan s y s t e m T y p e Airfoil c e n t r i f u g a l , s u p p l y a n d r e t u r n L o c a t i o n 3 3 r d f loor , 1 s t b a s e m e n t

Fan v o l u m e c o n t r o l Scro l l d a m p e r s , 1 0 0 p e r c e n t s h u t - o f f T y p e of e c o n o m i z e r 1 0 0 p e r c e n t o u t s i d e a i r , d r y b u l b l o c k o u t C o n t r o l s y s t e m C o m b i n a t i o n DDC a n d p n e u m a t i c S m a l l e s t o p e r a b l e u n i t One f l oo r Auxil iary coo l ing 2 5 0 t o n s c l o s e d - c i r c u i t c o n d e n s e r w a t e r In t e r i o r z o n e c o n t r o l Shu t -o f f VAV Ex te r io r z o n e c o n t r o l VAV wi th h o t w a t e r r e h e a t HVAC c o s t s

C o r e $ 4 . 3 0 pe r sq f t T e n a n t $ 3 . 1 0 p e r sq f t ( a v e r a g e )

M a i n t e n a n c e / r e p a i r c o s t s . . . $ 0 . 0 6 p e r sq f t p e r yr , S 0 . 5 0 p e r sq f t p r e s e n t w o r t h HVAC e n e r g y c o s t s $ 0 . 2 6 per sq ft per yr . $ 2 . 1 0 p e r sq f t p r e s e n t w o r t h T o t a l p r e s e n t w o r t h $ 1 0 . 0 0 p e r sq f t ( 1 2 p e r c e n t , 3 0 y r )

A l t e rna te s y s t e m p r o p o s a l Coo l ing s y s t e m

T y p e 1 0 0 0 t o n DX p a c k a g e s , 3 0 a n d 4 0 t o n s L o c a t i o n E a c h f loor

H e a t i n g s y s t e m T y p e Hot w a t e r - g a s - f i r e d b o i l e r s L o c a t i o n P e n t h o u s e ( 4 2 n d f l o o r )

F a n s y s t e m T y p e F o r w a r d - c u r v e d c e n t r i f u g a l , no r e t u r n f a n s L o c a t i o n E a c h f i o o r

F a n v o l u m e c o n t r o l Inlet v a n e s T y p e of e c o n o m i z e r W a t e r - s i d e , o p e n - c i r c u i t C o n t r o l s y s t e m C o m b i n a t i o n e l e c t r o n i c , DDC a n d p n e u m a t i c S m a l l e s t o p e r a b l e uni t One f loor Auxil iary coo l i ng Ex t ra c o n d e n s e r w a t e r loop c a p a c i t y In t e r io r z o n e c o n t r o l Shu t -o f f VAV E x t e r i o r z o n e c o n t r o l VAV wi th h o t w a t e r r e h e a t HVAC c o s t s

C o r e $ 4 . 3 2 p e r sq f t T e n a n t $ 3 . 1 0 p e r sq f t ( a v e r a g e )

M a i n t e n a n c e / r e p a i r c o s t s . . . $ 0 . 0 9 p e r s q f t p e r yr , $ 0 . 7 3 p e r sq f t p r e s e n t w o r t h HVAC e n e r g y c o s t s $ 0 . 3 5 p e r sq f t p e r yr , $ 2 . 8 2 p e r sq f t p r e s e n t w o r t h To ta l p r e s e n t w o r t h . . . . . . . $ 1 0 . 9 7 p e r sq f t _ ( 1 2 p e r c e n t , 3 0 y r )

Table 2—Comparison of equipment alternatives and costs for a build-ing located in Foster City, Calif. (Project 2).

