Article published in Home Energy Magazine
Issue Dated: May/June 1995
Written by: John Proctor, Zinoviy Katsnelson, and Brad Wilson.
A colleague of ours (we will call him Bill) approached us at a conference seeking advice on selecting an air conditioner for his renovated home. Our recommendations included, "Be sure that the cooling load is calculated and that the air conditioner is sized to that load." When Bill attempted to follow these instructions, only one of the four contractors would submit a sizing calculation (two others just wanted to know how many square feet there were in the house).
Bill hired the contractor who did the calculation and installed a high-efficiency four-ton unit. Is this a success story? Not really. The contractor calculated a total cooling load of 37,580 BTUs per hour at 105 degrees F outside and 70 degrees F inside. While the cooling load he calculated could have been met by a three-and-a-half ton air conditioner, the contractor convinced Bill to buy a four-ton unit "because then you will always have plenty of cooling."
Bill's air conditioner short-cycles (runs for shorter periods of time than it should) even during the hottest weather and removes very little moisture from the air. What went wrong? Four things:
ACCA has also produced Manual S for selecting equipment and Manual D for duct design (revised in January 1995). Manual S provides a method to select air conditioners based on the estimated sensible and latent load calculated for the particular house in the local climate.
If mistakes are made in the load calculations or the sizing method is flawed or incorrect inputs are used, the equipment will be incorrectly sized and will not perform as it should. Field studies have shown that most equipment is substantially oversized compared to Manual J specifications. In the Model Energy Communities Project, Pacific Gas and Electric Company (PG&E) found that 53% of the air conditioners checked were a ton (12,000 Btu/h) or more oversized and a study by Pacific Northwest Laboratories found a third of the air conditioners to be a ton or more oversized.
Because of the efficiency penalty of oversized air conditioners and because oversized air conditioners contribute substantially to utility demand peaks, in 1994, PG&E commissioned a study by Proctor Engineering Group to compare common load calculations and sizing methodologies to Manual J calculated values.
For instance, a 2.5% summer design temperature of 100 degrees F for Fresno, California, means that the temperature generally only exceeds 100 degrees F for 73 hours in the season (0.025 x 2,928 hours in the months of June through September). A theoretical perfectly-sized air conditioner will run continuously during those 73 hours. During the rest of the time the air conditioner will cycle and operate at less than its potential efficiency.
A properly sized air conditioner should provide maximum value to the customer as well as a reasonable profit and further customer referrals for the contractor. If an air conditioner is cycling even at four in the afternoon on the hottest days, it is a sure sign it is oversized. Incidentally, if the air conditioner is running continuously on hot days, it doesn't necessarily mean that it is the right size. It is more likely that the system is oversized and has one of three big problems: leaky ducts, improper charge, or low air flow across the coil (see "An Ounce of Prevention; Residential Cooling Repairs," Home Energy Magazine May/June U91 p. 23). or PG&E Appliance Doctor Pilot Project - Summer 1990 Activity
Customers depend on the expertise of contractors in selecting air conditioners. Yet contractors generally size air conditioners at least a half-ton larger than necessary and often oversize by a ton or more. Even the most conscientious contractor is driven to avoid call-backs (or even lawsuits). An oversized air conditioner can mask problems from duct leaks, improper flow across the coils, and improper charge. Unfortunately, many customers think that "bigger is better," so in a competitive situation, the contractor proposing the proper size unit may lose the bid. Contractors are hesitant to adopt an unfamiliar method of sizing when the methods they have developed over the years have served them well: "I've done it this way for 30 years and I've never had a complaint."
It is no surprise then that air conditioners are oversized; however, the advantages of a properly sized air conditioner are so large that these barriers need to be overcome. Customers pay a price for oversized air conditioners, and in many climates, lose comfort as well.
A properly sized air conditioner costs the customer less (see Figure 1). Bill's air conditioner cost him more money because it was too big. The contractor had the opportunity to discuss the value of the air conditioner based on the delivered efficiency and offer Bill equipment at a lower cost. He missed the opportunity.
Most of the cooling season the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Because of the short cycles, Bill's high-efficiency air conditioner is less efficient.
For short cycles, the coil does not have time to operate at the low temperature and when the unit stops, the moisture on the coil evaporates back into the indoor air. Thus, in humid climates, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units. Bill's oversized air conditioner could not remove enough moisture from the air, so his house was cold and clammy.
