This article was originally published in the March/April 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online March/April 1998
Efficient Cooling: Making It Happen
by Craig Sherman and Eric Hildebrandt
What's the best way to size a residential air conditioner? To what extent do air conditioners tend to be oversized? How can energy research and programs help promote optimal sizing of cooling systems? These questions can heat up a discussion, but a new study suggests solutions that help everyone.
effectively promote more efficient sizing of residential cooling equipment.
SMUD's research suggests that installing smaller HVAC equipment may save real money for contractors and homeowners alike. These first-cost savings alone may pay for the extra cost of rightsizing HVAC equipment. Between comfort, efficiency, and first-cost savings, there are a variety of selling points that can be used to promote proper sizing of HVAC equipment. While most utility programs have focused on training HVAC contractors, These selling points could also increase customer demand for rightsized cooling equipment.What's the Debate? When it comes to how air conditioners should be sized, there's a wide gap between the perspectives of residential energy researchers and HVAC installers. Most researchers agree that central air conditioners are frequently oversized--at least from the standpoint of overall operating efficiency. They say that air conditioners run most efficiently when they are allowed to operate for extended periods, rather than frequently cycling on and off. On a hot day, a larger air conditioner that has just been turned on can bring the house to the thermostat setpoint several minutes faster than a smaller unit. But under more typical conditions, the larger unit will usually operate much less efficiently, and won't usually provide better comfort (see If Bigger Is Not Better, What Is? HE Sept/Oct '96, p. 13).
To ensure proper sizing of air conditioners, these efficiency-minded experts recommend that contractors use Manual J, a methodology developed jointly by the Air Conditioning Contractors of America (ACCA) and the Air-Conditioning and Refrigeration Institute (ARI). Manual J is a building load calculation method based on complex modeling and analysis. It makes fewer simplifying assumptions than most other load calculation methods that contractors use. Still, some experts, such as John Proctor of Proctor Engineering in San Rafael, California, believe that Manual J results may actually size equipment larger than efficiency and comfort require, particularly in dry climates. In fact, Proctor has found evidence that Manual J may overestimate building cooling loads by as much as 50%.
Rather than using Manual J to determine the cooling load, most contractors use simpler, less time-consuming methods that err further toward oversizing. These methods provide wider margins of safety against equipment undersizing--at the expense of efficiency. Virtually every equipment contractor has a story to show why bigger is usually better for ensuring adequate cooling on the hottest days of the year, avoiding complaints, and satisfying customers.
Most contractors are not aware that Manual J already incorporates a margin of safety. And, its margin may not be large enough to accommodate poorly designed or malfunctioning air handling systems, leaky ducts, and inadequately charged refrigerant.
Some contractors believe that sizing units larger provides a margin of safety that can compensate for such problems and prevent customer complaints or callbacks. However, even significant oversizing of the air conditioning unit is unlikely to mask more serious problems in the air distribution system.Methods for Sizing Air Conditioners Contractors and engineers use many different methods to determine the cooling load. Methods range from Manual J calculations to simple rules of thumb. Some contractors have even developed customized spreadsheets based on building size and types. When an air conditioner rebate program at Pacific Gas and Electric Company (PG&E) asked contractors for their load calculation methodologies, the utility received over 40 different calculations. One common rule of thumb in the Sacramento area is to allow 1 ton of sensible cooling capacity for every 500 ft2 of conditioned space, or about 1 ton of total capacity per 650 ft2.
Contractors who do not use Manual J typically spend about 30 minutes assessing the type and size of equipment to install on a residential installation. They claim that including a detailed room-by-room Manual J calculation increases the time to about four hours--two hours to collect data on-site and two hours to enter the data into a software program and analyze the results.
For a typical home in the Sacramento area, contractors charge at least $200 for a Manual J load calculation. The cost can be significantly higher for large or unusual homes. On the other hand, in new construction, mechanical system sizing can be included in the energy code compliance calculations.
