Duct Leakage: How Much

January 01, 2001
January/February 2001
A version of this article appears in the January/February 2001 issue of Home Energy Magazine.
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Q:Is there a standard for energy-efficient ducts? I was called to a home built in 1955 that is under a warranty from a recent sale. The customer wanted us to “fix the ducts.” I didn’t see anything wrong with the ducts, but they claimed PG&E (the local utility) tested them and they were too leaky. Is there a standard and how do you test for it?

A: There are a number of state and federal standards that limit duct leakage based on different leakage test results. In the most common test, the registers are temporarily sealed and a fan is used to pressurize the ducts and measure the leaks.
        To understand the test differences, you should know these duct leakage terms:
        Total duct leakage. This refers to all the air that escapes from the duct other than through registers. A total duct leakage test measures all leaks, whether the air ends up inside or outside the house. This is the simplest and most widely used leakage measurement.
        Leakage at 25 Pascals (Pa). When the ducts are tested, they are either pressurized or depressurized with a fan. The test pressure most often used is 25 Pa (0.1 inch water column).
        CFM25. Leakage measurements in the United States are reported in cubic feet per minute (CFM). When the test is performed at 25 Pa, this is reported as CFM25.
        System flow. As you know, air conditioners and furnaces are designed to move a certain amount of air across the heat exchangers. People refer to the actual flow as the measured flow, and to the design flow as the rated flow. For example, the rated flow for most air conditioners is 400 CFM per ton. Many standards use a percentage of system flow as the maximum leakage.
        Percentage of conditioned floor area. A number of utility programs have used a percentage of conditioned floor area (such as 3% or 5%) as the standard. For example, a 2,000 ft2 home with 60 CFM25 duct leakage would just meet a 3% goal (2,000 x 3% = 60).
        Standards. There are a number of housing standards that address duct systems. One of the oldest ones is in the Uniform Mechanical Code (UMC). It specifies that ducts must be “substantially airtight.” Unfortunately, this requirement is vague and ill-defined. If you can’t measure the performance of the system against a standard, the standard essentially doesn’t exist.
         Fortunately, there has been a lot of progress in duct testing. Most recent standards specify some maximum total leakage at 25 Pa; the primary difference between the standards is how much the allowed maximum is (see Table 1). Many standards assume that the system flow is the same as that rated by the furnace or heat pump manufacturer, because this assumption simplifies calculations.
        Testing ducts for leakiness and then sealing them can be a profitable direction in which to take a portion of your business. This approach sets you apart from the rest of the contractors because it is more comprehensive and systems oriented. Besides learning to test and seal ducts, you need to learn about the pressures caused by duct leaks and the interactions with other systems (including combustion appliances).

                Mistakes to Avoid
        Don’t assume that you can see all the important leaks, so you don’t need to test. Many contractors think that they can tell how leaky a duct system is without testing it. “Just let me seal it,” they say. Time after time, programs across the country have found that if contractors don’t test to show the technician how they are doing, they miss significant leakage on many of the systems. If you can’t measure it, you didn’t do it.

                Lessons Needed
        Learn how to detect the indoor air pressures that duct systems cause. (To locate training opportunities, see “Training for Tomorrow: Are Your Contractors Certifiable?” HE Jan/Feb, ’99, p. 29. or at www.homeenergy.org.) Air distribution systems create powerful pressures in a home that can overwhelm the ability of combustion appliances to draft properly. In one recent New York tragedy, six people died in one home when their HVAC system pulled CO throughout the house.
        Learn how to respond to CO alarms as well as to check and repair combustion appliances. In the same New York incident, the homeowner had turned off the CO monitor because it kept going off.

                Proposed A/C Standards Bring Debate
        It’s an unusual day when a full-page ad in a national newspaper is devoted to new appliance standards. That’s what happened last October when DOE announced new proposed standards for the energy efficiency of central air conditioners and heat pumps. A coalition of consumer and environmental groups, utilities, and state governments lobbied actively during the subsequent 60-day comment period against the new standards as insufficient, especially given the technology available today.
        The use of air conditioners has increased from 23% of U.S. households in 1978 to 47% in 1997; it is now the largest single contributor to peak electricity demand. In many parts of the United States—notably California, New York, and Chicago—record peak demand has frequently brought the power system to the brink of collapse, and in some instances, it has precipitated outages. Consumers in the deregulated markets of Southern California and southeastern New York have seen the cost of their electricity skyrocket as demand has outstripped supply.
        Air conditioners are rated according to their seasonal energy efficiency ratio (SEER). Heat pumps, which can deliver heated as well as cooled air, are rated for SEER and also for heating season performance factor (HSPF). The proposed standards would require air conditioners to have a minimum SEER of 12, and heat pumps to have a minimum SEER of 13 and a minimum HSPF of 7.7. (Current standards require central air conditioners to have a minimum SEER of 10, although the shipment-weighted average unit sold today has a SEER of 11.) Raising the SEER for air conditioners from 10 to 12 represents a 17% energy savings.
        Seeking a stricter 30% efficiency increase for air conditioners are Pacific Gas & Electric Company, the California Energy Commission, the Natural Resources Defense Council, the National Environmental Trust, and the American Council for an Energy Efficient Economy.
        “Energy-efficient air conditioners are readily available, many up to 70% of the existing standard. It’s simply a question of updating the standards to where the technology has taken us,” says Andrew deLaski, Executive Director of the Appliance Standards Awareness Project. He also feels that the proposed standard is “a lost opportunity,” because once it becomes final, DOE won’t be able to tighten the standard again for ten years.
        Currently, consumers can choose from a large number of premium air conditioners labeled 13-SEER, at an average cost of $500 more than a 10-SEER. “A customer in San Diego would have made that amount of money back in one summer,” says deLaski.
        “The cost of the new air conditioners probably wouldn’t come down until these new standards are set, and that’s why they’re so important,” deLaski says. The new standards will not, however, take effect for manufacturers until 2006. In the meantime, making the federal legislative rounds is the Energy Efficient Buildings Incentive Act, a bill that would give tax incentives for efficient air conditioners.
        For comparison, the Japanese have already implemented even stricter air conditioning standards. The Japanese “Top- Runner” standards will require the most popular “mini-split” units to achieve energy efficiency ratios (EERs) above 17. The EER is a steady-state measurement, whereas the SEER includes performance at part load. These regulations go into force in 2003, but some units already meet that level.

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