With a properly sized air conditioner, it is easier to have sufficient supply and return grille area to keep the air speed low and the noise at a minimum. Common complaints about oversized air conditioners are that they blast frigid air and that they are noisy. A properly sized air conditioner, with proper ductwork and grilles, will provide longer cycles, more consistent temperatures, and better mixing of the house air.
ACCA Manual D specifies a maximum return grille velocity of less than 500 ft per minute and a maximum supply outlet of less than 700 ft per minute.
Figure 3 shows that for a standard 2' x 2' return grille, the 500 ft per minute requirement is exceeded with all units over 2 and one-half tons, with the resulting increase in noise.
In the second part of the study, Proctor Engineering Group compared four different equipment selection methods to determine how close the selected equipment capacity came to the calculated load. The capacity of an air conditioner is dependent not only on the outdoor conditions, but also on the actual indoor conditions (temperature and humidity). Proctor Engineering Group developed a procedure for estimating the expected indoor humidity at design conditions. By knowing both indoor and outdoor conditions, the capacity of the selected air conditioner was determined.
For both parts of the study, loads were calculated for buildings of different age and construction in two different climate zones.
Of the 40 load calculations that were submitted, we approved those that yielded building loads within 20% of Manual J as received. This group included four worksheets, one calculator method, and five computer programs. The approval process was interactive and led to many stimulating conversations. David, a contractor for over 20 years, shared some of the "seat of the pants" methods he had observed through the years.
One method was to "buy the distributor's overstock," another was to "install the rejected unit from a previous job," and still another was to "install the unit sitting in the truck or at the shop." David referred to these methods as "sizing by cost."
Contractors submitted methods that they sincerely believed would properly size air conditioners. Some of the methods, however, were based on information from as long ago as 40 years. These methods did not take into account the latest efficiency developments in building insulation, windows, and air tightness.
The methods were often handed down from the person who taught them the business. "I learned this from my father and it has always worked." Since the contractors had received few or no complaints of inadequate cooling, they considered their methods sufficient. Unfortunately, they were significantly oversizing units; particularly on newer more energy-efficient homes. In an effort to properly determine cooling load, some contractors had spent good money on computer programs, had developed their own methods from books in the library, or borrowed from other contractors in the area. Proctor Engineering and PG&E worked with these contractors to find ways to bring their methods within 20% of Manual J. With changes, an additional ten methods were approved. This second group included seven worksheets, one calculator method, and two computer programs.
Altogether, 50% of the submitted methods were approved for use in PG&E's service territory. Methods that will calculate loads within 20% of Manual J will vary with the climate because of the way latent loads are treated. Of the approved computer methods, RHVAC from Elite Software was the most user friendly. Right-J from Wright Associates faithfully followed ACCA Manual J. A number of the methods did not calculate the latent load of the home.
Many assumed that the latent load was 30% of the sensible load. The actual latent load is highly dependent on the air tightness of the home, the local climate, and the interior moisture sources (such as people). For hot, dry climates, the latent load will be far less than 30%, particularly if the house has a large amount of air leakage from the attic. For humid climates, the latent load can be higher than 30% of the sensible load if the house has a significant amount of air leakage. In all cases, infiltration loads (air leakage) were not specifically addressed or were calculated by an oversimplified procedure. Contractors often assumed that infiltration rates were the same in all buildings or only depended on floor area. With the widespread use of blower door testing, we now know that homes vary significantly in their leakage rate.
With the amount of data required to do an accurate load calculation, the possibility of errors is increased. Even the computerized methods of load calculation were seriously lacking in error checking procedures that could catch operator errors. For example, window areas can exceed wall areas, or wall areas facing north can be one square foot with a south wall of 300 square feet and east and west walls of 200 square feet. Many of the methods also oversimplified the process and gave insufficient options for climate, building assemblies (windows, doors, walls, etc.), and shading.
How should the loss due to duct leakage be taken into consideration when an air conditioner is sized? The answer of course is simple. Don't take duct leakage into account - fix the leaksdegrees
While this approach is rapid and simple, it does not account for orientation of the walls and windows, the difference in surface area between a one-story and a two-story home of the same floor area, the differences in insulation and air leakage between different buildings, the number of occupants, and many other factors. In some cases contractors attempt to cover these variables by categorizing the home as low (a new home in a moderate climate), average, or high (an old home in a hot climate) but this method also falls short of properly sizing air conditioners.
Figure 5 was produced with those types of categorizations.