Some researchers don't think it takes that long to complete Manual J. According to Proctor, it takes much less time and skill to do a whole-house Manual J than a room-by-room assessment, and for the purpose of sizing the air conditioner, a whole-house calculation is adequate. It takes about 30 minutes to gather the data in the home and enter it at the same time.Research in Sacramento To determine the extent of oversizing in the SMUD service area, we selected a sample of 40 homes that had received a new cooling unit in 1995 under SMUD's rebate, financing, and quality assurance program. During 1995, nearly three-fourths of customers replacing central cooling equipment in Sacramento participated in SMUD's program.
The sample used in this study included equipment installed by 22 different contractors. Together, these 22 contractors account for about two-thirds of the air conditioners and heat pumps installed through the program. The largest 5 of these contractors alone accounted for over half of all equipment installed with SMUD rebates or financing.
We looked at all system types installed through the program. The percentage of each type used in the study was proportional to the percentage of that type installed through the program. We inspected each home used in the study, collecting enough data to perform room-by-room Manual J load calculations using the RHVAC program by Elite Software.
We used these load calculations to estimate the sensible cooling load in each home. Calculations were performed using Manual J recommended design temperatures for Sacramento's climate. We then compared the sensible load estimate from Manual J to the sensible capacity of the installed equipment from manufacturers' specifications. For the sake of comparison, we also compared the size of the installed equipment to what would have been installed using the rule of thumb of 1 ton of sensible cooling capacity per 500 ft2. Finally, we researched the incremental costs of different sizes of cooling equipment and calculated the potential equipment cost savings from rightsizing equipment based on Manual J.Less Oversized Than Expected We found that equipment tended to be oversized relative to Manual J load calculations (see Figure 1). One-third of the 40 cooling units inspected were sized within .25 ton of Manual J, while 53% of the units were oversized by at least .25 ton. Only 15% of the sample were oversized by .75 ton or more. Overall, equipment was oversized by an average of .30 ton.
However, the percentage of units oversized by 1 ton or more was significantly lower than that reported in several other recent studies. We found only 10% to exceed Manual J calculations by 1 ton or more. A previous study in the Pacific Northwest placed this figure at 33%; another, in PG&E service territory in California, placed it at 53%.
Only about 13% of the sample was undersized by more than .25 ton, and only two of these were units undersized by more than 20%--the level of oversizing that seems to be built into Manual J (see How Big Is Enough, HE Sept/Oct '96, p. 17).How Good Are Simple Rules of Thumb? Rules of thumb are often thought to lead to equipment oversizing. However, results of SMUD's research indicate that using a simple rule of thumb (1 ton of sensible cooling capacity for each 500 ft2 of conditioned space) would result in only slightly more oversizing than methods used to size equipment in the 40 homes in the study. Fifty-three percent of the units in the study were found to be oversized by at least .25 ton compared to Manual J; 60% would have been that oversized if this simple rule of thumb had been used to size all 40 units. On average, units would have been oversized by .35 ton using this rule of thumb, compared to the average oversizing of .30 ton in the sample.
Sizing equipment with Manual J would result in significant savings to customers and contractors by reducing equipment and operating costs. Table 1 shows the estimated range of potential first-cost savings that could be achieved by installing the generally smaller equipment called for by Manual J calculations. Since the cost of installed equipment depends on the size of equipment and type of vendor (wholesale, high-volume contractor, low-volume contractor), savings were calculated based on a range of costs for different equipment and sales types.
For the 53% of customers in the study sample who could have downsized equipment by following Manual J, average first-cost savings would range from $515 to $770, based on an average reduction of .65 ton per unit. If all units had been sized with Manual J, eliminating both undersizing and oversizing, average first costs would still drop between $240 and $350 per installation.
The potential savings from downsizing cooling equipment appear to compare favorably with the additional labor cost of performing more detailed load calculations. Manual J calculations can be worthwhile, since on average, savings from reduced equipment costs are likely to at least equal the $200 to $250 that local contractors charge for a Manual J calculation. If local contractors were to follow Proctor's advice and do the simpler whole-house load calculation, the savings would be even more dramatic.
