hvac peak load calculation methods – history and comparisons

10/29/2015 HVAC Peak Load Calculation Methods – History and Comparisons

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HVAC Peak Load Calculation Methods – History andComparisons

by Bill Smith, president of Elite SoftwareCopyright © 2011

This article discusses both commercial and residentialHVAC (heating, ventilation and air conditioning) loadcalculation methods, and explains why results are oftenso different between methods. This article is intendedto be understood by both the lay person and the HVACprofessional. To help the lay person, some definitionsare in order.

When most people hear the word "load" they naturallythink of a load of firewood they carry into the house, ora load of dirt in a dump truck, or maybe even a loadheld up by a beam or structural member. All of thoseloads are a "weight" to be carried or supported in someway. Such loads are measured in pounds, kips, or

kilograms, depending on the units involved.

So what are heating and cooling loads? As pertains to heating and cooling equipment (anHVAC System), a load is a rate of heat transfer. Not a single quantity of heat, but anamount of heat that must be continually (at least for the worst case hour) removed or addedto maintain the desired indoor temperature. In the coldest time of winter, the peak heatingload is the amount of heat that must be added over an hour’s time to keep the space warm.In the U.S. imperial units system, this rate of heat transfer is in Btu’s per hour which iscommonly abbreviated as Btuh. In the metric system the equivalent unit is a watt and onewatt is equal to 3.41 Btuh. In the hottest time of summer, the peak cooling load is theamount of heat that must be removed in an hour to maintain a comfortable roomtemperature. Again, the cooling load unit is either Btuh or watts.

What causes heating and cooling loads? A summer cooling load is a heat gain to thebuilding. The sun provides all of the heat that comes in through the exterior of thebuilding. And within the building itself, heat is generated by people, lights, equipment, andappliances. A winter heating load is a heat loss that is caused by loss of heat from thewarm physical mass of the building to the cold air surrounding the building. Heat is lostthrough the walls, windows, roof, and through cracks and crevices where cold air seepsinto the building.

It is important to know the peak heating and cooling loads on a building so that the HVACequipment can be adequately sized. An undersized HVAC system will not be able tomaintain the desired indoor temperatures. An oversized HVAC system will be inefficient,and struggle to maintain comfort conditions, particularly with humidity control duringsummer months.

Differences in HVAC Load Calculation Methods

HVAC load calculations are most often done using computer programs, although somemethods are still able to do be done by hand. Many people are often alarmed to learn that



various computer programs and hand calculations can differ so widely on calculationresults. After all, if each calculation is done with the same outside and inside temperatures,same building material types, areas and quantities of roofs, walls, glass, people, lights,equipment, etc., why should the results ever differ by more than a few percent?

Complex Problem

The physics involved in the transfer of heat and energy between buildings, occupants, andthe environment is quite complex. The most current and best math models of this problemrequire significant input data and thousands of calculations in an iterative process. Thecalculations are so involved that researchers have been forced for years to create varioussimplified procedures that are doable by hand or within reasonable calculation time on apersonal computer. The cost of these simplifications from the theoretical best math modelsis accuracy.

In theory, one generalized HVAC load calculation procedure should work for all types ofbuildings. There would be no need for separate procedures for different types of structures,either residential or commercial in nature. In fact, such a generalized method has beendeveloped in recent years, and it is called the Heat Balance (HB) method. And it can beused on buildings of all types. However, there are various issues with HB, discussed later,that make it less popular to use for peak load calculations than many of the lesssophisticated methods that are designed only for residential or commercial buildings.

Early HVAC researchers learned that simplifications in calculation methods could be madeif generalizations about the subject buildings could be assumed. This led to HVAC loadcalculation methods that were designed only for residential or commercial buildings.

Residential buildings, in particular, were believed to be more simple in construction andusage. Home owners were traditionally considered to be less demanding than commercialbuilding owners, especially since commercial buildings usually cost far more than houses.For these reasons, residential HVAC load calculations procedures have traditionally beenmuch less sophisticated than commercial procedures.