G r o s s a r e a 4 5 0 , 0 0 0 s q f t N u m b e r of f l o o r s 17 o f f i c e , 3 o f f i c e / r e t a i l Area p e r o f f i c e f loor 2 0 . 0 0 0 sq f t ( a v e r a g e ) T y p e of s t r u c t u r e S t e e l F loor - to - f loo r h e i g h t 1 3 f t 6 in. wi th 9 f t ce i l ings Des ign c o n d i t i o n s

S u m m e r 8 4 F DB, 6 5 F WB W i n t e r 3 5 F _ _

Instal led HVAC s y s t e m Cool ing s y s t e m

Type 7 8 8 f u n s DX p a c k a g e s , 4 0 a n d 5 0 t o n s L o c a t i o n F loo r s 4 - 2 1 . e a c h f loor

H e a t i n g s y s t e m T y p e C e n t r a l w a r m air VAV, g a s b o i l e r s L o c a t i o n P e n t h o u s e

Fan s y s t e m T y p e F o r w a r d - c u r v e d c e n t r i f u g a l , no r e t u r n f a n s L o c a t i o n E a c h f l oo r

Fan v o l u m e c o n t r o l D i s c h a r g e d a m p e r s , (FC t a n s ) Type of e c o n o m i z e r W a t e r - s i d e , c l o s e d c i r cu i t S m a l l e s t o p e r a b l e un i t O n e f loor C o n t r o l s y s t e m C o m b i n a t i o n e l e c t r o n i c , D(DC, a n d p n e u m a t i c Auxil iary coo l i ng Ex t ra c o n d e n s e r w a t e r loop c a p a c i t y In te r io r z o n e c o n t r o l Shu t -o f f VAV Exte r io r z o n e c o n t r o l D u a l - d u c t VAV HVAC c o s t s

C o r e $ 3 . 6 7 p e r sq f t T e n a n t $ 2 . 6 5 pe r sq f t ( a v e r a g e )

M a i n t e n a n c e / r e p a i r c o s t s . . . $ 0 . 0 8 p e r sq f t p e r yr , $ 0 . 6 8 pe r sq f t p r e s e n t w o r t h HVAC e n e r g y c o s t s $ 0 . 3 1 p e r sq f t pe r yr , $ 2 . 5 0 pe r sq f t p r e s e n t w o r t h Tota l p r e s e n t w o r t h $ 9 . 5 0 p e r sq f t ( 1 2 p c r c e n t , 3 0 y r )

Al te rna te s y s t e m p r o p o s a l Cool ing s y s t e m

T y p e 7 2 0 t o n s — c e n t r i f u g a l w a t e r ch i l l e r s L o c a t i o n P e n t h o u s e ( 2 1 s t f l o o r )

H e a t i n g s y s t e m T y p e C e n t r a l w a r m air VAV, g a s b o i l e r s L o c a t i o n P e n t h o u s e

Fan s y s t e m T y p e Airfoil c e n t r i f u g a l s u p p l y , p r o p e l l e r relief L o c a t i o n P e n t h o u s e

Fan v o l u m e c o n t r o l V a r i a b l e - s p e e d d r i v e T y p e of e c o n o m i z e r 1 0 0 p e r c e n t o u t s i d e air , d ry b u l b l o c k o u t C o n t r o l s y s t e m C o m b i n a t i o n DDC a n d p n e u m a t i c S m a l l e s t o p e r a b l e un i t O n e f l o o r Auxil iary coo l i ng Closed c i r cu i t HX on t o w e r In te r io r z o n e c o n t r o l Shu t -o f f VAV Exte r io r z o n e c o n t r o l D u a l - d u c t VAV HVAC c o s t s

C o r e $ 4 . 0 1 p e r s q f t T e n a n t $ 2 . 6 5 p e r sq f t ( a v e r a g e )

M a i n t e n a n c e / r e p a i r c o s t s . . . $ 0 . 0 6 p e r sq f t p e r yr , $ 0 . 5 2 p e r sq f t p r e s e n t w o r t h HVAC e n e r g y c o s t s $ 0 . 2 4 p e r sq ft p e r yr , $ 1 . 9 3 p e r sq f t p r e s e n t w o r t h A d d e d p e n t h o u s e c o s t $ 0 . 2 8 p e r s q f t ( $ 1 2 5 , 0 0 0 ) To ta l p r e s e n t w o r t h $ 9 . 3 9 p e r s q f t ( 1 2 p e r c e n t , 3 0 y r ) _

