This article explores the nature of the efficiency tests required by the U.S. Department of Energy (DOE), and raises issues (not unlike those being raised by the DOE itself) about the validity and long-term future of these test procedures.
It is true that a contractor can talk about the SEER (seasonal energy efficiency ratio) of the heat pump or air conditioner, explaining to the homeowner that SEER is basically a ratio of the cumulative cooling output (in Btu) divided by the total electrical energy consumed over the cooling season (in kilowatt-hours). To obtain SEER, manufacturers must use standard tests and calculation algorithms prescribed by the DOE (see sidebar below). SEER is reported on the yellow EnergyGuide labels on products, through fact sheets provided by the manufacturers, and in industry association directories.
Given the growing concern regarding rising energy costs, does SEER provide a clear indication of how much energy a particular heat pump or air conditioner will use? In other words, does the testing and subsequent calculation of the SEER rating reflect how a system would operate in the real world?
In the world as we know it, manufacturers fulfill the requirements when a unit passes the DOE-prescribed muster. Frankly, by law, the current test procedures are the only game in town and are said to be fairly indicative of general performance by many industry stalwarts. And, as to whether the tests reflect how a system would operate in the real world: Do you really believe that a Toyota Prius gets 60 miles per gallon under any scenario other than “ideal” driving conditions during a simulation?
TIME FOR A SEER OVERHAUL?
Some argue that not only can the testing be flawed, but it doesn’t provide meaningful data, and the algorithm for calculating SEER is antiquated. A quarter century after it was developed, and after 15 years of mandatory use, is it time to look at what we have learned from the field and the lab?There are several elements of the current DOE testing and rating requirements that many consider to be out-of-step with systems installed in the field. These areas of concern include test airflow, latent capacity, external static pressure (ESP), ambient conditions used when testing, and the seasonal weather data used in the calculations.
Airflow across the evaporator is a critical ratings parameter, yet manufacturers are able to choose the airflow for SEER tests (up to a maximum of 450 cfm per ton) that can help to optimize the rating value. Steve Kavanaugh, professor of mechanical engineering, University of Alabama, noted that tweaking the airflow is one of the easiest ways a manufacturer can affect the test procedure.
“You can take a heat pump or air conditioner that has a medium-sized 3-ton outside coil, then you can take a great big 5-ton indoor coil and blow a little bit of air across it, and that can increase the SEER,” he said. “This setup wouldn’t bother someone in Phoenix, where it’s dry, but in Alabama, we’d be growing mold on the walls, because this scenario wouldn’t remove any moisture. Current SEER procedures do not penalize for poor dehumidification.” Kavanaugh would like to see the test procedure specify a standard airflow of 400 cfm per ton, so test conditions would be the same from one piece of equipment to the next and from one manufacturer to the next.
Kavanaugh’s comment also depicts an issue with regard to regional differences. The current SEER tests are designed so as to be applied uniformly throughout the United States. That means that certain products will likely perform better or worse in parts of the country that are not in close agreement to the test parameters. Think of driving the Prius in icy Alaska conditions all year long versus driving in sunny Phoenix. Actual performance in various regions can be considerably different than ideal test conditions.
DIFFERING OPINIONS
Alan Kessler, vice president of research and development at Rheem Manufacturing was intimately involved in the development of the ARI 210/240 certification standards and of the DOE test procedures on which ASHRAE 116 standard is based. He holds a more conservative opinion than do some who seek changes to the rating system.“I’m not aware of any loopholes in the DOE-approved test procedures. The history of the test development began with the ASHRAE 37 standard which ARI 210/240 used for the original steady state test points of 80°/67°-95°. These original conditions were thought to be a reasonable representation for the ARI 210/240 certifications. There was a desire among some people to have a seasonal energy efficiency ratio test procedure in order for consumers to have a standardized rating that worked across the entire country. The new SEER test procedures became a reality about 1978 and required additional steady state test points of 80°/67°-82° which are referred to as B-test conditions and 80/57-67 C (steady state) and D (cyclic) tests. These tests don’t completely represent the operation of the system, but it accomplished the goal of providing a standard rating point that is more nationally representative and could be used for a seasonal operating cost calculation,” said Kessler.
