BN-97-13-3 AIVC 10867 Situations to Consider When Variable Air Volume Is an Option

Mark D. Kukla Associate Member ASHRAE

ABSTRACT factory settings are not dependable and usually must be reset in the field. Just setting the proper flow at the VAV box does Variable-air-volume systems utilize some ofthe latest tech­ not mean that the diffusers are balanced (or even hooked up). nology available to control wne temperatures and save Testing and balancing of all diffusers is needed. energy. In fact, the energy savings can be quite substantial. Then why do engineers and building managers have reservations WHEN IS VAV THE PROPER APPLICATION? about using this type of system? Probably the memory ofpast experiences. Room volume in conjunction with the required determine minimum airflow needed for the The chain ofdesign, equipment selection, installation, and space. If this number is close to the maximum airflow needed air balancing must be tailored to a building '.s needs or conflicts at full load, then VAV is not prudent. The same holds true if will arise. There are many disciplines to consider when creating loads are fairly constant. Ifinstalled under either condition, the a VAV system, and if one link is weak, there can be trouble. Are VAV box would never modulate to a reduced airflow. more complicated systems the price we have to pay for saving The lower the minimum airflow, the greater the savings. energy? But how low can you go? Many designers go all the way to EVOLUTION OF VAV zero. This is a real saving but controversial from an standpoint. Is enough of a load created when occupants Variable-air-volume systems have come a long way since are present? After all, proper air changes are usually only their inception. This has been driven by the advancements in needed when space is occupied. the pneumatic and DDC control industry. Pressure-indepen­ If electric reheat is necessary, VAV might not be practical dent VAV boxes that automatically compensate for changes in unless the coil is offset from the primary airstream, as in a fan­ static pressure have made pressure-dependent boxes powered VAV box. This is because electric coils usually need virtually obsolete. Lower duct static pressure is needed for the at least 500 fpm face velocity (2.54 mis) to prevent overheat­ modem VAV box. More sophisticated transducers and ing. computer software have achieved a more precise control of air If precise room pressurization control is necessary, it gets volumes. more complicated. VAV return or exhaust is needed to track Above all, falling prices for variable-frequency drives the supply. In critical spaces such as laboratories, the controls (VFDs) have unleashed a premier energy-saving technology. and air balancing must be very precise to prevent room pres­ What used to be a luxury is now the norm. Fan volume control sure swings. is much more precise with a VFD. The fan runs quieter than If precise control is required, VAV makes with a vortex or system . Static regain design of duct­ controlling humidity more difficult due to the dynamics of work has also reduced fluctuating duct pressure and noise. continual airflow variations. Best results occur when special Proper testing and balancing is now specified more often. modulating humidity controls are used. Duct modulating-type This is the final link that can either make or break the system. humidity sensors along with an integrating device that resets VAV systems are by no means "self-balancing." In fact, they the output of the are recommended. This allows the are quite the opposite. There is more to go awry and numerous duct sensor to quickly respond to a quick rise in duct humidity components that must work together in harmony. Even the caused by reduced air volume. This arrangement, commonly

Mark D. Kukla is vice president of Air Systems Balancing & Testing, Rochester, NY.