Besides separate methods for commercial and residential buildings, another major factorthat has influenced load calculation methods over the years is the use of computers. Beforepersonal computers were widely available, simple procedures were even more important tohave, as doing calculations by hand is slow and tedious. Many procedures to this day stillhave simplifications for the reason of making hand calculations possible.

Commercial Load Calculation Methods

In the calculation of commercial building peak heating and cooling loads, many methodshave been developed over the years. ASHRAE has TETD (Total Equivalent TemperatureDifference), CLTD (Cooling Load Temperature Difference), TFM (Transfer FunctionMethod), HB (Heat Balance) and RTS (Radiant Time Series). ACCA has the Manual Nmethod, an updated variation of the ASHRAE CLTD method.

There are two other calculation methods primarily used for 8,760 hourbyhour annualenergy analysis that are also sometimes used for peak load calculations. All the variouscomputer programs such as EnergyPro, eQUEST, EnergyGauge USA, REM/Rate, andothers that are based on the DOE (Department of Energy) 2.1e program use what is knownas a weighting factor (WF) method. The most current and sophisticated energy analysisprogram was developed by the U.S. government, and is called EnergyPlus. It uses the HeatBalance (HB) method.



In general, the energy analysis methods, WF and HB, are more sophisticated than most ofthe methods used just for peak heating and cooling loads, such as TETD and CLTD.Although the WF and HB methods were primarily intended for 8,760 hourly analysis, theycan also find peak heating and cooling loads during the course of a year for all types ofbuildings, both commercial and residential.

Residential Load Calculation Methods

ACCA has a residential load calculation procedure called Manual J. It has been updatedover the years, and is now on its eighth edition. As ACCA upgrades its procedures, it doesnot create new names for the procedures. The name stays as Manual J and only the editionnumber changes. The current ACCA residential method is commonly called MJ8.

ASHRAE has offered residential load calculation procedures for many years as well.Unlike ACCA, ASHRAE creates a new name for a procedure when it undergoes a majorchange or replaces an earlier procedure. In 1985, ASHRAE used the Design EquivalentTemperature Difference (DETD) approach, and from 1989 through 2001 the procedureswere called the residential CLTD method. In 2005, ASHRAE introduced both theResidential Load Factor (RLF) method and the Residential Heat Balance (RHB) method.

While ASHRAE introduced two new residential methods in 2005, the ASHRAEHandbook of Fundamentals (HOF) in 2005 and 2009 supplies tables and shows anexample calculation using only the RLF method. In fact, the ASHRAE 2009 HOF statesthat RHB is a researchoriented implementation of the HB method and that it is expectedthat RHB will one day be incorporated into thirdparty software. As of the time of thiswriting in April 2011, there is no software available that implements the ASHRAE RHBmethod.

As explained above for the commercial procedures, the sophisticated WF (weightingfactor) and HB (Heat Balance) methods used in several building energy analysis programscan also be used for calculating peak loads on residential buildings.

Result Differences

Once you realize that significant simplifications have been made in all the popular HVACload calculation methods, it is easy to understand why there will often be significantdifferences in results from those procedures. Over time, these various methods have beenapplied to the same buildings, with results sometimes differing by as much as 40%.Inevitably, the question becomes, which results are the most accurate?

The greatest difference between calculation methods always happens when one method ismisapplied. For example, entering a complex building like a hospital as equally as possibleinto an ASHRAE RTS computer program and into an ACCA MJ8 program will yieldhugely different results, with the RTS loads being much higher. This is easilyunderstandable, as the RTS program calculates hour by hour, and the MJ8 program usessimplified weighted average factors for roof and wall loads. In such an example, there isno problem at all in acknowledging which results are more accurate.

Let's consider a more realistic case where the ACCA MJ8 procedures are used on the samehouse as the ASHRAE RLF procedures. In this case you have two residential specificprocedures used appropriately on their intended type of building, a house. Additionally,both procedures are based primarily on the use of lookup tables such that the calculationsare still doable by hand. Will the results from these procedures differ? Absolutely!