"system effect" pressure drop typi-cally on the order of 0.3 in. WG, but as high as 1.0 in. WG, depending on discharge arrangement, according to one manufacturer's catalog data

for fac tory- ins ta l led discharge plenums. The water economizer precooling coil adds another 0.25 in. WG pressure drop. Finally, the effi-ciency of the floor-by-floor fan sys-

tem itself is less than that of the central fan system. The typical floor-by-floor air handler will in-clude a forward-curved fan with inlet vanes and a relatively small

HVAC systems

Case studies 3

P r o j e c t 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 com-petitive 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 static efficiency on the order of 45 to 50 percent (including inlet vane and belt drive and motor losses). The central fan system, consisting of an airfoil centrifugal fan with an AC frequency variable-speed drive and large high-efficiency motor, will have an overall efficiency of about 50 to 60 percent (including drive and motor losses).

Even with a somewhat higher full-load fan power requirement, the central VAV fan system may have lower annual energy costs than

the floor-by-floor system. The pri-mary reason is the superior part load performance of the variable-speed drive compared to the for-ward-curved fan with inlet vanes. (Variable-speed drives may also be used on floor-by-floor fan systems, but they are not as cost effective as when they are applied to large cen-tral systems. We have found their use in the floor-by-floor application to be uncommon at this time, but this may change as the cost of the drives continues to fall and drives are included as a factory-installed

option.) Another reason for the better

part-load performance of the cen-tral fan system is building diversity between floors. For instance, some floors may be near full load while others may be only lightly loaded. It is more efficient to operate a single fan at, say, 75 percent than to oper-ate two fans with one at 50 percent and the other at 100 percent. This is due to fan laws (the power required is proportional to the cube of the velocity) and to reduced fan system efficiency at low loads.

Projec t 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 heat-ing 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 floor area and height of this building made it ideal lor the floor-by-floor system from an initial-cost, standpoint. The typical floor area was within the optimum range for packaged units. Mechanical costs were higher for the central system since it is generally not efficient, to serve as many as 20 stories from a single fan room. The architec-tural impact was also more significant—the central system duct shafts were larger than the area required by floor-by-floor equipment rooms, and the penthouse space required for fan rooms significantly impacted building architectural appearance and first costs.

The decision to use a floor-by-floor system primarily resulted from the following factors:

o The original design scheme included floor-by-floor fan rooms, and the architectural changes required to accommodate a central system were substantial.

o First costs were more important to the owner than annual energy and maintenance costs.

o The owners like'floor-by-floor and felt it was easier to lease because of a a perceived tenant preference for systems with individual floor units for efficient off-hour operation. Although the system works well, there have been occasional packaged unit failures. There is no redundancy on the individual floors; when a unit or its control system fails, cooling capability is totally lost on the floor until the unit 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 maintenance costs estimate. But in other of our buildings, the repair costs have been higher than predicted.

The ability of an HVAC system to take advantage of the San Fran-cisco Bay area's very mild climate is probably the single most significant factor in determining overall energy performance. The dry bulb tem-perature around the Bay in a typi-cal year is between 40 and 70 F during more than 80 percent of a typical building's operating hours. (This figure rises to about 92 per-cent for San Francisco, the "air-conditioned" city.) With such mild weather, heat recovery systems are generally not cost effective, and

most HVAC designs use either air-or water-side economizers to reduce cooling energy use.

In this climate, the most efficient type of economizer is the air-side economizer. In some colder cli-mates, energy used for humid-ification systems can substantially offset air economizer savings. But the weather is seldom cold in this area, and comfort problems due to very low relative humidity caused by cooling with outdoor air are vir-tually nonexistent. Very few com-fort HVAC systems in this area

inc lude humid i f i ca t ion capabil i ty. The water economizer is essen-

t ial ly an ind i rec t evaporat ive cooler. Water is circulated through a cooling tower where it is evapo-ratively cooled, then circulated through cooling coils to cool supply air indirectly. Although water econ-omizers can substantially reduce cooling compressor energy, their overall efficiency is almost always less than that of air-side econo-mizers in the Bay area climate. This is primarily due to the parasitic energy use of the cooling tower fans and pumps, which operate almost continuously at full load whenever the system is on, unlike the air econ-omizer, which provides truly "free" cooling d u r i n g cool and cold weather. Furthermore, because of the heat exchange inefficiencies of the cooling tower and cooling coil, the water economizer will not gen-erally provide as large a fraction of the cooling load as an air econo-mizer, except in very dry, mild weather, which seldom occurs in this area due to the influence of the bay.