THIS PREPRINT IS FOR DISCUSSION PURPOSES ONLY, FOR INCLUSION IN ASHRAE TRANSACTIONS 1997, V. 103, Pt. 2. Not to be reptlnted in whole or in part without written permission of the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 1791 Tullle Circle, NE, Atlanta, GA 30329. Opinions, findings, conclusions, or recommendations expressed In this paper are thoae of the author(s) and do not necessarily reflect the views of ASHRAE. Written questions and comments regarding this paper should be received at ASHRAE no later than July 18, 1997. referred to as "anticipating control," allows the duct sensor to 1':.' ,. override the space and provide uniform controL . : To benefit from the energy savings of variable-frequency .. drives, the air-handling system needs to be of moderate capac­ ·~ ity. The larger the fan motor, the.lower the minimum energy .. draw percentage. Motors in the range of 20 to 100 hp generally .. can go down to 35%-40% offull-load amps. Motors froni 5 to 20 hp can generally reduce to 40%-60% of FLA and from 1 to 5 hp to around 60%-80%. Slowing down a small motor does r·• not save as much energy as slowing down a large one.•; ~ .. '• There is no doubt that VAV systems save energy.when ... compared to constant-volume systems. All zones of a building " generally do not reach their peak loads simultaneously· and, " therefore, will never need a fan running at total capacity. ... r·~nn~t~n1--un111mf" 1.!UQtrmQ rnn f"nniin11t\11t.1l\/ uT T}H-"'\.U• f-"'Vf"P\.!- ...... ,...... _. ..,.J ..,. .. - ...... ~- ...... _ ...... -~-~ ..J -- ...... _..., __4 ___ ..., ll:DOAM , Noon :S:DOPM l:OOPM sively high air volumes and waste a great amount of energy. 1 On average, a VAV system runs at 60% of full load during Figure 2 Actual maximum occupied simultaneous load vs. ,,, • . , occupancy operation. This can translate to generous energy design load. 1 savings when variable-frequency drives.are installed because for the pressure loss between the sensor and the farthest tl!rmi­ the fan electrical energy decreases with the fan volume nal. Locating it at the end of the ductwork or too close ~o an ~i""i""nrrlinn,tn thP. r•11hP f!.tn hn11 . :1 isolated terminal with local pressure swings will cause a ripple ---~·- .... ow -·- ---- ·-· ·-· .. Since the total of all peak loads wiH never occur at the' effect throughout the system as the VA:Ydampcr modulates. same time, the sum of all the peaks will.never bt:: needed and, There are a· handful· of methods for 'controlling system therefore, the equipment does not need to be sized Lhal largt:. static pressure·. Some work better than: others. There is a diversity.factor that permits the si;i:e ofrthe cooling, 1. System Bypass-a relief damp~r opens and dumps excess heating, and air-distribution equipment to be selected on lower air into' the return dl.lct br plenum. With this type of c6tit:rbl, actual maximum occupied simultaneous loads rather than a fan motor energy savings are nonexistent. This low-budget higher design load, The diversity factor can be 70% to 80% in ~ys(em will control space temperatures properly, but the many situations. relief damper is usually a source·df air leakage and noise. Existing constant-v1;:1lume systems can be retrofi~ed to This type of control is still p;opular for small office sys'fems variable air volume and beadded to.without increasing the size and DX coil units where constant coil velocities must 'be of the central .equipment. The diversity of VAY· makes this maintained in the air-handling unit. possible (see Figures 1 and 2). . • 1" 2. DisCbarge , u~peµari . aut!'mati~ .. , ~~per at . fan discharge. Only used on forward-curved fans. They are riot SYSTEM STATIC PRESSURE CONTROL as efficient as a vortJx d~per .ind. ate a sotirc~j of noi~e. Static pressure control is needed to prev~nt overpressfii'.3 increased . pressilre drop,.' and system effect. :Discharge izing the duct system ·and is the key to energy, ~avings.· The dampers are rnrely usel:1 anyn1'ori£ 1 ·~1 '.,:' ' · · ·· 1.; ;· 1 sensor should be located about 75% of the di.stance· from .the' '· r,,• • 1 ~t 1:· J 1 .; ,. l ! ~ '. T,__~~' ,.,·-)\ 3.i::, yQqe~ qp~peJ"-