Assuming all areas, temperatures and other data have been entered as equally as possible,



result differences between MJ8 and RLF will vary much less than between disparatemethods such as RTS and RLF. However, loads will still tend to be higher with MJ8because it has hourly load factors on glass (but not on roofs and walls), while the RLFmethod uses averaged load factors (no unique hourly load factors) for all exterior buildingsurfaces, including glass.

The above two examples illustrate relatively straight forward reasons for differences inresults between methods. Some load calculation procedures have very obvious differencesin methods. The method using the more advanced technique is usually more accurate.

More Reasons for Result Differences

There are many difficulties in trying to compare HVAC load calculation methods.Assuming that all quantities of load components (roof, wall, and glass areas, number ofpeople, lights, equipment, etc.) are made equal, and that all indoor and outdoor designtemperatures are equal, there is still a great deal of variability for many other input items.

For example, one method like the CLTD procedure offers only seven types of walls incategories AG. You can assign what ever Ufactor you like to a wall, but every wall in theworld must fall into one of the seven categories that is primarily distinguished by thethermal mass of the wall. Other methods like HB, allow for smooth and infinite variationsof wall types with different thermal mass.

When using the CLTD method, many real world walls could be legitimately selected aseither of two CLTD wall type categories, as they are not clearly in one category or another.The same thing happens for roofs under the CLTD method, where only 13 roof types areprovided.

The MJ8 method is derived from the CLTD method, and thus has similar limitations on theroof and wall categories. The ASHRAE RLF method has even more limited selections forroofs and walls. These finite "type" selections cause abrupt differences in loads calculatedfor a roof or wall of the same Ufactor and same indooroutdoor temperature differences.The more simple the load calculation method, the fewer the "type" categories provided forroofs and walls.

There is also a chance for great disparity on glass loads because there is both atransmission and solar component to deal with. Many of the tabular based methods makegreat simplifications, especially with the solar gains. Even fairly sophisticated methods canshow significant differences on glass solar gains.

More than actual calculation method differences, the most common reasons for differencesin load calculation reports from different computer programs on the same building arefrom quantity discrepancies. Sometimes there are just outright mistakes on roof, wall, andglass areas. Same goes for the count on people, lights, and equipment. Confusion over howto handle fresh air in terms of ventilation or infiltration causes significant differences aswell.

Even when expert load calculation software users compare results on the same buildingthere will be differences. As discussed above, there are judgment calls on building materialtype selections that experts can disagree on, and yet can defend their selections with equalvalidity.

Which Method is Most Accurate?

The first question many people often ask is, which HVAC load calculation method is most



accurate? There is unanimous agreement that the Heat Balance method most accuratelyreflects the true physics involved in heat gains and losses to buildings. And yet at this time,there is no dedicated residential or commercial peak load computer program that uses theHB method. Only the EnergyPlus software uses the Heat Balance method and EnergyPlusis used primarily for building energy analysis, not peak load analysis.

There are two main reasons HB is not yet widely used in HVAC peak load calculationsoftware. First, there is the complexity of the HB method. A designer must be prepared toenter more data and more details for all the load components. This is time consuming, andrequires more thought and engineering judgment. Second, and most importantly, is that theHB procedures "blend" all the component loads (roofs wall, glass, etc.) together during thecalculation process. At the end of the calculation process, only a single number as a gain orloss in heat results. There is no way to see how much effect there was from the windows orthe people or the lights, etc.. Everything is blended, or rather "balanced," before a finalresult is obtained.

Since so many economic decisions are made as to whether to use certain building materialsor not, HVAC designers often find it imperative to be able to review component details ofan HVAC load calculation. All nonHB load calculation methods preserve this ability. TheRadiant Time Series (RTS) method is the closest in accuracy to the HB method while stillproviding component load details.

At this point in time, the RTS method is only available as an ASHRAE sponsored methodprimarily for use on commercial buildings. The RTS method is robust enough that, like theHB method, it could theoretically be used on both commercial and residential buildings.However, the RTS method relies heavily on the conduction time series (CTS) concept,which helps to model the time lag of heat transfer for various building materials. Very lightpoorly insulated materials can transfer all the heat from a test heat "pulse" in as few as fourhours, whereas well insulated heavy materials might need a full 24 hours to transfer all theheat from a test pulse. The CTS series for a material describes that reaction to heat.