Manual S provides a process for selecting equipment that will meet the sensible and latent loads at Manual J design conditions. Its primary strength is that it guides the user to select an air conditioner that has a sensible capacity between 100% and 115% of the calculated sensible load. This is a major improvement over a number of other methods. The two main weaknesses of Manual S are that it dictates using 50% (or 55%) indoor relative humidity and it sets no upper limit for latent capacity. In dry climates the infiltrating air carries less moisture into the house, the indoor relative humidity is lower, and the latent load is lower. With less moisture in the house air, the air conditioner runs at a higher sensible capacity.
John Proctor is president and Zinoviy Katsnelson is a senior research engineer with Proctor Engineering Group. Brad Wilson is a senior program development manager at Pacific Gas and Electric Company in San Francisco, California.
The report discussed in this article is available from Proctor Engineering Group, 818 Fifth Ave., Suite 208, San Rafael, CA 94901.
This article is reprinted from a series on energy-efficient remodeling, which is being funded by the Environmental Protection Agency and the Department of Energy.
Short Cycles
Air conditioners are very inefficient when they first start operation. It is far better for the air conditioner to run longer cycles than shorter ones. The efficiency of the typical air conditioner increases the longer it runs. 
Figure 2 illustrates that if the on-time of an air conditioner is only 5 minutes the efficiency (EER) is 6.2. If a properly sized air conditioner half the size were used instead, the same amount of cooling would take place in about 9 minutes, and the efficiency would rise to 6.9. This represents a savings of 10% for the customer.
Moisture Buildup
The ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain.
Noisy Operation
The speed of the air blowing through the supply registers and the air being drawn into the return grille affects an air conditioner's performance. If the air speed is too high, it will be noisy and uncomfortable, and the return grille filter effectiveness will be reduced. The speed through the grilles depends on the size of the air conditioner (a larger unit has more air flow and higher air speed) and the area of the grille (a smaller grille causes higher air speed).
Sizing Up the Sizing Calculations
To qualify for PG&E's air conditioner rebates in 1994, contractors were required to submit their load calculation methods, and they had to submit the actual calculations for each job. We compared over 40 different load calculation methods submitted by manufacturers, distributors, and contractors to Manual J. Manual J was used as a baseline because it is the most widely accepted load calculation methodology and is generally recognized as providing a safe estimate of cooling load. (Some experts believe Manual J consistently overestimates the load by about 20%, as a built-in "safety" factor.)
Most Contractors Oversize
The submitted calculations were all over the place (see Figure 4). In the extreme, the calculated load was three times the Manual J calculated load.Don't Duck the Duct Factor
The effect of duct leakage has only recently been investigated to any significant extent. As a result, cooling loads due to duct leakage are not included in any of the methods. Duct leakage has three effects on design cooling load. First, a supply leak is a direct loss in capacity. Second, a return leak will often bring in superheated attic air. Third, the difference between supply leakage and return leakage will cause increased infiltration. While it is tempting to treat duct leakage as additional infiltration, the effect is actually more complex.
Sizing by the Square Foot
The "square-foot-per-ton" sizing method avoids calculating the cooling load of the building and proceeds directly from the square footage of the building to the size of the air conditioner. No contractor submitted such a method for approval but a number of contractors reported that they often used this method, or knew others who did. In a study by the Florida Solar Energy Center, 25% of the contractors reported that they size by floor area (see "How They Size Air Conditioning Systems in Florida," above).
Selecting Equipment with Manual S
Manual J (or other methods) gives a contractor both the sensible and latent design loads for the house. A common, but wrong, practice is to divide the total cooling load by 12,000 Btu/h per ton and choose an air conditioner with that nominal tonnage. Nominal tonnage does not indicate capacity under differing design conditions.
As shown in Table 1, Manual S could result in the selection of the same equipment in both humid and dry localities, with very different results.
Both Manual J and Manual S are methods that "safely" oversize air conditioners. In dry climates, Manual S will oversize even more. If these two methods are applied there is absolutely no reason to build in additional safety factors when selecting air conditioning equipment.
Problems with Manufacturer's Data
Air conditioners selected based on standard indoor conditions of 80 degrees F with 50% relative humidity (which is the standard ARI capacity rating condition) will be incorrectly sized for 75 degrees F. Unfortunately, many of the major manufacturers provide information only at 80degreesF. It would be a great improvement if the manufacturers provided tables that presented the sensible and latent capacities at 75 degrees F for a variety of indoor humidities.
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