In addition to savings from reduced equipment costs, using Manual J to size air conditioners reduces energy costs by increasing operating efficiency. Proctor estimates that a unit oversized by 1 ton may operate up to 7% less efficiently than a unit sized based on Manual J. For a typical home in Sacramento, which may use 1,200 kWh per year for cooling, this represents additional savings of about $8 per year, or a discounted present value of $65 over a 15-year life of the equipment.Working with HVAC Contractors In the past, most efforts to promote proper sizing of cooling equipment have been focused on HVAC contractors working with customers through utility rebate or financing programs. But even utilities that do not offer rebates or financing for HVAC equipment can encourage proper equipment sizing by providing customers with direct contractor referrals and lists of approved contractors. Utilities and other organizations may use a variety of approaches to work with contractors to promote better equipment sizing.
One innovative utility program was PG&E's 1994 air conditioning rebate program. To qualify for the program, contractors were required to submit the methodologies they used to perform load calculations. Methods were approved if the utility found they yielded building load calculations within 20% of Manual J. Contractors were then required to submit actual load calculations, using the approved methodology, for each installation.
Another option for utilities is to require load calculations only when the size of newly installed equipment varies from the size of previously installed equipment, and for installations in homes that did not previously have central cooling. However, it can be difficult, costly, or impossible for utilities to actually verify the size of previously installed equipment.
Another variation on this approach would be to offer an incentive to contractors to get them to submit the methodologies they use and some specific samples to the utility. The utility could then compare these results to load calculations based on Manual J and find ways to improve the methodologies to result in better equipment sizing.
Perhaps the ideal option would be to develop more simplified sizing methodologies that compare favorably with Manual J for the local climate--but require less time to perform. Contractors participating in energy efficiency programs could be either required or encouraged to utilize this type of simplified calculation. Most of the time required to perform a Manual J calculation is spent collecting room-by-room information and entering the data into a computer. A computer can perform the actual Manual J calculations in seconds. Simplified methods may reduce data collection and inputs; hopefully, this would not come at the expense of precise load calculations.Creating Customer Demand for Proper Sizing Utilities and other organizations promoting energy efficiency don't have to work only with contractors. They can employ a variety of approaches for creating increased customer demand for properly sized HVAC equipment (see Homeowners: Who Will Demand Rightsizing?). Relatively little seems to have been done to pursue this customer-driven approach; utility programs and other informational efforts have been focused almost exclusively on HVAC contractors. Programs can offer rebates, financing, contractor referrals, or information to customers. The agency sponsoring the program can then provide targeted information and recommendations to customers early in the decision-making process.
Most importantly, perhaps, customers should be made aware of the tendency for cooling systems to be oversized, and the fact that oversized equipment costs more to purchase as well as to operate. For instance, local customers should know that about half the new systems in our study were oversized, and that on average an oversized system costs at least $515 more than a unit sized according to Manual J. These kinds of numbers might pique homeowners' interest in rightsized equipment.
Homeowners can be encouraged to question sizing recommendations. They can ask contractors the size of proposed new equipment and the size of existing equipment. When a homeowner who is satisfied with an existing system is offered larger equipment, he or she should be prepared to ask for a more detailed explanation. In such cases, homeowners can also ask contractors to provide cost estimates for both sizes of equipment in order to quantify the higher first cost of oversizing.
To estimate the approximate size of the unit they need, homeowners in the Sacramento area can use the rule of thumb of 1 ton of sensible cooling capacity for every 500 ft2 of conditioned space in their home. This method would let a homeowner know if the unit is grossly over- or undersized. It might be possible to develop rules of thumb with similar levels of reliability for other climates. If the contractor proposes installing a unit that is larger than the local rule of thumb would indicate, the homeowner should ask him or her to explain why. The homeowner can also ask the contractor to provide cost estimates for both sizes of equipment.
Homeowners can also ask contractors to explain and document of the method used to determine what size equipment to install. Although it may be difficult or impossible for most homeowners to assess the accuracy of different sizing methods, asking for this information may help differentiate between the contractors who follow very simple rules of thumb and those who do more detailed sizing calculations.
In addition, since contractors frequently offer customers more than one sizing option, efficiency programs can advise customers about the potential tradeoff between overall efficiency and rapid cooling. Homeowners can also be educated about alternatives to oversizing, such as programmable thermostats and dual-speed air conditioners.
Craig Sherman and Eric Hildebrandt are with Energy Services' Research and Evaluation Section at the Sacramento Municipal Utility District in Sacramento, California.
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