This article was originally published in the January/February 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online January/February 1998
Pressure Pans: New Uses and Old Fundamentals
by Jeffrey Siegel and Bruce Manclark
Pressure pans are often misused, and the information they provide can mislead the inexperienced. Bruce Manclark of Delta-T Incorporated (Eugene, Oregon) and Jeffrey Siegel, formerly of Ecotope Incorporated (Seattle, Washington), investigate how mistakes are made, and share their research into one strategy for more accurate pan readings.
Contractors working on HVAC systems need to be able to measure duct system leakage. Before sealing, they need to determine whether the retrofit will be cost-effective. Getting correct information is the key to quality control; getting it quickly is the key to cost-effectiveness. Spend too much time, and the labor budget gets busted. Make the wrong decisions, and systems that need sealing don't get sealed, while other ducts get unnecessary work done.
Retrofitters have many tools for measuring leakage. Two of the most common are pressure pans and duct pressurization devices. Duct pressurization devices like the Duct Blaster can give precise measurements of duct leakage. Pressure pans measure the ratio of leaks to the indoors versus leaks to the outdoors. Pressure pan readings are difficult to interpret, and the same number can reflect quite different leakage rates in different houses. Currently, duct leakage standards with numerical targets for duct tightness require duct pressurization testing.
Delta-T and Ecotope, funded by the Eugene Water and Electric Board and Puget Sound Energy, are developing a new way to use pressure pans, which may eventually speed up duct sealing (see Not Your Daddy's Duct Sealing Method, p. 44). Based on retrofits in the Northwest, we have come up with an equation that uses pressure pan readings to provide estimates of duct leakage to the exterior. In certain cases, we have used quick pressure pan measurements to get data that are almost as reliable as the numbers from Duct Blasters. However, the equation still needs development. For now, what is most important is that technicians using pressure pans receive adequate training in duct diagnostics.Conventional Wisdom Because pressure pans are quicker to set up, retrofitters often try to use them instead of duct pressurization devices. This causes problems. Conventional wisdom suggests that a system with an average pressure pan reading of greater than 1 Pascal (Pa), or any individual register that has a reading of 2 Pa or greater, is leaky enough to require sealing. This rule may work well in some duct systems, but it can also lead to serious errors.
The potential for trouble was demonstrated in two houses sealed by a crew from Delta-T. In the first duct system, pressure pan readings were all 1 Pa or less. Based on the pressure pan test, this house seemed like a poor candidate for duct sealing. However, a duct pressurization test revealed that there were 375 cubic feet of leakage per minute to the outside with the ducts pressurized to 50 Pa (CFM50), an amount that was unacceptably high to the sponsoring utility. This duct system was sealed. A second home's duct system had pressure pan readings that were all between 3 and 4 Pa. This seemed like an ideal candidate for duct sealing. However, a duct pressurization test revealed only 125 CFM50 of leakage to the outdoors, too low to justify a duct sealing retrofit. If the crew had based its decisions solely on pressure pan tests, they would have made the wrong decision at both of these houses.
Similarly, a crew in our area recently sealed the ducts in a house a utility (using pressure pans) had determined needed sealing. The crew's initial numbers were considerably lower than those reported by the utility. What had changed? Upon investigation, the crew found that the homeowner had replaced a very dirty furnace air filter the day before they arrived. The crew had also found and opened several dampers in the system. These are just a few examples of the factors that can affect pressure pan readings.
The duct pressurization test directly measures how much air leaks out of the system at a given pressure. This can be mathematically related to the equivalent leakage area in a system. The leakage area can be correlated with energy loss, giving the retrofitter useful information. The pressure pan test measures pressure in the duct, which is the result of interactions among leaks to the indoors, leaks to the outdoors, the physical details of those leaks, and the geometry of the duct system. For example, registers close to other registers will have low pressure pan readings, while dirty filters push all readings higher.The Duct Pressurization Test Duct pressurization tests are well documented; they have been used for several years to measure duct leakage to the outdoors. While there are several duct pressurization tests, this article uses the term to denote the test procedure described below. This test is also called an exterior duct leakage test, a blower door/duct tester test, a Duct Blaster test, or a leakage to exterior test.
First, all supply and return registers are sealed, usually with masking tape or garbage bags, and a blower door is installed. A duct tester, which is like a miniature blower door, is attached and sealed to the duct system, usually at the furnace cabinet. The air handler fan is off for the entire test. A Pitot tube or similar pressure tap measures the pressure of the duct system with respect to the house, and a tube is set up to measure outside pressure with respect to the ducts. The blower door pressurizes the house to about 50 Pa with respect to the outdoors. With the air handler off, the duct tester blows air into the duct system until there is no pressure difference between the house and the ducts. With no pressure difference between the house and the ducts, all air going through the duct tester is going to the outdoors through duct leaks. The duct tester shows how many CFM it is blowing to maintain a pressure difference of 50 Pa to the outdoors.
