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A S H RA E J O U R N A L

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About the AuthorsM.H. Hosni, Ph.D., is professor of mechanical en-gineering and the director of the Institute for Envi-ronmental Research at Kansas State University. Hehas been the principal investigator (PI) and Co-PIfor RP-822 and RP-1055. He is the chair of the Hand-book Committee for ASHRAE Technical Committee(TC) 5.3, Room Air Distribution. ChristopherWilkins, P.E., is group leader for Hallam Associatesin Burlington, Vt. He is a member of TC 4.1, LoadCalculation Data and Procedures, and TC 4.12, Inte-grated Building Design.

Heat GainFrom Office Equipment

By Christopher Wilkins, P.E.Member ASHRAE

The independent research by Wilkinsand McGaffin produced significant dataon the overall building load as measuredat panels serving distinct areas within abuilding. Data was also collected on themeasured power consumption of indi-vidual items of office equipment. The TC4.1 research as executed by Hosni et al.,expanded on this by obtaining data in amore controlled and formal manner.

Hosni�s work for TC 4.1 also includedmeasurement of the radiant and convec-tive split of the heat gain from the equip-ment. Documentation relative to radiantand convective split is relevant whenusing advanced load methods.

Research Project RP-1055 obtainedheat gain measurements from office, labo-ratory, and hospital equipment. The finaleffort of this research was to take the col-lected data and identify patterns or gen-

SHRAE Technical Committee 4.1, Load Calculation Methods,has completed two recent research projects and the results willbe of interest to engineers who perform cooling load calculations.

ASHRAE Research Project RP-822 focused on development of a methodby which the actual heat gain and radiant and convective split from equip-ment in buildings could be measured [Hosni et al., 1996]. This methodol-ogy then was incorporated into a second research project, RP-1055, wherethe technique was applied to a wide range of equipment [Hosni et al., 1999].A research team at Kansas State University, led by M.H. Hosni, completedboth of these research projects. This research was followed up indepen-dent research by Wilkins and McGaffin, 1994.

eralizations that could be widely used forcommon applications. It was discoveredthat results for general office equipmentcould be generalized, but results fromlaboratory and hospital equipmentproved too diverse.

Here we will present generalized guide-lines based on the results of all previouslymentioned research for office equipment.The reader is encouraged to consult theproject�s final report and the technicalpaper (Hosni et al., 1999) for detailed dis-cussions of results for laboratory andhospital equipment.

Nameplate vs. MeasuredIt is now well documented that name-

plate data overstates the actual powerconsumption of office equipment. Powerconsumption of this type of equipment isassumed to be equal to the total (radiantplus convective) heat gain. Many engi-neers would find it convenient if a stan-dard number or ratio could be applied toall nameplate data to obtain a useful esti-

mation of the actual heat gain. All re-search completed to date, however, sug-gests that this is not possible.

In Hosni et al., 1999 work as part of RP-1055, they found that for general officeequipment with nameplate power con-sumption of less than 1,000 W, that theactual total heat gain to nameplate ratioranged from 25% to 50%. When all testedequipment was considered, the range wasbroader. The conclusion was that if thenameplate was the only information knownand no actual heat gain data were avail-able for similar equipment, then it wouldbe conservative to use 50% of nameplateand most accurate if 25% of nameplate wereused. Wilkins and McGaffin also found awide range in this ratio.

Use of this type of blanket ratio couldintroduce a large degree of error into cal-culations. Nameplate data for similarequipment varied widely but the actualmeasured heat gain data was consistent.Applying a generalized ratio could intro-duce an error of 100% or more. Much bet-ter results can be obtained by consider-ing the heat gain as being predictablebased on the type of equipment, notbased on nameplate data.

Results by Equipment TypeThe data collected in RP-1055 for all

M.H. Hosni, Ph.D.Member ASHRAE

and

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equipment tested was sorted and reviewed in an attempt toidentify trends or generalizations that could be presented foruse by practicing engineers. It was discovered that clear pat-terns could be established for office equipment but that labora-tory and hospital equipment was too diverse to be generalized.Office equipment was grouped into categories such as comput-ers, monitors, printers, facsimile machines, and copiers. Resultsfor the measured heat gain of equipment within a given groupwere then analyzed to establish patterns.

ComputersHosni et al. tested a total of eight com-

puters of Pentium or 486 grade. Four weretested together with a monitor and fourwere tested alone. The measured maximumranged from 52 W to 70 W. The nameplatepower ranged from 165 W to 759 W. Theheat gain from computers tested with moni-tors was determined by subtracting a typi-cal value for a monitor from the total of thetwo. Wilkins and McGaffin reported dataon 12 computers of 486 grade and older.The average heat gain for the 12 was 56 Wand the average nameplate was 391 W.The average heat gain for all 20 computerstested was 55.6 W.