2 BN-97-13'3 method of all. Inverter maintains motor efficiency regard­ This can lead to job delays, animosity between the balanc­ less of rpm or horsepower. The motors do not lose their ing and control contractors, and to a system that has multiple power factor and stay at about 95%. Some VFDs may cause shortcomings or•just does not work. New products can be a electrical noise that can be filtered out with an isolation hea4ache. Generally, the DDC hardware and software that transformer. have not been radically changed in a while prove to be the most VPDs have,pecome more reliable in recent years and are reliablen , less sensitive io;!'ower surges. They still can tfip off, so it is not - There· are many different VAV control vendors, and their a good idea to .~ e them for critical systems such as fume hood produots!are continually being updated, so the balancing tech­ exhaust, unlesflr.~ an alarm system is included. In situations nician often needs assistance to start the job. The necessary where consta" . stack velocity is required, outside .air inlet hardware and software also need to be provided. This should dampers are u s ~H instead of varying the fan speed. · be reflec_ted in the control contractor's job specification. Some balanCing; tlontractors now own laptop computers that can Prices foi:g;~FDs haye dropped dramatically and a.re now interface with a growing number of systems, but control only about 2~ more than a vortex damper system. The contr.aptors are reluctant to loan out proprietary software. system still must be large enough to make the additional investment worthwhile. Figum•3 shows annual energy use DDC VAVs have not changed the need for air balancing. with dif{erent flpw-throttling techniques. Even though the computer screen may read 1262.5 cfm (595.5 \ ·.\ l":. .• . • 1 .·, '1!_\ t') ; Lis), the m;:tual airfloW'could be several hundred cfm higher or ·-r: CONTROL EQUIPMENT lowen This happens ..quite< ifrequently because of the duct • .•.. (j . . ·, _, !·:. configuration, transducer accuracy, or program writers who DOC takes Over .,, just ballpark the flow coefficient or make :errors. There are .. ; • ' " · .i literally hundreds of lines of text:, and one tiny mistake can . . IDirect di~al conttol·;(DDC) .products arei much mote havea large impact.1 ,. ,- , • " , • u advanced ,. ;cost~effective, and .. reliable than just·111 few years · .: Control techniciaris are expected to be experts at HVAC ago. Building managersrJmon;karn1@Jove facmty.widcr inte­ systems, electrical wiring; pneumatics, c:romputer program­ .!V~~ion ... ~n~ data acc~~~tp.g from , ~ singl~ 19~ti9i~~ .. fvent ming, customer-relations, iltld all their comp·any's latest prod­ sc~".(i.ulmg, ; da~ loggmg, and. alai:fll ,ffPp~ ,ng .arri~a~y once ucts. This is a lot for anyone to master. Also consider the time they fimre out how to :use the software.. "' ' • I ~., ,. f 1 o , / 1 • , ; •, they> are given to dd a.proper check on what was just installed . • 1 ;~ New; produ~ts can b~ ;. ex,cipng 7 ~itJi-; :~any _pps~.bilities Whether the construction is behind schedule and the pe©ple and_more flexibility. Cgntrol.technology i~ evolvJrg .qpickly, are ready to move in or the control technician has too many sq1.q11ickly that it doe hP,()~ .~ways . ~i;em .io have .b.'?en field other,plhaes to be, the DDC VAV controls are, more often than t,esij!,cf;, ._. , ,. .·. .r , · not; W.0tkingimproperly when the balancer is told "everything Amid the dust and wide teIQpei;atiµ:~ . swings of.~ construc­ is ready:i' By oooperating·with each other, the balancer and the \i~>n sjte, YAV boxes are install ~d with the micro 7 ~lectronic control technician can calibrate the DDC VAVs to produce c?,p?::?.1~ ~~~~}I~: "2,0lJ~ted.; ~ _~ t}vh~t .wbi: 'eq ,fi,ne l~ the ,~~bo~ accurate readings. \( r ~J.WY -9Ji~~ e a~ ~ ~b~ ~Vife at ~e job site ...·• :. _: :: . ;, ~ne.u.matic VAV :: '~; 1.;-; Con_trol ~i_:chni 9j.a.f!:s . fe ~t~y, , have ~~,ir. ~or~pu,~ q~t for ~ I) .. • • '• . " tllem with these new pr($'j\l}Fts 1 . !~>'. . afr ~.t:teP. .left tcying to '.' rl :: P11.eumatic VAV is simpler and ·rmore user-friendly. figl.) re out how the system works under· an aggressive sched­ Rveryt>lie is fiat at the mercy of the control contractor. The ule, .wi;fll pebple r~aay t8 ni. M~ in aHd1fui-aii-' ba1ihi~e'r tile~~ing t t 1 • }_'\ I I , •· •• • 1• I' t I I • • • Jlialancing techniCiartis usually able to troubleshoot the:limited down' th~fr rit cks: '" - ' · • · ~ · - 'J • ' . ,. ·" -" number of problems without assistance of the control techni­ cian. •'J • ~· ' '•· • 4 .... t .. .. ' '• •; ' A manometer can be hooked up to the differential pres­ tt " t.' sure taps. This is good for troubleshooting but is not accurate ... enough for balancing. Often the inlet duct configuration will give a skewed reading. The pneumatic"controllers contain a separate minimum and m~imum adjustment knob to make ·a:Yercorning these poor inlet conditions e11sier. DDC VAV boxes typically have only one flow adjustment. They are balanced at maximum and then set to minimum to be checked. Minimum flow is hope­ fully correct because no further adjustment is available. . .. V ~i ..•l • 111...., • ., r.-....,...... _ Y Pneumatic VAV systems are more rugged and have fewer r' • t" components that can fail, although they do need frequent cali­ figur,e ~. Annual energy use witfl different flow throtfling bration. Their major drawback.is that they cannot easily or ~· f.t!chniques. ., , accurately obtain the degree 6'fenergysa~ings' that come from