For instance, at 55 F outside air dry bulb temperature, an air econo-mizer would provide the entire cooling load for a VAV system with a fixed 55 F supply temperature. Assuming a coincident wet bulb temperature of 50 F (typical of the area), for the water economizer to be equally effective, the combined tower leaving water temperature approach to wet bulb and cooling coil leaving air temperature ap-proach to tower water temperature would have to be 5 F or less. This is

HVAC systems

generally not possible even with an oversized tower and coils. (With both air- and water-side econo-mizers, we have found that it is generally cost effective to reset supply air temperatures upwards in these mild weather conditions to prolong the time during which the economizer can meet 100 percent of the load, as well as to reduce zonal reheat losses, despite the resulting increase in fan energy. A side bene-fit is an increase in air circulation rates, which can be uncomfortably low with VAV systems at part load.)

The energy performance of the water economizer is very dependent on system component design and sizing and on how well the water side of the system is maintained. The tower should be oversized com-pared to normal air conditioning duty to provide as close an ap-proach to wet bulb temperatures as economically possible. The tower should be the type using propeller fans (either draw-through or blow-through) rather than centrifugal blowers, unless absolutely pre-cluded by acoustical concerns. Propeller fans generally require half the horsepower of blower fans in tower applications due to the low static pressures required.

Coil selection is also critical. In-creased economizer coil heat-trans-fer effectiveness, which improves economizer cooling performance, must be balanced against increased air-side pressure drop, which in-creases parasitic fan energy. Since an open-circuit tower is generally used for highest cooling efficiency, it is generally advisable to use me-chanically cleanable economizer coils for ease in maintenance, and an automatic, well-maintained wa-ter treatment system is essential.

Besides reduced energy costs, air-side economizers offer an addi-tional advantage over water-side economizers: increased outside air intake. During more than 85 per-cent of the building's operating hours in the typical Bay area cli-mate, the air economizer controls will cause introduction of more than code-minimum amounts of

outside air. This reduces the like-lihood of tight building syndrome, a building malady caused by in-creased concentrations of indoor air contaminants that is becoming in-creasingly more common in today's energy-efficient buildings. The causes of TBS are not generally known, but the cure almost always involves increasing outdoor air ventilation levels. This increase is essentially built into systems with air-side economizers. In addition, the efficiency of air filters is typi-cally higher for central systems be-cause space constraints usually preclude the use of bag filters on floor-by-floor systems. (To be fair, it should be noted that we have yet to have a "sick" building even among those with code-minimum outside air supplies and pleated fil-ters, but the added outside air in-take capability of the air econo-mizer and the high-efficiency fil-tering typical of central systems certainly minimizes the likelihood of air quality problems and in-creases our design liability comfort level.)

A factor often overlooked in cal-culations of floor-by-floor system energy usage is the use of hot gas bypass, found as a standard feature of most units, to ensure frost-free, stable, low-load operation. This control essentially causes the unit's energy demand to be constant at the lowest step of unloading, even when much lower coil loads are ex-perienced. For this reason, units with more steps of unloading will be more efficient at low loads. In-credibly, some air conditioning units found on the market have large compressors with only one or two steps of unloading, resulting in extremely inefficient part-load operation. On the other extreme, recently introduced units using scroll compressors instead of recip-rocating compressors are able to operate at low loads without the use of hot gas bypass, resulting in al-most linear part-load performance, degraded only by cycling losses.

Tables 1 and 2 summarize pre-dicted energy use of the two case

study buildings, with both floor-by-floor DX and central systems simulated using the DOE-2.1C program along with a custom plant program used to model the water-side economizer (which the DOE program cannot do). The central system outperforms the floor-by-floor system primarily due to the superior performance of the air-side economizer.

Off-hour energy costs In the past, central systems were

very inefficient at serving a par-tially occupied building. If only a few floors were occupied, say during late hours or over the weekend, the system would have to condition the entire building. Floor-by-floor sys-tems are inherently more efficient in this regard since they may serve each floor independently.