The test is often repeated with the house pressurized to about 25 Pa. The data from such a two-point duct pressurization test can be inserted into a formula to get a duct leakage curve. Such a curve allows the retrofitters to estimate leakage at actual operating pressures, which is more important than the leakage at 50 Pa. However, two-point tests can be time consuming, requiring an experienced crew to spend as much as two hours on testing, in addition to their sealing work.
The duct pressurization test results can be expressed in CFM50 or they can be mathematically manipulated to estimate leakage area. These numbers are directly comparable between different houses. A duct system that has a leakage rate of 300 CFM50 is leakier than a duct system that has a leakage rate of 200 CFM50. Every contractor and utility duct sealing program has a different standard for determining what is an acceptable leakage rate and what requires duct sealing. For example, in the Pacific Northwest's smaller homes and manufactured homes, leakage of more than 250 CFM50 usually means that a duct retrofit is warranted.Testing Techniques Technicians in need of a faster estimate of duct leakage (or working to help pinpoint duct leakage) often conduct a pressure pan test (see How to Pan for Pressure Gold). The pressure pan test requires a blower door and a pressure pan, which is a modified cake pan with a gasket on the bottom and a pressure tap on top. A duct tester need not be installed, and the registers do not have to be sealed. The blower door is used to depressurize the house to -50 Pa with respect to outdoors. Alternatively, if the ducts are all in one space, the house can be depressurized to -50 Pa with respect to the space the ducts are in. Then the technician covers one register at a time with the pressure pan and records the pressure difference between the house and the ducts at each register, with the air handler off.
With the house depressurized to -50 Pa with respect to the outdoors, the duct system will have a pressure that ranges from 0 to 50 Pa with respect to the house. A duct system at 0 Pa is entirely within the pressure envelope of the house and has no leaks to the outdoors. A system approaching 50 Pa is essentially outside the pressure envelope, meaning that it has catastrophic leakage to the outdoors. Three sources of pressure can influence a pressure pan measurement.
In the Eugene houses where pressure pans gave the wrong result about the duct system tightness, the problem was that the ratios were thrown off by unusual situations. In the house with low pressure pan readings but high leakage, the problem was unusually high internal leakage. In the house with high readings but relatively little leakage, the problem was a single register that blew much of its air outdoors, causing the duct pressure to be excessively influenced by outside pressure.
It takes about ten minutes to complete a pressure pan test in an average sized house, once the blower door is installed. This speed allows technicians to repeat the test several times during a retrofit to evaluate the effectiveness of a particular sealing action, such as sealing the leaks at register boots. Also, because using a pressure pan does not require sealing registers or opening and sealing the furnace cabinet, technicians do not have to worry about damaging wet duct seals in these locations.Can a Pan Be Enough? The disadvantage of the pressure pan test is that it is more art than science. As in the Eugene houses, pressure pan results are often hard to interpret--the physical significance of the readings is not always clear. It is even harder, if not impossible, to make meaningful comparisons between homes based on pressure pan results. The one exception is when homes have very similar duct geometry and installation, as is the case with manufactured homes or identical homes in a subdivision.
The best way to interpret pressure pan results correctly is to understand the physics behind the test. This, in turn, requires a firm grounding in the duct pressurization test.
Still, we have found that a technician with adequate background knowledge can get enough information from a properly executed and analyzed pressure pan test to make valid field decisions on certain homes. The standard pressure pan test assumes that the technician knows the pressure in the space where the ducts are located and duct leakage to the interior--numbers that normally come from doing a duct pressurization test. However, some populations of homes have consistent relationships among duct leakage to the exterior (or whatever space the ducts are in), duct leakage to the interior, and house leakage to the crawlspace. Experienced technicians can develop an intuitive sense of these relationships. Using a pressure pan test and plenty of experience with a given housing stock, some technicians can judge whether a house will be cost-effective to duct seal. We have developed a new mathematical technique so that experienced technicians can rely on something more than intuition.
If crews can make correct decisions using only pressure pans, they will save time and increase cost effectiveness. To see whether this is possible, we analyzed pressure pan data from 44 homes. The data showed trends relating pressure pan numbers, duct leakiness, and local construction styles. We analyzed these trends and developed the equation below, which helps relate pressure pan results to leakage. This formula allows similar homes to be compared on the basis of pressure pan tests.