BN-97-13-3 3 1 night setback, multiple minimum and maximum airflows, and , , Drawbacks include the additional .c,:ost and the mainte­ history logging. nance required for. apd~P: mptors in.t,b,Q sys.~e]I1.. Wide airflow Probably one of the biggest limitations, when it comes s~ings can be notiqeabJe,w.l;len ~e te~al }s switching from time to sell a pneumatic ~ontrol system, is that mai~tenance heating to coolfng, ,e!;\peciaj,ly .if !@:fan te~inal is oversized. personnel are not able to troubJeshoot problems from . ~ office There is a new prpdµct that controls the flow by monitmj.ng the computer. Pneumatic VAV boxes are not capable ~f ~ending duct pressure on the discharge side of the terminal box to avoid data through a local area network for ystemwide ~haring of this. , information. Sophisticated building control strategi~s cannot Series-Not often used because of the high energy be. performed. The pneumatic VAVs cannot bfo! accessed penalty from.rans running for long periods!; ln the occupied th~ough a computer or via a modem over teleph~~e! . line~~ mode, th~ int~mal fans must run continuously: ·· ' '1 Analog Electronic VAV lhduction Analog electronic VAV systems are an improv~m~nt Made to increase ~~om circulation by mixipg 'plemim !11-r over the old pressure-dependent electric units. A~:,anaiog with primary duct air. Induction is created liy pi:imaf'Y ·air VAY sysrem has a veiocity sensor and contruiler that wili nozzies with small orifices that crelite jet streams that draw maintain proper air volume after balancing ha~>: , ;beerr) plenum air mto the termi:ii.al;box. Induction ratios can vary completed. . from25%fo65%. ·'1 " . , • • ! ' P · One advantage .an analog system .~as over a pneumatic ::J One limitation of this·1box is tllat full ' shut-off •is nbt one is that it can be ~esi~eci to, se~,se upstream d,uct te~~~r­ usually an option, and this can lead to subcooling of the space, ature and make a decision whether. this air is needed in· the especially since colder discharge'·filr '.retnperanir~s from'..the ., . . 1 space. This is · n"ext to impossible tO. do .with a pneumatic air-handling unit are usuallr nece,&sary. A slightly hl.gher static sy tern, which requires a peating"coi) "at each ·termi.rliil box it pr.essuie_in the duct must also be maintruned compared to .a 1 heat i ~ necessary. With aci analog system, onfy a heating coi1 co,nv~ntional VAV. ·' . '· ,:; · "' .... J •• .• : in the air-handling unit is required. Of course, the. 6tijldlng I • ' • • -' : .: ,I J I- ' . .lJ ·;; l ·-~ ~ must b~ . zoned properly because heating and cooling "are not J•• t available at the same time. . ' -· · ; , ~ · • .- ~ . ,' i . ' . ,, ; Wm:ks yv(\11 fQr a . p~~al VAV sy:~tem in ar<;f!s,such as~ Beside that one bright spot, analog electronic VAV conference room or comer qffice. Th~y ar;~ ~elf-COJ?.taiD~~ , ~d systems have many of the disaqvantag~ . {hat pneum~tic :1µ1d --· ..• ·~ .. , .. . ,,, ...., require no pneumatic or electric connections. They even have DDC systems have. Like pneumatic systems, analog systems their own built-in:_~ennp,~y.t put ·do. .P.?t sense a.ip'Ww.; r, i) I;. . , are' limited in their ability to communicate to th~ '-building's The next generation of VAV diffusers do sense airflow energy management system. They also contaiD.'sensiti'veelec­ but catmot measrire 'it, so they are stlU pres·sure tlependoot. A troniC components that'are more likely to faiJ!, sirnilart-0 DDC: ~ lOw voltage tie-in isiretjt:iited. Teth'peniture·setpoints are more precise ·can be adjusted ·via a hand-held remote control.: TYPES OF VAV TERMINAL BOXES and , i ; ,;:·. '; ~ ;'. Cooling Only, Single _Duct Bypass Terminal '.:; . ! ·~. ' 1 Great for interior spaces of multiple-story·buildings;·But I! r ., Unwanted air is dumped ·lfito 'the .retum plenum, which earl sometimes still dfop back'dowff.mtb. the 0Ccupiedspace1, subcooling can be a problem if the minimum is JJ.Ot z~ro an!f IAQ can be a concern ifthe minirnunl'ts 'ilto'. :" ·:i ., ; ,;,.· i) '.' '1.ifhis-appibach dbe"snofreduce the fan airflow demand. In tiifu, ttlere are·db fan·m&for endrgy;sa\lings," and diversify can Reheat, ~ingle . [)~~t not be 'taken 'ad~anllige of: . i. '1 ;·· ; ' ' ·' ' ~· ' "'. I ';; Usually used for exterior arid under-roof areas-Of build­ Damper Sectio'°' ings that require heating as well ' as · ~ooling. The trouble is that reheat water is often needed all year long to prevent subcool~ 'l'Wo-po'Sition actuation with no flow measurement. This ing from the minimum airflow. i, application has limited use. " ~ \ ,' I .I' Fan Powered Air Valves

