HVAC Load Calculations, Old And New

August 1, 2002
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HONOLULU, HI — In commercial, residential, industrial, and institutional HVAC design, it is critical for design contractors and engineers to perform a load calculation. Not only is it important for occupant comfort, it is also needed in order to avoid indoor air quality (IAQ) problems, which can lead to costly, even ruinous litigation.

The importance of running a load calculation is not a new concept — but the load calculations are. And even the calculations are not entirely new. They are based on older load calculation methods, which we can now compute much more quickly than when they were first devised.

Lynn Bellenger, P.E. (Pathfinder Engineers, Pittsford, NY), opened the ASHRAE seminar “Review of Load Calculations Procedure” with an overview: “How Did We Get Here.” It is probably the best way to understand and appreciate the current load calculation methods.

FROM THERE TO HERE

Bellenger described the evolution of the current load calculation methodology as “a continuing process” that started in 1967.

It began with Total Equivalent Temperature Difference/Time Average (TETD/TA) methods, which Bellenger said proceeded in logical steps and were easy to understand. The equations were “time consuming but clear,” she said. And the results validated field results.

Their disadvantages: They were “repetitive, time-consuming equations,” Bellenger said. And although their mathematical results were validated in the field, they were still “really an approximation.”

Then came the Transfer Function Method (TFM), which factors together heat gain by conduction through exterior walls and roofs; conversion of the cooling load from heat gain; and use of room transfer functions. The method had the benefit of being flexible for load variations, Bellenger said. However, “the calculations were formidable,” and the computers available in 1972 were too costly for most engineering firms to purchase and maintain.

A few years later, the Cooling Load Temperature Difference/ Cooling Load Factor (CLTD/CLF) method was devised, offering greater simplicity due to its single-step calculation, tabular data, and manual method.

“It was simple to use and easy to teach,” Bellenger says, but it didn’t give engineers a feel for the cooling load process. Moreover, it had a limited range of application. In 1984, caveats were announced that stated limitations to the manual method and guidance for its use.

The latest step in evolution has resulted in the Radiant Time Series (RTS) and Heat Balance (HB) methods, which “more accurately model the real-world situation by taking maximum advantage of a powerful, inexpensive computational tool — the computer,” stated Thomas Romine, a past chairman of Technical Committee 4.1, last year. “The change in approach is comparable from using the Farmer’s Almanac to computerized weather models for forecasting.”

Bellenger described the RTS method as rigorous and accurate due to the degree of detail it includes. RTS cooling load calculations include lights, people, equipment, walls/roofs, and fenestration components, among many other specific design details, such as orientation.

INTERNAL LOADS

Christopher Wilkins, P.E. (Hallam Associates, South Burlington, VT), described internal load calculations as “the area of greatest engineering judgment.” These calculations include people, computers, and lights. He also introduced some exciting data on computer equipment loads (the “plug load”) based on a small-scale study in Burlington, VT.

For space cooling loads in Burlington, he stated that people, lights, and equipment are about 60% of the load. However, these data are generally “the least amount of information available to you at the design stage.” This is probably true for most parts of the country.

Regarding the people load: “If you know how many people will be in a given space, that’s a good start,” he said. Knowing their activity is critical. ASHRAE does have information designers can use to figure occupant activity levels into the load calculation.

Regarding the plug load: Nameplate wattage output is generally much higher than the actual output, Wilkins said. “I think everyone is aware now that the nameplate data is quite misleading.”

The surprising thing was, all PCs seem to come out in the same output range (50 to 70 W). Printers and copier wattage output varies a lot, he said, but PCs can be figured quite accurately into the load calculation if the designer knows how many there will be.

Based on real output, most office spaces today would be 1 or 1.5 watts per square foot, he said. “You could probably could get away with 0.5 in some cases,” Wilkins said. “Even for spaces in Silicon Valley [CA], I’ve never found a commercial office higher than 1.25 watts per square foot.”

Regarding lamps: Their types are more variable, depending, of course, on what has been installed. “You need to check if the lights use electronic (newer, lower watt loss) or magnetic ballasts (older, higher watt loss),” he said. High-pressure sodium lights put out the lowest wattage per square foot. Incandescents put out the highest.

HEAT BALANCE COOLING LOAD CALCULATION PROCEDURE

Curtis Pedersen, Ph.D. (Hastings, MN), took the audience through more of the specifics of the “Heat Balance Method” (also the title of his presentation).

All load calculation methods start with some kind of computational model, stated Pedersen. All the models that have preceded HB/RTS have been approximations, albeit good ones. But using the simpler methods, the designer couldn’t see exactly where the load was coming from.

“Computers have come a long way,” he said. Since the number-crunching capability exists, designers should perform the HB/RTS method for the most accurate, most flexible load calculations possible.

The HB procedure, Pedersen said, is one in which air is well-mixed (has a uniform temperature throughout the zone) and heat gains from various sources interact. It is room or zone oriented, rather than surface oriented.

The HB framework consists of:

  • Walls;

  • Ceiling/roof;

  • Floor;

  • Extra “thermal mass” surface; and

  • Windows.

    The framework assumes conditions in which each surface temperature has a uniform temperature, radiates diffusely, and has one-dimensional heat conduction; the variables are the inside and outside face temperatures.

    The essential thermal characteristics needed, Pedersen said, are:

  • Equivalent area and orientation of surfaces; and

  • The construction of each surface.

    Heat Balance is preferred over other methods, Pedersen said, because it permits the investigation of the effects of real parameters, such as carpeting and schedules.

    RELATION TO RTS

    Steve Bruning, P.E. (Newcomb & Boyd, Atlanta, GA), described the “RTS Method” as the “son, nephew, or cousin,” of Heat Balance.

    The problem with the HB method on its own, he said, is applying it in the typical design process. “It just ain’t that simple” as computing cooling loads based on detailed data, Bruning said. “It’s important for engineers to understand the components of the load and the impact their assumptions have on the result.

    “Building design is complex and schedule driven,” Bruning said. “We must be able to make quick decisions based on our best judgment. However, sometimes when we make our decisions, the type of building material (steel or concrete) hasn’t even been made yet.”

    The HVAC system’s size is a critical decision, “and people do make mistakes,” he said. System designers and retrofitters need to know what the impact of certain assumptions are — and the impact of corrections.

    RTS, he said, is able to compute a load calculation model from parameters gained from the HB method. These radiant time series parameters convert radiant heat gains at 1-hour intervals, into the resulting convection gain at that hour and later on. RTS provides an “easy-to-understand basis of assumptions,” Bruning said.

    In most modern buildings, “Appliances and lighting are much more important than walls and ceilings,” Bruning said.

    “RTS is a culmination of years of ASHRAE research,” he summarized. Three out of four major software manufacturers have plans to incorporate it into load calculation software.

    Are more changes in store? Probably. Consider this exchange from the end of the seminar:

    “I’ve been teaching duct loss at 1% of the load, and I know that’s wrong,” stated an audience member. “That was in the CLTD method in the 70s.”

    The Load Calculation Data and Procedures Committee, TC 4.1, is compiling a list for the 2005 ASHRAE Handbook, Bruning said. “We’re always looking for new information to include.”

    Publication date: 08/05/2002

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