User Tip Index
- Tip #1: Using the communication ports on the DG-700 and DG-500 gauges to perform fully automated Blower Door tests and to data log pressures.
- Tip #2: The location of the outside end of the building pressure tubing is very important when setting up your Blower Door.
- Tip #3: Leakage area calculations.
- Tip #4: Using the Blower Door subtraction method for estimating duct leakage.
- Tip #5: Measuring total air handler flow with a Duct Blaster.
- Tip #6: Using the DG-700 and TECLOG3 to measure worst case fan depressurization and appliance draft.
When installing the building pressure tubing, be sure the outside end of the tubing is at least 5 feet to the side of the exhaust airflow from the blower door fan. Although it is common practice (and was our recommended installation procedure for years) for blower door operators to insert the open end of the tubing just a few inches through the patch on the nylon panel, and leave it, we have determined that this set up practice can produce inaccurate building pressure readings due to the exhaust airflow from the fan hitting the end of the tubing.
A good location for the end of the building pressure tubing is at the base of the building where it meets the ground. We have redesigned our nylon blower door panels with two access holes near the floor to make it easier to properly install your building pressure tubing. If the fan is exhausting to a porch, garage or other enclosure, it is best to install the end of the building pressure tubing outside of the enclosed space.
Once the leakage rate for a building has been measured, it is useful to estimate the cumulative size (in square inches) of all leaks or holes in the building's air barrier. The estimated leakage area provides us with a way to visualize the physical size of the measured holes in the building. This can be particularly important when explaining the results of a test to a building owner. Leakage area calculations are also used in infiltration models to estimate the building's natural air change rate (i.e. the air change rate under natural weather conditions).
TEC's airtightness test analysis software calculates two separate leakage areas, based on differing assumptions about the physical shape of the hole. These leakage area calculations are compatible with the two most commonly used infiltration models. Energy analysis or rating software that require the user to input airtightness test results typically specify one of these two leakage areas.
The Equivalent Leakage Area (EqLA) is defined by Canadian researchers at the Canadian National Research Council as the area of a sharp edged orifice (a sharp round hole cut in a thin plate) that would leak the same amount of air as the building does at a pressure of 10 Pascals. The EqLA is used in the AIM infiltration model.
Effective Leakage Area (ELA) was developed by Lawrence Berkeley Laboratory (LBL) and is used in their infiltration model. The Effective Leakage Area is defined as the area of a special nozzle-shaped hole (similar to the inlet of your blower door fan) that would leak the same amount of air as the building does at a pressure of 4 Pascals.
Importantly, when using leakage area calculations to demonstrate physical changes in building airtightness, we recommend using the Canadian EqLA measurement. Typically, EqLA more closely approximates physical changes in building airtightness. For example, if you performed a blower door test, and then opened a window to create a 25 square inch hole and repeated the test, the estimated EqLA for the building will have increased by approximately 25 square inches from the initial test result. The EqLA is also easier to measure, especially in windy weather, because the measurement is taken at a higher building pressure than the ELA.
For years, technicians have been using a simple Blower Door comparison test to estimate residential duct leakage to the outside. The technique, called Blower Door subtraction, involves conducting two whole house Blower Door airtightness tests with and without the supply and return registers and grills sealed off from the house. A subtraction of the sealed register test from the unsealed register test provides an estimate of duct leakage to the outside.
Researchers now realize that the Blower Door subtraction test has a number of drawbacks with respect to the accuracy of test results. Accuracy is reduced for two reasons. First, subtraction will typically underestimate duct leakage due to connections between the duct system and the interior of the house which are not sealed when the registers and grills are temporarily sealed. The Blower Door subtraction method assumes that once the registers and grills are taped off, the duct system is effectively outside of the pressure envelope of the house, and as a result the Blower Door does not measure any duct leakage in that configuration. However, connections between the duct system and the interior of the house cause a certain amount of duct leakage to be measured even with the duct system sealed from the house. The amount of underestimation due to this phenomenon is a function of how well the duct system is connected to the structure of the house.
Secondly, because Blower Door Subtraction involves subtraction of two separate Blower Door test results (using the same Blower Door), the accuracy of the test result is a function of the repeatability of the Blower Door measurements. The error due to repeatability further clouds the accuracy of your subtraction test. So now that we know there are shortcomings to Blower Door subtraction, is there anyway to improve our leakage estimate using the subtraction technique?
There is a technique to alleviate the underestimation problem associated with Blower Door subtraction. The subtraction duct leakage estimate can be modified by taking one additional pressure measurement. The pressure measurement needed is the pressure between the duct system and the house with the registers and grills sealed and the Blower Door depressurizing (or pressurizing) the house to the target pressure of 50 Pa. This measurement can be taken at the supply or return plenum, or at a supply register or return grill by punching a small hole through the masking tape and inserting a pressure tap or hose connected to a pressure gauge. Once this pressure is measured, an appropriate subtraction correction factor can be used to modify your original subtraction duct leakage estimate. A table of correction factors for Blower Door subtraction is shown below.
|House to Duct Pressure
in Pascals (taped off)
Let's look at an example to illustrate use of the subtraction correction factor (SCF). You have performed a subtraction test and found the following:
- CFM50 (ducts open) = 3,250 *
- CFM50 (ducts sealed) = 2,825 *
- Pressure between the ducts and the house (ducts sealed) = 40 Pa
- From the Table above, the SCF for 40 Pa is 1.54
- Initial Duct Leakage Estimate = 3,250 - 2,825 = 425 CFM50
- Modified Duct Leakage Estimate = 425 CFM50 x 1.54 = 655 CFM50
* If ducts run through attics or crawlspaces, be sure the attic or crawlspace is effectively outside when the blower door is operating - i.e. attic to outside pressure=0.