ASHRAE provides about 35 suggested CTS values for mostly commercial buildingmaterial combinations. For the RTS method to be used properly for residential peak loadcalculations, CTS values would need to be generated for more residential roof and wallmaterials, and this has not yet been done.

If the HB method is the acknowledged most accurate method, but has the drawbacksmentioned above that make it so rarely used for peak load calculations, what then are themost accurate practical peak load calculations methods available today?

For commercial buildings, it is this writer’s opinion that the ASHRAE RTS method is thebest overall method, as high accuracy is obtained while also preserving the details of loadcontributions from all the various internal and external components. Good results can beobtained from older and simpler methods such as the ASHRAE CLTD and TFM methods,as well as the ACCA Manual N commercial method. So stating a preference for the RTSmethod is in no way saying that the other methods should be avoided. An experienceddesigner using those methods may very likely calculate more accurate results with thosemethods than a novice using the RTS method.

For residential buildings, the ACCA Manual J 8th edition method is the best overallmethod. It definitely lacks in sophistication compared to the ASHRAE HB and RTSmethods, but those methods are not fully developed for use on residential buildings. TheACCA Manual J method has been honed for many years on residential structures, and hasa very successful track record.



Comparison of Residential Methods

It is not uncommon for individual HVAC designers to have a favorite HVAC loadcalculation method that they use on all buildings. Consulting engineers often like usingASHRAE commercial methods on all their projects, both residential and commercial.HVAC contractors, on the other hand, often like to use the ACCA MJ8 method on all theirprojects both commercial and residential. Both types of HVAC professionals generallyhave a long history with their preferred method and know how to adjust the method forvarious types of projects.

I am going to go out on a limb and state some broad generalizations that are based purelyon my own anecdotal experiences and what I have heard from thousands of HVACdesigners served by Elite Software for over 31 years. It is important to understand thatthese generalizations have no scientific basis and could be completely wrong.

Skilled users of any load calculation method are able to adjust inputs in subtle ways thatcan steer results high or low. All of my observations are based on a knowledgeable userentering data as equally as possible, and with no attempt to influence the results.

Unlike with commercial methods, where there is more competition amongst the methods,the Manual J method has been extremely dominant over the years with its number of usersversus anything else. In my opinion, the ACCA MJ8 method produces the most accurateequipment sizing information for houses, even though it uses one of the more simplisticmethods (a simplified variation of CLTD). With many successful users of Manual J overmany years, I have developed a high confidence in Manual J. Thus, the current MJ8method is what I will compare all other residential methods to.

The feedback I have from the very few people who have done ASHRAE RLF comparisonswith MJ8, indicates reasonable closeness of results, with RLF usually being slightly lower.In the case of peak residential load calculations involving a DOE2 based program on ahouse and using the weighting factor method, the MJ8 method will tend to be 1520%higher. The same thing happens when compared with the Heat Balance method. MJ8 tendsto be higher in the same range.

When MJ8 is compared to the ASHRAE CLTD or RTS method on a house, those twocommercial methods tend to be 1525% higher than MJ8. Consulting engineers are oftenconflicted about this. They know the ASHRAE commercial methods have builtinassumptions for commercial buildings, but they wonder whether that matters much. Manyengineers prefer using the higher loads from the ASHRAE commercial methods andbelieve there is no problem with using those methods on houses.

As a final note on residential methods, a lot of questions come up about how MJ8compares to MJ7. When MJ8 was first released in 2003, many MJ8 calculations werecoming out higher than MJ7 for the same house. It turned out that the way users wereinterpreting duct inputs in MJ8 caused those loads to go much higher than MJ7. AddendaC of MJ8 added additional duct inputs that tended to reduce the duct heat gains. Afterfurther testing, it was found that MJ8 and MJ7 tended to agree within plus or minus 5%,with MJ8 sometimes being lower and higher than MJ7.