Installed in ceiling plenums and used for spaces th~.t The valve has a cone assembly with a staged spring require heating as well as cooling, they are excelleil.1'. fdl: main- inside. As static pressures change, the cone moves to maintain taining room air circulation. '' the set airf19w. ,The valve is accuratelr, calibrl\terrosio:q~i:esi~tant design and : _q,uic~. respo11s.eJime. ;,., heilt !generated ;by lights. If this is not suffieient, an 0ffset Due to their cost, they ~e usually limited to l~borittories, reheat coil ,(Ian be·.added .. ; . : ' isolation rooms, and process air. \ .

4 BN-97-13-3 ·" Everi though vatiable-frequency-0.rives are 'not usually Vortex Shedding used on fume . hood eXhaust fans, ' energy saved. by not Ui-e This mass flow sensor is very accurate even at low veloc- eXhausting large amounts'. o:feonditioned:filt out of building ... 1, . • a ities: If Is .expensive and only seen on hjgb end systems such is an even greater'benefit.· to-'ffiree dollarslper cfm ($0.47 1 one as those designed for laboratories. The sensor'sends out digital to $L42·per'L/s) aimu~savings can be ~xpected. pulse oJ tputs at a freqJb~cy that is linear to the airflow. The ·.:1 I f:· · pii.lse by vortices of the air created as air moves • r·i ~' cr• ~atcdr Dual Duct " ' through'the sensor. .! . .;With a,hot: all~ · cold duct, unwanted mixi,n~ ~an w~ste Subsfantial straight duct upstream is highly recom­ energy as well as ,cause discomfoq i~, the space: .This is caused mended because resetting calibration to match actual airflow by a leaky damper or downstream air sensors. Inlet air sensors is very cucib~f~ome and time consuming. The vortex shed­ are favored instead of a downstream one because· low.: duct ding sensor cannot be located, upstream or downstream, near pr.es,sp~e . f~ c.ause ~.~ unwanted deck to open_ trying to main­ any source of excessive turbulence. For example, a VAV t~n proper voJ1Jme. · darlipt!r at minimum can cause enough turbulence to make this . I l " I ' . . • : l. ~ ! SenSOf unreliable. (I ~• I ~, In, certain. parts of the com;t:ry :l!uJI!idi ty control can ht? a ,.. ·' ''" problem-. D~Jlg the sumJUer m~mths, the hurnW, outside air. Fluidic · passes through the hot deck with nothing to drop out the moi~~ , tup;. Th~~pauses tµe bp~ldill-g to be a little o~, ~h~ h_umid side. : Similar to the vortex shedding sensor, the fluidic type is a mas~ · flow sensor that is accurate even at low velocities. In , ' :·!. , . ~ ' : ·I cont¥ast to vbrtex it'is "'ith pneumatic INLEJ AIRFLOW .SENSORS sh~ddirig,, \.1 ~ed 'o~y sY.s}em~ and'is yerf quic.K. ~and . ~,~~~ to bal~ce . It consumes ·. 1··1 ··: . r· :· r = · • 1·.r " ·, · ~ -· !' ' · . '. is to small ,.ffP.ounts of pneumatic air in its measurement process:· ·A~ moving ) mro~fh f\;'AV . ~ox not ~ ea~1 ~e~~ure. 1 The aiiflow sensor, usually located at the_ mlet, must be The pngumatic air must be clean and dry' at~ all times or' the designed to contend with the turbulent movetrtent of'mi f'ourid matcHed controller will fail. 7 r. ··r' ' ' #" in most duct systems. Reliable flow measureml'