The heat gain from computers showedlittle reduction when idle versus opera-tional. The exception was computersequipped with the Energy Star energy-saver feature. This feature will place acomputer in a �sleep� mode if it remainsidle for a preset period of time. Hosni etal., 1999 found that the heat gain reducedto a typical value of 18 W when in sleepmode. This sleep mode on an individualpiece of equipment likely will not affectthe peak-cooling load but it could affectthe diversity factor and maximum heatgain of larger areas within a building.

Two conclusions can be drawn fromthese data. The first is that nameplate dataon computers should be ignored when per-forming cooling load calculations. The sec-ond is that a typical value for heat gainfrom a computer can be established andapplied in cooling load calculations to ob-tain practical results. Engineers typicallywant to be conservative in cooling loadcalculations. Table 1 allows engineers tochoose heat gain values for computerswith varying degrees of safety factor.

MonitorsThe magnitude of the nameplate power

consumption for all monitors tested byHosni et al. ranged from 168 W to 565 W.

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Table 1: Typical heat gain from computers.

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The measured maximum total heat gain of all monitors testedranged from 53 W to 86 W. The monitors tested ranged from 14in. to 20 in. (36 cm to 51 cm). Hosni et al., 1999 found that themeasured total heat gain correlated closely with monitor size.Hosni et al., 1999 developed the following relationship to esti-mate the heat gain from monitors as a function of monitor size:

Heat Gain = 5 × S � 20

Where S is the monitor size in inches and heat gain is in unitsof watts. For example, a 15 in. (38 cm) monitor would have a heatgain of 55 W.

Wilkins and McGaffin did not sort their data by monitor size.They presented data on 10 monitors (13 in. to 19 in. [33 cm to 48cm]) and found that the average value for the heat gain was 60W. Their testing was done in 1992 when DOS still was used andthe Windows operating system was being introduced. Theydiscovered that monitors displaying Windows consumed morepower than monitors displaying DOS. Table 2 is a quick refer-ence for engineers who may prefer a table to an equation. Theenergy-saver mode for monitors reduces the power consump-tion and heat gain to zero. Table 2 is derived from research byboth Hosni and Wilkins.

Laser PrintersHosni et al., 1999 found that the power consumed by laser

printers, and therefore the heat gain, depended largely on thelevel of throughput for which the printer was designed. Table 3presents data on four general categories of laser printers. Hosniet al., 1999 opined that smaller printers are used more intermit-tently and that the larger printers may run continuously forlonger periods of time.

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Table 3: Typical heat gain from laser printers.

Table 2: Typical heat gain from monitors.

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These data can be applied in at least two ways. The mostobvious is to take the value for continuous operation and thenapply an appropriate diversity factor. Diversity factors are dis-cussed further later. This likely would be most appropriate forlarger open office areas. Another approach could be to take thevalue that most closely matches the expected operation of theprinter with no diversity. This may be appropriate when consid-ering a single room or small area.

CopiersHosni et al., 1999 presented data on a total of five copy

machines. Copy machines were considered to be of two types,desktop and office. Office-type copiers represented freestand-ing office grade copiers. Larger machines used in productionenvironments were not addressed. Table 4 represents a sum-mary of the results. Hosni et al., 1999 observed that it would beunlikely that desktop copiers would be operated continuouslybut that office copiers were of the type that often are operatedcontinuously for periods of an hour or more.

Freestanding office-type copy machines often are installedin rooms outside the primary occupied area of an office. Thesecopy rooms generally can tolerate a short-term increase in tem-perature caused by a period of continuous copier operation.Engineers must consider each application and determine theappropriate mode of operation.

Miscellaneous EquipmentTable 5 lists a few other types of equipment that may be

encountered. Values for facsimile machines and image scan-ners are based on data from Hosni et al., 1999. The values pre-sented for dot matrix printers are compiled from data presentedby Hosni et al., 1999 and by Wilkins and McGaffin, 1994.

DiversityThe actual peak heat gain for all equipment in a common area

of a building is less than the sum of the peak for each becauseof usage diversity. It is important to have a clear understandingof what diversity is, if the data presented here is to be appliedaccurately. Diversity, as discussed here, is not related to the

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Table 4: Typical heat gain from copiers.

Table 5: Typical heat gain from miscellaneous equipment.

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discrepancy between nameplate data and the measured heatgain. Diversity enters into the equation when some equipmentis idle or turned off and is not contributing its maximum heat orpower to the overall cooling load of a given space or systemwithin a building.

Wilkins and McGaffin were able to measure diversity with acombination of measurements at dedicated equipment powerpanels and detailed inventories of equipment in the areas servedby the panels. Their work encompassed 23 areas within fivedifferent buildings totaling more than 275,000 ft2 (25 550 m2).The first step was a survey to account for every piece of equip-ment in the space and to measure the power consumption ofeach. The peak power consumption (assumed to be equal topeak total heat gain) of all equipment was summed to provide avalue for the maximum possible total heat gain of the equipmentin the area.