To improve the efficiency of off-hour central system operation, vir-tually all of our recent central sys-tem designs have included the abil-ity to shut off air supply to unoccu-pied areas or floors. (This capabil-ity is required under the latest draft of ASHRAE Standard 90.IP.) In most of our designs, this is accom-plished by using smoke dampers at supply duct shaft penetrations at each floor. In some jurisdictions, such dampers are required for life-safety purposes, and we have been allowed to use them for off-hour floor isolation, provided the fire-men's control of the dampers takes precedence over any automatic controls. For those buildings where smoke dampers are not required, the cost to convert shaft fire damp-ers (which are almost always re-quired in buildings over two stories) to combination fire/smoke dampers (without end switches and fire-men's override controls, which should not be required in this case) is typically very small. An alterna-tive approach for these buildings is to use normally closed VAV boxes and control air EP switches to shut off air to unoccupied areas. New VAV boxes with DDC controls al-low this type of off-hour control in software without additional hard-

ware expenditures. The central system with off-hour

isolation capability must be able to operate stably at the very low loads that might occur when only one floor or area is being served. Of primary concern are the fan system, which may be forced to operate in unstable or surge portions of its characteristic fan curve, and the central water chillers, which may short cycle or shut off on safety devices if operated for long periods at low loads.

Fan stability is a function of fan type as well as the VAV duct pres-sure control device. If large airfoil centrifugal fans are used, inlet vanes should not be used for pres-sure control. Vanes leak signifi-cantly at low loads, causing over-pressurization of (and sometimes significant damage to) duct sys-tems. In addition, at low loads the fan will almost certainly be forced to operate in an unstable region of its fan curve, often causing an an-noying pulsating rumble and vibra-tion. For airfoil centrifugal fans, we have successfully used variable speed drives as well as variable scroll dampers, both of which have proved to allow stable low-load operation.

Variable speed drives offer the additional advantages of very en-ergy-efficient and incredibly quiet part-load operation. Although the fan technically operates in surge at low loads, we have found it difficult to detect in practice. (Our primary fan supplier believes that this sta-bility will occur provided the fan static pressure control set point is less than about 1.5 in. WG.) The cost of variable speed drives has fallen to the point that they are easily life-cycle cost justified (less than 2 yr payback periods) versus inlet vanes for large fan systems (50 hp and greater) and may be so for smaller system's as well.

Variable scroll dampers do not provide the very efficient part-load operation of variable speed drives, but they do offer the advantage of 100 percent fan shut-off, which may serve as an effective backdraf t

damper for parallel fan systems. The problem of chiller low-load

stability can often be solved by us-ing multiple chillers staged as re-quired by the load, provided the minimum stable-load capability of the single chiller is on the order of expected off-hour loads. Where lower loads can be expected for long periods of time, hot gas bypass should be installed (typically on only one compressor to reduce first costs).

Central systems with off-hour isolation capability, especially those with variable speed fan drives, can be very efficient at serv-ing even a single floor at a time. It is often difficult to convince owners and tenants of this, however—they cannot imagine how the huge supply fans could not always re-quire huge amounts of electricity.

In fact, the central system can be more efficient than the floor-by-floor system in the off-hour mode. This is partly due to the superior performance of the air economizer but is also due to the energy con-sumed by floor-by-floor system cooling tower fans and pumps. While they may comprise only 3 to 5 percent of the total connected cool-ing equipment full-load electrical demand, the pumps are generally constant volume, which means constant energy demand, and the tower fans run frequently to main-tain cold water for economizer oper-ation. When serving only one floor of, say, a 20-story building, pumps and tower fans may use more energy than the floor air handler and com-pressor.

On projects where a significant amount of off-hour operation is ex-pected, it may be cost effective to install an automatic shut-off valve at each floor-by-floor air condi-tioning unit, interlocked to shut off flow to the unit when the unit is off. Pumps may then "ride the pump curve" (i.e., run wild), or they may be staged, variable speed driven, or a combination of the two to reduce energy usage.