While MJ8 results compared closely with MJ7 results on equivalent building materials,new materials is where MJ8 surpassed MJ7. MJ8 includes hundreds of more roof, wall,and glass material types than MJ7. With some modern material types, like insulatedconcrete forms, MJ7 simply can’t calculate well for those, while MJ8 can. Besides materialissues, infiltration and duct gains and losses were made more accurate in MJ8 as well. Sowhile overall results on projects that both methods can address aren't huge, MJ8 was still a



big improvement over MJ7.

Comparison of Commercial Methods

In the comparison of commercial load calculation methods, there is more controversy overwhich method is best than there is with residential methods. The three most popularcommercial methods are ASHRAE CLTD, TFM, and RTS. ACCA has a commercialmethod called Manual N, which is a variation of the CLTD method, so all commentsconcerning the ASHRAE CLTD method are applicable to the ACCA Manual N method.

The HB method is not used in any dedicated commercial peak load calculation program atthis time, but it is used in the EnergyPlus energy analysis program, which is sometimesused by a very small percentage of designers as a peak loads program. The WF methodused in all the DOE2 derivative programs is also used by a small percentage of designersfor the calculation of peak loads

The ASHRAE CLTD method is the oldest of the current popular commercial methods, andwas first introduced back in the 1977 HOF. It was further explained in the 1979 ASHRAEGRP158 manual. It is actually a simplification of the TFM method much in the same waythat RTS is a simplification of the HB method. The basis for the TFM method was firstpublished in the 1972 ASHRAE HOF. However, the TFM method is more math intensivethan CLTD, so it took much longer to develop into useable a form for HVAC designers.

Being a simplified method derived from TFM and able to be tabularized, the CLTDmethod was able to be used by designers with slide rules, early calculators, and pencil andpaper. It was also simple enough that it could be automated on very early small 64kmemory personal computers (called microcomputers in those days). Elite Software was thefirst to implement CLTD on personal computers back in 1979. In the mid 90’s, personalcomputers gained enough memory and computational capability that the TFM method wasable to be implemented.

The CLTD method enjoyed popularity right from 1977, and if it was derived from TFM,shouldn't a true TFM method become more popular than CLTD? In my opinion, the TFMmethod did not become more popular than the CLTD method. Carrier exclusively adoptedTFM into its software, while Trane offered it as an option along with CLTD and othermethods.

Elite Software never adopted the TFM method, as there were a lot of discrepancies withCLTD results on glass solar loads, especially with the south orientation. Some peopleargued that TFM, being more math oriented, should be trusted over CLTD in those cases,but many engineers believed that the reduced TFM solar loads, sometimes as much as 40%lower than CLTD, were not believable. This controversy was large enough that EliteSoftware never implemented the TFM method into its software.

Besides questions about glass solar load accuracy, another reason so many HVACdesigners continued to prefer the CLTD method was that it was manually verifiable. Adesigner was still able to verify by hand the calculations of any computer program thatused the CLTD method. With the TFM method, the calculations were so involved thatthere was no practical way for a designer to manually verify them. Results from acomputer program using the TFM method were essentially "black box" results that had tobe accepted on faith alone.

As time passed, HVAC designers became more comfortable accepting results that couldnot be manually verified. This helped increase the popularity of TFM, but also opened thedoor for accepting other improved methods that could not be easily hand verified.



In the 2001 ASHRAE HOF, the HB and RTS methods were introduced. As mentionedpreviously, the HB method is theoretically the most accurate method of peak loadcalculations. Elite Software studied these methods and learned why two methods wereintroduced simultaneously, something ASHRAE had never done before. HB wasdeveloped for maximum accuracy while RTS was derived from HB to preserve componentload contributions in the final results that the HB method does not preserve.

In discussion with various developers of the HB and RTS methods, Elite Software wasable to determine that RTS was preferred by most consulting engineers. And just asimportant, it was discovered that glass load results from RTS did not differ as much fromCLTD as TFM differed from CLTD. Thus, anecdotal confidence in the accuracy of theRTS results was high, and Elite Software became the first software company to implementthe RTS method in commercially available software in 2004.