BN-97-13-3 5 carbon dioxide to reach a level at which the control system will Flow stations can; be a lot more than: 5 % ,off. This is due to the

act upon it. To avoid potential IAQ pr:oblems, the C02 concen­ lack bf proper straighLdu:et, which is usually a factor. tration should not be used to drop the ventilation rate below' the The flow station strategy alsG>.cannot take into consider" quantity that is required to satisfy the scheduled occupancy ation the fact that most air-handling units leak air. It is not profile (Janu et al. 1995). At the present time, a serisor that usually apparent because of the negative pressure they are reads, carbon dioxide and volatile organic compounds is being under. In this scenario, air "would be developed. assumed to-be outside air. n < The carbon dioxide concentration method, in contrast, is 'I ' Mixed Air p'i~mum Static Press~re c<)tjtrol a practical method of instituting continuous measurement and f; closed-loop control of outside airflow (Jariu et al. 1995),: The In a typical system, outside air is drawn in' because of the method is established on a tracer gas concentration balance negative air pressure produced'by the mixed air plenum. If this that uses carbon dioxide as the tracer gas. " pressure: is reduced; so is the'outside air. This is exactly what The outdoor airflow rate (in cfm or Lfs) is determined as happens when a VAV system is not at full load. .\ . ~ follows: If the control system is designed to maintain a· constant static pressure in the mixed air plenum, this wbuld gua11Mk:~

C02 ra-C02 sa XCFM sa a constant volume of outside air. A static pressure: sensor CFM Qa "' , , . -_, (1) localed in Ll11:: milled air plenum woultl n:posilion Ll1t: rt:lurn air CU2 ra-"" CU2 oa · automatic damper to maintain the proportions of retuf-h~k arld outdoor \int~e that. iidequate air, quality requires (Cohen A single C02 sensor is used for reasons of aceuracy. Samples of supply, return, and out~ide air ar~- pumped to the 1994).'.ThroUling the return a;ir,automatic damP.er maint!lins a constant outside air intttk:e. , , seiisor alternately via athree-way valve. C02 is measured in parts per million. These data aie'stfot to the controhystem, , _As long as the mixed air dampers are prQperly sized, ithis which uses this infomiation t6 determirie the positiofi of the type ofOA control can be added' to an'1rxisting I:IVAOsystem. out'side Air damper. %is method's ·success is heavily"depen­ Tt ,crmhi'··twen he inst1Jllr.nnon p1wkae;ecl HVAC e:quipment dent on the long-term reliability of the CO~ sensor:'Periodic without modifications to the ductwork or ceilings. ,: , r·' ; , \ 'lJ ,, • calibration of the sensor needs to be part of the maintenc!mc!e ~I,'. I • ( < program. '1 Cons~ant Spiic~ ~~t!•sure This direct approach measures building static pressure Flow Stations and compares it to outdoors. The return fan capacity or the return and relief dampers are adjusted by the control system in This method of continuous airflow measurement in the response to the static pressure signal. The problem is that wind system uses pitot flow stations, pressure drops across orifices, can have an adverse effect on the sensor that is located heated thermistor grids, or vortex shedding devices to monitor outdoors. Windows and doors that are left open would also the supply, return, and/or outside air. It is difficult to measure challenge the system. This method of outside air control is outside air directly, due to low velocities, wide temperature more for maintaining the building pressure than providing swings, and limited space. The best arrangement calls for flow adequate ventilation. It should only be used in facilities that stations in the supply and in the return back to the AHU. Relief have a generous amount of exhaust, such as laboratory build­ air is not included. The difference between these two readings ings. should give the actual outdoor airflow volume. Or does it? With all these methods of controlling outside air, it is TABLE 1 important that ASHRAE Standard 62 be followed. Proper Compounded Errors with Flow Stations ventilation is very important from health and energy stand­ points. People spend large portions of their lives in buildings, Design *Actual and they should be provided a healthy environment. On the other hand, building owners spend large amounts of cash on SupplyCFM 10,000 9,500 conditioning outside air, so it is a delicate balance. ReturnCFM 8,000 8,400 Outside Air CFM 2,000 1100 CONCLUSION %ErrorOA 0 45 It appears that there are numerous situations that need to be considered when creating a VAV system. They cannot be * Assuming a - 5% error for supply measurement and a+ 5% viewed like a constant-volume system. VAV systems are not error for return measurement. constant but a dynamic, ever-changing assemblage of compo­ To convert cfm to Lis, multiply cfm by 0.472. nents that must be custom equipped to satisfy the needs of the occupants and owners. As you can see from Table l, an error of only 5% in the The complex technology to do this is now here. All we flow readings causes a 45% error in outside air calculations. have to do is manage it properly. Most of the past VAV prob-

