Evaluation of the Repeatability of the Delta Q Duct Leakage Testing Technique Including Investigation of Robust Analysis Techniques and Estimates of Weather Induced Uncertainty
The DeltaQ test is a method of estimating the air leakage from forced air duct systems. Developed primarily for residential and small commercial applications it uses the changes in blower door test results due to forced air system operation. Previous studies established the principles behind DeltaQ testing, but raised issues of precision of the test, particularly for leaky homes on windy days.
Details of the measurement technique are available in an ASTM Standard (ASTM E1554-2007). In order to ease adoption of the test method, this study answers questions regarding the uncertainty due to changing weather during the test (particularly changes in wind speed) and the applicability to low leakage systems. The first question arises because the building envelope air flows and pressures used in the DeltaQ test are influenced by weather induced pressures. Variability in wind induced pressures rather than temperature difference induced pressures dominates this effect because the wind pressures change rapidly over the time period of a test. The second question needs to answered so that DeltaQ testing can be used in programs requiring or giving credit for tight ducts (e.g., California's Building Energy Code (CEC 2005)).
DeltaQ modeling biases have been previously investigated in laboratory studies where there was no weather induced changes in envelope flows and pressures. Laboratory work by Andrews (2002) and Walker et al. (2004) found biases of about 0.5% of forced air system blower flow and individual test uncertainty of about 2% of forced air system blower flow. The laboratory tests were repeated by Walker and Dickerhoff (2006 and 2008) using a new ramping technique that continuously varied envelope pressures and air flows rather than taking data at pre-selected pressure stations (as used in ASTM E1554-2003 and other previous studies). The biases and individual test uncertainties for ramping were found to be very close (less than 0.5% of air handler flow) to those found in for the pressure station approach.
Walker and Dickerhoff also included estimates of DeltaQ test repeatability based on the results of field tests where two houses were tested multiple times. The two houses were quite leaky (20-25 Air Changes per Hour at 50Pa (0.2 in. water) (ACH50)) and were located in the San Francisco Bay area. One house was tested on a calm day and the other on a very windy day. Results were also presented for two additional houses that were tested by other researchers in Minneapolis, MN and Madison, WI, that had very tight envelopes (1.8 and 2.5 ACH50). These tight houses had internal duct systems and were tested without operating the central blower — sometimes referred to as control tests. The standard deviations between the multiple tests for all four houses were found to be about 1% of the envelope air flow at 50 Pa (0.2 in. water) (Q50) that led to the suggestion of this as a rule of thumb for estimating DeltaQ uncertainty. Because DeltaQ is based on measuring envelope air flows it makes sense for uncertainty to scale with envelope leakage. However, these tests were on a limited data set and one of the objectives of the current study is to increase the number of tested houses.
This study focuses on answering two questions:
- What is the uncertainty associated with changes in weather (primarily wind) conditions during DeltaQ testing?
- How can these uncertainties be reduced ?
The first question is addressing issues of repeatability. To study this five houses were tested as many times as possible over a day. Weather data was recorded on-site — including the local windspeed. The result from these five houses were combined with the two Bay Area homes from the previous studies. The variability of the tests (represented by the standard deviation) is the repeatability of the test method for that house under the prevailing weather conditions. Because the testing was performed over a day a wide range of wind speeds was achieved following typical diurnal variations of low wind in the early morning and greatest winds in the late afternoon/early evening. Typically about ten tests were performed in each house.
To answer the second question, different data analysis techniques were investigated that looked at averaging techniques, elimination of outliers, limiting leak pressures, etc. in order to minimize the influence of changing wind conditions during the test. The objective was to find a reasonable compromise between test precision and robustness — because many of the changes to the analysis to make the test more robust limit its ability to examine wide ranges of pressures and leakage flows.
A secondary goal of this study is to show that DeltaQ uncertainties are acceptable for testing low leakage systems. Therefore houses with low duct leakage were deliberately chosen to be tested. This is important for utility and weatherization programs that give credits for tight ducts and for codes and standards that may refer to DeltaQ testing. In particular the following organizations/standards bodies are thinking about adopting DeltaQ testing, but before doing so they want to see DeltaQ applied to the low leakage situations they wish to address: California's Building Energy Code (CEC 2005), ASHRAE Standard 62.2 (ASHRAE 2007), RESNET (Residential Energy Services Network) and the US EPA EnergyStar homes Program. This issue is always going to somewhat subjective, but a key criterion will be having repeatability uncertainty below the low-leakage limits being proposed. These low leakage limits are typically 6% of air handler flow, or a range of about 60 cfm to 120 cfm (30 to 60 L/s) depending on system size.