Evaluation of the Repeatability of the Delta Q Duct Leakage Testing TechniqueIncluding Investigation of Robust Analysis Techniques and Estimates of Weather Induced Uncertainty Page: 3 of 58
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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.
DeltaQ Test outline
Just like an envelope leakage test, the DeltaQ test measures the pressure difference across the building envelope
while simultaneously measuring the airflow through the blower used to change the envelope pressure difference.
The DeltaQ test uses the fact that changing the pressure difference across the house envelope also changes the
pressure difference across duct leaks and therefore changes the duct leakage flows. The magnitudes (and for some
leaks, the direction) of airflow through the duct leaks are different when the forced air system blower is on or off.
The ramping technique gradually increases the envelope pressure difference from zero to about 50 Pa and back
down to zero over a period of about 90 seconds.
This ramping procedure was applied to the four parts of the DeltaQ test:
1. House depressurized with forced air system blower OFF
2. House depressurized with forced air system blower ON
3. House pressurized with forced air system blower ON
4. House pressurized with forced air system blower OFF
Because the orifice plates used in most blower doors can only be used over a limited range of air flows and resulting
envelope pressure differences, it was found that two orifice plates need to be used. The ramping procedure needs to
be applied for each orifice. Therefore, in each of the four parts there are actually two ramps: one for each orifice
plate. Figure 1 shows the envelope pressures from an example DeltaQ ramping test. In addition to the envelope
pressures during the test, Figure 1 also shows the baseline pressure measurements that are made at the beginning and
end of each test. These baseline pressure differences serve two purposes: the first is that they are averaged and
subtracted from the other envelope pressures to remove the effect of stack and wind pressures from the envelope
pressures because the pressures used in the analyses must be only those induced by blower door operation. The
second is to serve as a guide to determining if a test has too much wind speed variability resulting in excessive
measurement errors. This will be discussed in more detail later. The baseline pressures were averaged for 30
seconds before and 30 seconds after the ramping.
Figure 2 illustrates the pressure-flow data pairs corresponding to the envelope pressures in Figure 1 after baseline
subtraction3. A slight difference between the data taken with the two orifice plates due to inaccuracies in their
calibrations (The manufacturer's accuracy specification is 3% of flow.) illustrating the necessity of pairing central
fan ON/OFF data only for a given blower door orifice, i.e., not using central blower ON data from one orifice with
central blower OFF data from another orifice.
In most cases the baseline pressures are small and the influence on the leakage flows is negligible. In very windy
cases the baseline pressure changes can lead to changes in estimated duct leakage air flows up to 30 cfm (15 L/s) by
adopting the correct baseline pressure subtraction.
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Dickerhoff, Darryl & Walker, Iain. Evaluation of the Repeatability of the Delta Q Duct Leakage Testing TechniqueIncluding Investigation of Robust Analysis Techniques and Estimates of Weather Induced Uncertainty, report, August 1, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc893638/m1/3/: accessed October 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.