According to Kessler, SEER is a good relative indication of system efficiency and relative cost of operation. “SEER is the only measure of residential equipment that gives the consumer a common basis of comparison for operating costs across an air conditioning season. This does not mean that it will be the operating cost in a particular circumstance, but it will provide relative comparisons to operating costs between equipment choices of various manufacturers.”
Harvey Sachs, director of buildings programs, American Council for an Energy-Efficient Economy (ACEEE).
TEST VALUES QUESTIONED
The values used for ESP in the test procedures can also be incorrect, according to Harvey Sachs, director of buildings programs, American Council for an Energy-Efficient Economy (ACEEE). “The assumptions about the resistance to airflow of the ductwork are way off. The assumptions in the test are at less than half the average external static pressure measured in the field. That means the airflow is reduced because you just can’t push the air as hard.”Kavanaugh added that only using a simple, clean fiberglass filter will influence the ratings as well. “The test procedure states that for a 3-ton unit, all that is needed is 0.15 inches of ESP and a clean air filter. If you run your fan under that test condition, you can run it hard without kicking the fan wattage up too high, because there’s so little restriction. But when you go out in the field, you’re not going to run the fan at that low load. You’re also going to put in a more efficient filter, and it’s going to get loaded up, then the ductwork will be more restricted. You’re probably going to triple the amount of pressure that’s required from the fan in the field compared to what is required of the DOE test procedure.”
To eliminate that problem, Kavanaugh would like the test procedure to mandate an ESP of 0.5 inches of water (IWP) with a MERV 6 filter. As Sachs noted in his paper, “Can SEER Be Saved?”, field tests consistently show average values of 0.5 IWP for residential systems. The low values of ESP in the test procedure mean that fan power is lower than actual values observed in the field and actual seasonal efficiency is lower than the SEER value would predict. Higher-than-expected ESP can be attributed to a combination of factors, particularly upgraded air filters with high pressure drops and constricted ductwork.
Kessler said, “As with all applications there are variations which may be due to misapplication or selection of more or different accessories.” He disagrees with the contention that increased restriction of airflow (high-efficiency filters) results in fan wattage increases. “The physics are such that when you increase the static on a fan, the airflow is reduced and the fan watts decrease, not increase,” said Kessler. However, he added, “Fan watts will increase if a different higher speed tap is employed to provide increased airflow.”
Another element of the test procedure that some in the industry would like to see changed for humid regions is the indoor ambient conditions used during laboratory cooling tests. These conditions are indoor dry-bulb and wet-bulb temperatures of 80°F and 67°, which correspond to a 50 percent relative humidity (rh). Most homeowners keep their thermostats between 73° and 77°, so the 80° test condition falls on the high side for expected return air supplied to the air handler.”
Some stakeholders also question the DOE-approved shortcut SEER approach used when rating most air conditioners and heat pumps having a single-speed compressor. This shortcut method relies on tests conducted at only one outdoor temperature, 82°. Kessler defended the current method. “SEER based on the 82° temperature was derived for an average of the climate zones. It is not weighted in one direction or another.”
Brian Dougherty, a mechanical engineer with the National Institute of Standards and Technology (NIST), acknowledged that some stakeholders would like to change the distribution of fractional bin hours used in calculating SEER for two-capacity and variable-speed units. Fractional bin hours designate how much time the unit is expected to operate within each 5° outdoor temperature bin, i.e., the fraction that the average hourly outdoor temperature falls within 65° and 70°, 70° and 75°, 75° and 80°, etc., all the way up to 100° and 105°, over the entire cooling season. The current distribution (see Table 1) is felt by some stakeholders to be too weighted towards the mild, and therefore most favorable, outdoor temperatures.
Table 1. (Click on the chart for an enlarged view.)
ONE SIZE DOESN'T FIT ALL
Sachs maintains that the current SEER value is “the wrong way to go for the next quarter century.” He would like to see an alternative regional rating method implemented, so that the seasonal efficiency approach embodied in SEER could remain a useful way to help consumers select appropriate equipment for their needs. In this type of alternative system, ratings methods would have to be adopted to reflect regional needs, such as hot and dry temperatures in the Southwest and hot and humid temperatures in the Southeast.“The country doesn’t all look alike. Until we get realistic about the differences among regions, we’re mandating stupidity in parts of the country,” said Sachs. “For example, most people in the West don’t care about humidity control. What they and their utilities do care about desperately is performance at very high temperatures. From central California to Dallas, you can get weeks of 100°-plus peak temperatures. SEER itself, particularly for single-speed equipment, is essentially based on measurements at 82°.”