The formula is intended to help a crew decide whether to seal ducts in a home. Once the work is underway, continued use of pressure pans and the formula may also help them decide whether they have sealed enough leaks to consider the retrofit finished.
The formula is an empirical curve fit, rather than a mathematically derived truth. It should be used with caution. It assumes that the duct technician can make an accurate assumption about unintentional leakage to the inside and it is intended to provide a general result rather than a precise value.
Typical 4-inch x 10-inch or 4-inch x 11-inch floor registers found in most Pacific Northwest homes have free areas (Ai) of 24.1 square inches. The free area for any register can be found in the manufacturer's register catalog. The formula is not particularly sensitive to the area value, so assuming a free area of 25 square inches per register usually works. While Ai is supposed to be the total interior leakage to the indoors, adding all registers together results in an overestimate, because one register is covered by the pressure pan. This overestimate is intended to account for some of the unintentional interior leakage that always seems to exist. If a crew has used Duct Blasters enough to have a good idea of the interior leakage in the housing stock they're working on, they can just add the area of this interior leakage to the sum of all of the register free areas. This will make the formula more accurate.
The most important part of the equation is the exponent, b. It summarizes the relationships among leakage to the indoors, house leakage, and leakage to the outdoors. Therefore, the exponent must be empirically derived for a given local housing stock. The exponent has the most influence on whether or not the equation works. In order to derive it, one needs duct pressurization test results (leakage to exterior in CFM50) and the total pressure pan result for a sample of similar houses. Using algebra, the exponent is then determined for each house. All the exponents are then averaged, resulting in an exponent that should predict valid results for similar homes.
The equation does a good job of predicting duct leakage to the outdoors on a wide variety of Pacific Northwest homes. We applied the formula to a sample of seventeen site-built and manufactured homes. With duct pressurization test and pressure pan results for these houses, we determined the average exponent to be 0.668. When this average exponent was used with the pressure pan results for each home, the predicted leakage was within 10% of the actual leakage in nine houses (53%) and within 20% of the actual leakage in 14 houses (82%). There were only two sites where the predicted leakage varied by more than 25% from the actual leakage. Both of these sites were homes where we knew that there were large amounts of duct leakage to the indoors (due to panned floor joists or open duct boots), which is the variable most likely to throw off the results. If you suspect large amounts of leakage to the indoors, it is probably best to conduct a duct pressurization test, unless you can tolerate a lot of uncertainty in your measurements.
In addition, we went back and looked at data from 13 older manufactured homes and 14 natural gas-heated site built homes. The average exponent for these 27 homes was .712. Pressure pan measurements were not the focus of the research at these homes and more measurement error was evident. Compared to the first 17 homes, the agreement between predicted leakage and measured leakage on the second sample was not quite as good. Of the 27, 63% (17 homes) had predicted duct leakage within 20% of the Duct Blaster measured value. In most of the other 10, there was a relatively large amount of interior leakage, so we didn't expect the result to be correct.
Regardless, the stated purpose of this equation is to make decisions in the field, not to calculate duct leakage. Twenty-six of the 44 homes in this study received retrofits because duct pressurization tests showed they had over 250 CFM50 (in manufactured homes) or over 300 CFM50 (in site-built homes). In all but three of the 44, crews using the pressure pan results and the formula would have made the correct decision on whether to seal the ducts. This shows that the equation can help the duct sealer in the field.
The formula still has limitations. It cannot be relied on in homes with a large amount of unintentional interior leakage. Since it takes supernatural help to measure interior leakage without a Duct Blaster, every duct sealing crew needs one of these, as well as a pressure pan, on their equipment list. Duct technicians should become familiar with the Duct Blaster test and should also learn the extent of interior leakage in housing stock in their region. A Duct Blaster is also necessary for determining the exponent on as many houses as possible. Remember--in order to determine the exponent, both duct pressurization test results and pressure pan results are needed. Once a crew is comfortable with their locally derived exponent and has verified that it works well for their housing stock, then they can rely more on pressure pan testing.
While it may be possible to save time in many homes by substituting a pressure pan test for a duct leakage test, there are no shortcuts when it comes to health and safety tests. These require time and patience, and are worthwhile regardless of the cost.
We are continuing to test every possible retrofit candidate with both duct pressurization tests and pressure pan tests. We are developing and verifying exponents for a variety of housing stocks in the Northwest while attempting to find a more satisfying physical basis for the equation. Ultimately, we would like to develop a fast and easy test that measures duct leakage at typical operating pressures.
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