Continuous measurements were taken for a period of oneworkweek at the equipment panels serving the area. Care wastaken during the survey to assure that only receptacles wiredto the equipment panels were powering equipment. The peakpower consumption recorded at the equipment panel repre-sented the actual peak total heat gain of all equipment in thespace. The ratio of the measured peak at the equipment panelsand the sum of the maximum of each individual item of equip-ment is the usage diversity.

Diversity was found to range between 37% and 78% withthe average (normalized based on area) being 46%. Figure 1illustrates the relationship between nameplate, the sum of thepeaks, and the actual with diversity accounted for. Figure 1 istaken from Wilkins and McGaffin and is based on the averageof the total area tested. Data on actual diversity can be used asa guide but diversity will vary significantly for spaces withdifferent occupants. The proper diversity factor for an office ofmail order catalog telephone operators will be different fromthat of an office of sales representatives who travel regularly.

Heat Gain per Unit AreaWilkins and McGaffin found in the areas that they tested

that the actual heat gain per unit area ranged from 0.44 W/ft2 to1.05 W/ft2 with an average (normalized based on area) of 0.81W/ft2. These data were compiled based on 275,000 ft2 (25 550m2) of office space in five buildings. These spaces were fullyoccupied and highly automated with a computer and monitor atevery workstation. Table 6 presents a range of load factors

Figure 1: Load factor comparison.

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Table 6: Typical equipment load factors.eciveD naF tnaidaR evitcevnoC

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Table 7: Radiant-convective split.

with a subjective description of the type of space to which theywould apply.

Wilkins and McGaffin performed tests in 275,000 ft2 (25 550m2) of highly automated spaces, comprising 21 unique areas infive different buildings. The maximum load factor they reportedwas 1.08 W/ft2. This corresponds to a medium load densityspace based on the subjective classifications presented in Table6. It is likely that the medium load density will be appropriate formost standard office spaces. Medium/heavy or heavy load den-sities may be encountered but can be considered extremelyconservative estimates even for densely populated and highlyautomated spaces. Other research supports this, including work

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H E A T G A I N

by Komor in 1997. Komor performed aconsolidation of data obtained from sev-eral sources and in all cases, his findingswere consistent with Table 6.

Radiant Convective SplitOffice equipment produces both radi-

ant and convective heat gains. Convec-tive heat gain is converted instantly tocooling load while radiant heat gain is

absorbed first by the building mass andthen converted to cooling load over time.This distinction can impact the time andthe magnitude of the peak cooling load.Hosni et al., 1998 developed a method tomeasure radiant heat gain from equipmentusing a net radiometer mounted on anarticulating arm.

Hosni et al., 1999 found that the radi-ant-convective split for equipment was

fairly uniform. The most important differ-entiating feature was whether or not theequipment had a cooling fan. Table 7 is asummary of Hosni et al., 1999 results.

Future TrendsThe data that we have presented here

is based on contemporary equipment. Therelevance of these data in the future iscertainly a legitimate question. TheLawrence Berkley National Laboratorysponsored research in 1995 and reportedthat equipment energy intensity will de-crease until 2002 and then begin to in-crease slowly through 2010. To date, thisprediction has proven accurate. It is likelythat the data presented here will be rel-evant for several more years.

ConclusionsHeat gain from equipment is an impor-

tant contributor to the overall heat gainof a space. The information presented inthis article should be a useful tool to en-gineers performing cooling loads or en-ergy analyses. We also hope that equip-ment manufacturers understand the im-portance of nameplate values for coolingload calculations and take appropriatesteps to provide more realistic power con-sumption information.

ReferencesHosni, M.H., B.W. Jones, J.M. Sipes, and Y.Xu. 1996. �Test method for measuring the heatgain and radiant/convective split from equip-ment in buildings.� Final Report for ASHRAEResearch Project 822-RP, Institute for Envi-ronmental Research, Kansas State University,October.

Hosni, M.H., B.W. Jones, H. Xu. 1999. �Mea-surement of heat gain and radiant/convectivesplit from equipment in buildings.� Final Re-port for ASHRAE Research Project 1055-RP,Institute for Environmental Research, Kan-sas State University, March.

Wilkins, C.K. and N. McGaffin. 1994. �Mea-suring computer equipment loads in officebuildings.� ASHRAE Journal, 36(8):21�24.

Komor, P. 1997. �Space cooling demandsfrom office plug loads.� ASHRAE Journal,39(12):41�44.

Koomey, J., M. Cramer, M. Piette, and J.Eto. 1995. �Efficiency improvements in U.S.office equipment: expected policy impacts anduncertainties.� LBNL, Berkeley, Calif., Decem-ber. n

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