Which brings up another interesting point. High SEER values (above 14 or 15) cannot be achieved with most DX single-speed equipment: For higher SEERs, two-speed or variable-speed technology is generally chosen as the most cost-effective path. That’s because at more moderate conditions, the equipment operates at a lower speed more continuously, which results in a higher SEER. Variable-speed equipment is typically optimized for low speed operation, not maximum speed, which is often required in various parts of the country.
“At extreme conditions, many times a single-speed system is more efficient than the two-speed or variable-speed system, because those systems are often optimized for part-load, lower speed operation. By default, fixed-speed systems are optimized for the one speed at which they operate,” said Dougherty. “Utilities are wondering if they should be promoting these very high SEER values in their locations, because maybe they’re not helping with their key issue, which is peak demand.”
Dougherty added that SEER reflects seasonal efficiency, which translates to seasonal energy consumption. “A seasonal descriptor such as SEER is appropriate for homeowners who historically are charged based on the amount of electricity that they use. When the electric bill is only based on kilowatt-hours, a higher SEER air conditioner and heat pump should save the homeowner money. However, as electric utilities start to implement pricing programs where the homeowner’s bill reflects some type of power demand charge - via real-time pricing or a time-of-use schedule - then the current SEER becomes less useful to the consumer.”
JUST THE FACTS, PLEASE
Kavanaugh offers a solution to all these issues surrounding SEER. “I’d like to see the manufacturers test their cooling equipment at three points, and those are outdoor air temperatures of 104°, 86°, and 68°. Not only would this cover more areas of the country, but they would also match up with the metric system designations of 40°, 30°, and 20°C.”Then Kavanaugh would like to obtain all the data concerning how the equipment performed at each of those temperatures. “As an engineer, just give me the numbers, and I can figure out how well equipment will work in a certain location.”
Kavanaugh would ultimately like to see all the manufacturers release this data, so that computer programs could incorporate the information for contractors. “All contractors would have to do is click on a city, and the weather data from that city would be dropped into a spreadsheet. Then contractors could select the type of equipment they want to install, along with the temperature that the homeowner wants the house at, and the program would show how much energy the machine will use and what the peak demand is.” This data is not consistent from one manufacturer to the next, so it’s difficult to know whether a manufacturer is giving the net total capacity of a system or the gross total capacity of a system. Unless it is certified, there’s no way to know for sure. According to Kavanaugh, manufacturers do not currently provide all the data that he is suggesting.
However, Kessler said that the application data requested by Kavanaugh is provided by responsible manufacturers, typically at 75 - 115° outdoor ambient temperature in 5° increments, and various indoor wetbulb conditions showing total, sensible and latent capability along with power.
“Regarding the mix-matching of coils to achieve various ratings, manufacturers provide the information so that consumers know exactly what they are buying. Each contractor has access to the application tables to determine the efficiency at a given temperature. Some of the application information designates operating efficiencies as high as 115°. Consumers can even go to the ARI Website (www.ari.org) to check particular combinations. There, someone can be assured of a matched system and can print out an ARI Certificate of Certified Product Performance.”
While there is much discussion going on as to whether today’s DOE-approved test methods are the best way to determine the efficiencies of air conditioners and heat pumps in the future, the rating system isn’t going away anytime soon. That is because manufacturers are used to designing equipment based on the SEER system, and changing the ratings system might force them to redesign products and change the way their equipment is developed and sold. That type of change takes a long time. As Dougherty noted, “Understandably, manufacturers don’t like the yardstick to change, but they might be open to substantive changes to the DOE testing and rating procedure if such changes were technically justified and the industry was given sufficient time to transition.”
Kessler summed his views up this way, “Regardless of the test parameters, engineers will always optimize the system and attempt to minimize the degradation coefficient. As an example, a 14-SEER unit rated at 80/67-82 will not operate the same in the hottest part of the country as it does in cooler conditions. Today’s rating system is designed so that half of the people in the country will be below and half the people will be above the rating point. That is what we expect to happen.”