The energy cost estimates in Ta-bles 1 and 2 assume 10 percent of

the office floors operate after hours each weeknight from 6 to 10 PM and on Saturdays from 9 AM to 2 PM to account approximately for system off-hour efficiency.

Maintenance costs The central system has several

large-expense maintenance items. The chillers require both regular maintenance and a more extensive regular inspection or teardown. Repair or replacement of one of the major pieces of equipment, such as fans or chillers, due to a total failure can be extremely expensive.

The higher-efficiency bag filters we typically use on central systems are much more expensive than the pleated filters used in the floor-by-floor systems. However, they need to be replaced less frequently and, as some filter manufacturers claim, may actually cost less to maintain when labor costs are included.

The floor-by-floor system main-tenance and repair costs center upon the packaged water-cooled units (as well as the cooling tower, which is common to both central and floor-by-floor systems). Com-pressor failures and control failures are all too common in our experi-ence. On some of our more recent projects, we have started using screw and scroll compressor floor-by-floor units and are optimistic that they will prove to be more reliable than traditional recip-rocating compressor units. Despite the increased likelihood of equip-ment failure with floor-by-floor equipment, the impact of a failure is generally less severe than that for a central system failure because only one floor would be affected. This problem with central systems can be mitigated by using multiple or redundant primary equipment, such as multiple fans, chillers, and pumps. Despite the higher risk of catastrophic failure of primary central system equipment, the overall life-cycle maintenance and replacement costs of the central system are usually less than those of the floor-by-floor system.

The maintenance/repair costs (or

HVAC systems

guaranteed maintenance costs) shown in Tables 1 and 2 are based on our experiences with current projects. Obviously, it is difficult to predict the exact repair costs or the time when a piece of equipment must be replaced. The annual costs shown are those expected for the first few years of service; repair ex-penses will increase as the system ages and repairs and replacement become more frequent. The costs assume maintenance by an in-house engineering staff, with major repairs and inspections by an out-side service company.

Subjective factors Owner and general market per-

ceptions of the relative merits of floor-by-floor and central systems can be more significant in system selection than the results of rig-orous design and cost studies. A chilled-water system is generally perceived to be of significantly higher quality and reliability than a direct expansion system. We have seen cases where major prospective tenants did not want to rent space in a building because of potential reliability and noise problems associated with packaged DX re-frigeration systems. (The improved reliability and lower noise of the new screw and scroll compressors may eventually change this percep-tion, however.)

A floor-by-floor system is gener-ally perceived to be easier and more efficient to use on weekends and after hours—a key feature in mar-keting the building to increasingly energy-conscious tenants. But this perception may not be correct if the central system includes the capa-bility for off-hour isolation of floors and is properly designed for low-load operation. However, reality does not change the perception, and this perception is often a factor in the decision to use a floor-by-floor system.

The floor-by-floor system is also perceived to reduce exposure to equipment failure because failure of a single component is unlikely to affect more than one floor of the

building. But this perception ig-nores the fact that large central system equipment is generally in-herently more reliable, making failure less likely. A properly de-signed central system will also have redundant equipment to minimize the impact of equipment failure. This redundancy is impractical with floor-by-floor systems, for which equipment failure generally means the entire loss of cooling for a given floor.

Conclusions As with many design options, the

choice between a floor-by-floor and a central system requires weighing advantages and disadvantages of each system with the requirements of the specific project. The system choice can have a major impact on the building's appearance, first costs, operating costs, and leas-ability.

While each project is different, we have identified the following trends for high-rise offices in our area:

• The overall building costs of a central system will be less than those of a floor-by-floor system for buildings 8 to 10 stories and less. Above about 15 stories, cost con-siderations generally favor floor-by-floor systems.

• The ideal floor size for a floor-by-floor system is between 15,000 and 25,000 sq ft. Outside of this range, a central system will proba-bly be less expensive.

• Central systems are usually less expensive to operate and main-tain than are floor-by-floor sys-tems.

• Central systems with off-hour floor isolation capability should be as efficient to operate as floor-by-floor systems for partial occupancy operation.

• The choice between a central and a floor-by-floor system more often than not is based on subjec-tive factors, such as biases of the design team or perceived tenant preferences, rather than on the re-sults of rigorous design and cost studies.

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