In this case, our modified duct leakage estimate is 54 percent larger that the original Blower Door subtraction estimate. This means we would have underestimated duct leakage to the outside by 54 percent without using the correction factor.
Now how about repeatability error. On a day with only slight wind, our experience is that the repeatability of manual Blower Door test is about +/- 3% of the unsealed whole house CFM50 value when using the same gauges for both tests. For the example above, a repeatability error of 3% means we have an error of approximately +/- 97 CFM50 (0.03 x ,3250 CFM50) in our leakage estimate. But we must also apply the correction factor calculated above to the 97 CFM50 which increases the error to +/- 149 CFM50 (97 CFM50 x 1.54). Thus our final subtraction leakage result is 655 CFM50 (+/- 149 CFM50). This means the actual leakage in the duct system is somewhere between 506 CFM50 and 804 CFM50. And in very windy weather, repeatability error will increase much larger than the 3% shown here.
Note: If you are using an APT system to conduct your Blower Door test, repeatability errors will typically be reduced below the 3% quoted above, and the APT system will provide you with a estimate of the measurement uncertainty.
Do all of these problems mean that I shouldn't be using the subtraction method to estimate duct leakage? Not at all. If you need to quantify duct leakage and are comfortable with the relative imprecision of the subtraction technique, subtraction makes good sense. Of course you should always use the correction factors listed in the table above when using the subtraction method, to account for underestimation. If accuracy is important, much greater precision of duct leakage estimates can be achieved by using the Minneapolis Duct Blaster and directly testing the duct system.
And a final word of caution when using the subtraction method, if the measured duct to house pressure is less than about 20 Pa (meaning the duct system is very well connected to the house structure), we suggest that Blower Door subtraction (modified or not) can not be relied upon to provide meaningful duct leakage estimates. This commonly means that Blower Door subtraction can not be used in houses which use building cavities for a significant part of the duct system (e.g. basement houses which use panned under ceiling joists for return ducts). In these cases, the duct system must be tested directly with a Duct Blaster.
In applications where a duct leakage estimate is not needed at all, use of a blower door and a pressure pan is probably the ideal setup. In this case, duct leakage can be quickly checked by depressurizing the house to 50 Pa and taking a quick pressure pan reading at each register or grill. Inspection standards can also be set using this procedure. For example, the duct system might pass inspection if all registers and grills have a pressure pan reading of 1 Pa or less. This technique provides crews and inspectors with excellent measurement feed-back, but simply does not quantify the leakage in the system.
This test is used to measure total air flow through the air handler. First make sure all supply and return registers are open and untaped (keep filters installed if they are reasonably clean). Set up a pressure gauge to measure the duct pressure WRT the house at the supply plenum or a few feet away from the supply plenum in a main supply trunk. Use a static pressure probe to measure duct system pressure and be sure the static pressure probe is pointing into the air flow. Turn on the air handler and measure the normal operating duct pressure WRT the house. Record the normal operating duct pressure and turn off the air handler. Do not move the static pressure probe used to measure duct pressure because we will need to use it later in this test.
Open the air handler cabinet access panel and seal off the return opening in the cabinet from the air handler fan using tape and cardboard. Now install the Duct Blaster system in the access panel opening of the air handler cabinet. This is typically done by attaching the Duct Blaster's square transition piece to a piece of cardboard that has been cut to fit over the opening and taped in place. In this configuration, all return air flow will be moving through the Duct Blaster fan, with the return ductwork effectively sealed off from the supply system.
Turn the air handler fan back on and re-measure the duct pressure WRT the house. Now turn on the Duct Blaster fan and adjust the fan speed until the duct pressure equals the normal operating duct pressure measured above. Once adjusted in this way, determine the flow through the Duct Blaster fan by measuring the fan pressure and using the flow table. The measured Duct Blaster fan flow is your estimate of the total system air flow including flow through return registers, plus return duct leakage, plus leakage at the air handler access panel. The only component of total system airflow that is not included in this measurement is any leakage on the return side of the air handler cabinet (other than the air handler access panel).
If more fan flow from the Duct Blaster is needed to complete this test, remove the flexible extension duct from the fan and connect the exhaust flange from the fan directly to the access door opening using tape and cardboard.
Note: If you have installed the Duct Blaster in an unconditioned space (attic, garage or crawlspace airhandler), open a door from the house to the outside to prevent pressure changes in the house when the Duct Blaster is operating. Also open any vents or doors between the unconditioned space and outside to prevent that space from experiencing pressure changes from Duct Blaster fan operation.
Single Return Systems:
In single return systems, there is a simplified procedure which can be used under certain circumstances. If you are already hooking up the Duct Blaster to the central return to conduct an airtightness test, and you verify the duct system is tight (following retrofit or as part of a new construction performance test), then the air handler flow test can be performed without moving the Duct Blaster fan to the air handler cabinet. In this case you will need to measure the normal operating duct pressure prior to installing the Duct Blaster system to the return grill. Once your airtightness test has been completed, then remove the temporary register seals, turn on the air handler fan, and adjust the Duct Blaster fan to achieve the same normal operating pressure measured earlier.
The main advantage to this procedure is that you do not have separate the return side of the system from the air handler fan, which can be a laborious and time consuming process.
- Click here for a PDF file on using the DG-700 and TECLOG3 for combustion safety testing.
- Click here to download the sample TECLOG3 data file used in the article above.
- Click here for a PDF of TECLOG3 features.