It may be safe to say that this conversation will continue for some time to come. However, with the recent efforts in the U.S. House of Representatives and the Senate to draft new energy code legislation, the dust may actually settle before all the talking is through.
Sidebar: Testing and Rating
In the United States, federal law requires that the energy efficiency of central air conditioners and heat pumps up to 65,000 Btuh be measured and expressed by the SEER. The DOE test procedure used for determining SEER accounts for how the equipment’s efficiency changes with outdoor temperature and, if applicable, compressor modulation.The procedure accounts for standby power consumption and transient losses that occur when the unit cycles on and off in trying to match the building load. The federal test procedure is published annually in Appendix M, Subpart B, Part 430.23 of Title 10 of the Code of Federal Regulations (available at: http://www.access.gpo.gov). The identical procedure is also published as an appendix within ARI Standard 210/240-2006, Performance Rating of Unitary Air Conditioning and Air-Source Heat Pump Equipment.
In accordance with federal requirements, manufacturers are required to test a minimum sample of each model of outdoor unit, if a split system, or each model of a single package system in order to establish the equipment’s SEER rating. The federally mandated testing is conducted at the manufacturers’ expense, with the majority being conducted at the manufacturers’ own test facilities and the balance being contracted out to independent testing laboratories.
Each split system outdoor unit can be installed with several different indoor units. The OEM of the outdoor unit typically sells several different indoor sections that can be combined with each outdoor unit. In addition, there are third-party independent coil manufacturers (ICMs), who just make indoor sections. There are typically several ICM indoor units that can be applied with each OEM outdoor unit.
Although each model of the outdoor unit may be sold with a variety of indoor units, typically each outdoor unit is only tested with one particular model of indoor unit. Most, if not all, other combinations with the same outdoor unit are not tested as complete systems.
According to Brian Dougherty, a mechanical engineer with the National Institute of Standards and Technology (NIST), “The SEER for these untested combinations is determined using alternative rating methods (ARMs) that typically take the SEER of the tested combination and then adjust it up or down to account for differences between the alternative indoor unit versus the indoor unit of the tested combination. This approach to getting SEER ratings is needed to keep the test burden at a reasonable level and is justified because SEER is predominantly affected by the outdoor unit.” Each manufacturer must provide validation data and get their particular ARM approved by DOE before using it to generate SEER ratings for untested combinations.
It is worthwhile to note that the indoor sections of a split system combination come in two main types, those designed to be installed with a hot-air furnace and those designed to be autonomous (i.e., no furnace). In the first case, the air conditioner or heat pump relies on the furnace’s fan to blow air across the indoor coil; thus, the indoor section of the air conditioner or heat pump is not sold with an integral fan. In the second case, the indoor section comes with its own fan. With respect to the SEER rating, those systems sold without an indoor fan are tested within a test facility that can force air to flow through the indoor section. The SEER calculations are then adjusted for the power consumption and the amount of heat added to the indoor airstream by the indoor fan of a hypothetical hot-air furnace.
Where the federally-mandated requirements mostly end, work by the industry’s trade association, the Air-Conditioning and Refrigeration Institute (ARI), takes over. Michael Woodford, ARI assistant vice president engineering and manager of the domestic standards program noted that, “Since 1959, the Air-Conditioning and Refrigeration Institute’s certification programs have been assuring consumers that HVACR products will perform in accordance with a manufacturer’s published claims.”
He added that manufacturers voluntarily submit their products for independent testing in order to obtain the ARI Performance Certified mark. On an annual basis, samples of manufacturers’ products are randomly pulled for testing by an independent laboratory under contract to ARI. Many of these annual equipment selections are units whose rated SEER were obtained using an ARM, so the ARI program greatly expands the number of units whose ratings are backed by testing.
“To maintain certification status and the integrity of the program, ARI tests a representative and random sampling of products annually. Typically, the number of tests corresponds to 30 percent of a manufacturer’s basic model groups produced each year, as outlined in the certification program operations manual,” said Woodford. “If a product fails the certification testing, it may be retested and annual testing may be increased during the following year. In some cases, if a product’s first-test failures exceed 10 percent, it will be subject to penalty tests, in which additional fees and tests are imposed.”
Publication date:07/23/2007
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