6 BN-97-13-3 lems have now'been.solved, but there:is still: a long way to go can Society of ·• Heating; Refrigerating and Air­ in implemcniting a:1successful strat1egy, ..consistently.1 More ': Conditioning Engim.eers, Inc.\11 complicated systems areindeed·the·.prirewe have to pay for ASHRAE. 1988. ANSI/ASHRAE Standard 111-1988, Prac­ saving energy. . " • ;: , ·; ticesfor measurement, testing, adjusting, and balancing .• J ·• • ! ,.... ', .r · :. oj building. . heating, ventilation, air-conditioning, and REFERENCES , .. !i· ,r: . ' : : , systems. Atlanta: American Society of ASHRAE. 1995. 1995 ASHRAE handbook-applications, Heating, Refrigerating and Air-Conditioning Engineers, p.38.2 Atlanta: American Society of Heating, Refriger­ Jµc . . . ating aii(i1J\1£conditid'ning Engl.neers, iNc'. · :· ·i'iil Janu,-,G.J., J.J:?. Wenger, and C.G. Nesler. 1995. Outd9or air Cohen, ; ;r, J 99.4; Providing constant ventilation in variable . : ·flow control for VAV systems. ASHRAE Journal, April, airwolume systems. ASHRAE Journali.1May; pp. 38-40. .•. pp. 62-68. Janu, G.J.,,J.D.,Wenger, and C.G. Nesler.. 1995: Outdoor air Milewski, L. 1994. Computerized balancing: Garbage in, flow conttoLfor VAV systems.•ASHRAE Journal, April, gai;b,ag~ out. Heating/Piping/, May.

· p164(1)~l . ') it' I . , _,,_ , \; NEBB. i991. Procedural standards for testing, adjusting and Wendes.;H.i199.4. Variable:air volume manual. .Lilbum, GA: balancing of environmental systems. Gaithersburg, MD: < ·• FainnontPress,·p. 9. -,; National Envll;onpiental Balancing Bureau.

.; J ' Richardson, G. 1994. Commissioning of VAV laboratories ~IJ;JIJQ~RAPHY . .i and the problems encountered. ASHRAE Transactions ASHRAE. 1995. 1995 ASHRAE - handbobk~-applicativns, ~ ()(),, pi42,19 Atlanta: Ambrican :Society of'Heating1 Refriger~ SMA,CNA. 1987. 1!Y1P,.. $ysfo/.f!s--4pplications. Chantilly, ating and Air-Conditioning Engineers, Inc. , : ' •< . 1 J,y~:., Sheet Metal ~d Air~ f:,<)Ulditioning ypntractori;;' ASHRAE. l996. 1996 ·ASHRAE handbook:,,_:_HVACSystems .:· . J'{ational Ass,ociation1.Inc'. , , .. , ! , . .'a11d--Equipment, ·pp. 2~6 ·and 20.9 .Atlanta: American §~ACNA. 199}. IJVA..,<:; ~ystems,n'festing, Adjusting and

1 • : Society o'f Heating, ·Refrigerating .•and Air-Conditioning ·" ." Bqlf1.!1cing. C~!lllµtly, VA: Shee,t Metal and Afr Condi­ Engineers,. Inc. · •:·: .ii:. ,tioning Contractors' National Association, Inc. ASHRAE. 1989. ANSI/ASHRAE Standard 62-1989, Ventila­ \\{endl'.s, H. 1994. Variable air volume manual. Lilburn, GA: 1 tion for acceptable indoor aiP qidi'Ut:f Atl~ta: • Ameri- Fairmont Press. t ' .. ,, ._ , ·' 1.' : -;. · · :! '

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