In my observations over the years and across a broad spectrum of projects and locations,the CLTD method has tended to calculate overall higher than TFM, typically about 1015%. As mentioned before, specific glass solar calculations could be higher by as much as40%. The CLTD and RTS methods compare fairly closely, and either method can be plusor minus 510% of each other. This close agreement happens when making close matchesof CLTD material types with RTS material types, i.e., first selecting a CLTD buildingmaterial, and then matching it to an RTS material.

If you go in the other direction from RTS to CLTD, RTS allows so much variation andsensitivity on material variations that you can’t always get a good match with a CLTDmaterial. A major benefit of RTS over CLTD is the provision for absorptance, emittance,Delta R, and Houtside coefficients. The effects of these factors are built into the CLTDtables and are not specific user inputs as they are with RTS.

When compared to peak loads obtained from the DOE2 WF method and the EnergyPlusHB method, it always seems to me that those methods calculate lower than CLTD, TFM,and RTS, usually in the range of 1025% lower. This is very easy to explain for winterheating loads. The energy analysis programs when used as peak load programs always takecredit for any solar gains, people, lighting and equipment loads to help offset winter heatlosses.

The specific peak load methods are more conservative and do not give credit for anyoffsetting loads to winter heat losses. To me, the conservative approach is justifiable, asthe peak winter heat loss can easily occur in the dark of night with no one at home and nolighting or equipment operating.

Why the energy programs using WF and HB methods tend to always calculate low on peakcooling loads compared to MJ8, CLTD, TFM, and RTS is harder to understand. Onereason that it is hard to analyze is that the energy programs don’t give a good break out ofthe individual component loads. So you can’t compare differences in the roof, wall, andglass loads at the worst case outdoor design conditions.

Another reason I believe there are frequent differences in the peak loads of energyprograms is that it is harder for a designer to control what the extreme upper and loweroutside temperatures are. Many designers don’t make sure the outside design temps areequal when comparing peak loads from an energy program to a dedicated peak loadsprogram. The energy programs tend to use less extreme outdoor temperatures and thisreduces the peak loads they calculate.

Whatever the reasons for these differences, many designers do not trust sizing HVACequipment based on the peak heating and cooling loads obtained from software primarily



developed for building energy analysis. I agree with that concern, as there is a high risk ofunder sizing HVAC equipment when it is sized based off the peak load calculations fromvarious energy analysis programs. This concern is valid for equipment sizing in bothresidential and commercial applications.

In my opinion, the ASHRAE RTS method is the best overall commercial peak loadcalculation method available today, as it provides both high accuracy and component loadcontribution (roof, wall, glass, etc.) reporting. Perhaps someone will develop an approachin the future that allows the HB method to retain component loads in the final results.Perhaps designers may evolve to where component load details are not so important tothem. Perhaps a completely new theory will be developed that is more accurate than HBand with preservation of component loads. But until then, I believe RTS is the best overallcommercial HVAC peak load calculation method available today.

Summary

In the residential world, the ACCA MJ8 procedures remain the most popular residentialsizing method. The MJ8 procedures are not technically advanced compared to thededicated commercial methods such as TFM and RTS. And MJ8 is certainly not asadvanced as the methods able to do both residential and commercial peak loads and energyanalysis such as the WF and HB methods.

But what MJ8 has going for it is years of dedicated specific residential structure heattransfer analysis. This has allowed the development of hundreds of load factor tables thatrepresent very well what happens in peak HVAC loads on residential structures. As ACCAcontinues to refine its procedures, I think the Manual J method will see big changes in thecoming years as it catches up on calculation theory with the more advanced commercialand energy analysis methods.

As mentioned before, the ASHRAE RTS method is a great method in that it employs onlya small compromise from the HB method in order to maintain component loadcontributions. I think the RTS method has a very long future, with only small refinementsneeded.

About the Author:

Bill Smith is the president, owner, and founder of Elite Software. Mr. Smith wrote the firstcommercially available load calculation program using ASHRAE CLTD procedures in1979. Elite Software was the first company to sell an ACCA Manual J program based onthe 6th edition in 1984. Mr. Smith invites your comments on this article, and will continueto update it as more developments occur.

Mr. Smith welcomes your email about this article. email

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hvac peak load calculation methods – history and comparisons

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