Because differences in air temperature between indoors and outdoors create changes in house pressures, we need to measure and adjust for this baseline stack effect pressure during a Blower Door test. Failure to account for a baseline stack effect pressure of –2.5 Pa would cause an underestimation of the house leakage measurement (CFM50) by about 3%.
Airtightness is really a property of the building enclosure, exactly analogous to moisture permeability and U value. It doesn’t depend on the volume, but does depend on the surface area. The air permeability of materials is typically measured as a flow per area at a given pressure difference across the material. U value is similar. If we want a metric to use to measure the airtightness quality of the exterior enclosure of buildings it just makes sense to use something very similar.
Here is an example to help demonstrate how volume is not proportional to surface area:
Comparison between ACH50 and CFM50/ft² for a 2000 ft² home at 3 ACH50
House Is 50 X 40 X 8
Volume = 16,000 ft³
Surface Area = 50 X 40 X 2 + 180 X 8 = 5440 ft²
CFM50 = (3 X 16000)/60 = 800 CFM
CFM50/ft2 = 800/5440 = 0.147 CFM50/ft²
Increase height to 2 story at 3 ACH50
House Is 50 X 40 X 16 Volume = 32,000 ft³
Surface area = 50 X 40 X 2 + 180 X 16 = 6880 ft²
CFM50 = (3 X 32000)/60 = 1600 cfm
CFM50/ft2 = 1600/6880 = 0.233 CFM50/ft²
In this example, when the volume doubled, the surface area increased by 26%. And when the ACH50 stayed the same, the CFM/ ft² of surface area increased by 58%.
The use of Air Changes per Hour at 50 Pa (ACH50) started at least 60 years ago by researchers who were interested in ways to predict the natural infiltration rate of buildings, which at the time was most commonly measured in Air Changes per Hour. The use of this metric when studying air quality in buildings does make some sense. If a pollutant is suddenly released in a building, the time for the concentration to decay by a certain percentage does depend on the infiltration measured in air changes per hour. The analysis of a tracer gas decay test gives a result in air changes per hour. So when they started measuring airtightness, for use in estimating natural infiltration in air changes per hour, it made sense to use ACH50 as the metric. But two homes with the same volume can have very different surface areas and the purpose of measuring is to determine something about the construction quality. We think ACH is the wrong metric.
Many standards are now trending in the direction of using square foot of enclosure area instead of ACH. Examples include US Army Corp of Engineers, LEED, Swedish and US Passive House.
The location of the outside end of the building pressure tubing is very important when setting up your blower door.
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.
The answer is it depends on the leakiness of the building. The maximum flow through a Model 3 fan at 50 Pa is about 5,800 CFM. A 10,000 square foot house that was built with airtightness in mind may not have 5,800 cfm of leak while a 1,200 square foot Georgian with huge holes to various parts of the building may not be able to reach 50 Pa with the Model 3 fan at full speed. With this information, that is why our fans are rated based on the amount of flow each can generate.
Of all of the environmental effects, wind has probably the largest impact on blower door testing. If you have to test under windy conditions, try to set up the Blower Door on the leeward (downwind) side of the house. Also place your outside reference tube toward the center of the leeward side of the house close to the house. If it looks like it might rain, tape the tube to the house with the opening pointing down and close to the ground.
Use long time averaging periods for both the pre-test baseline house pressure reading and for the CFM50 reading. To do this, use the baseline function on the DG-700 and let it count up for 30 to 60 seconds. Also use the CFM@50 mode on the DG-700 and when the building is at about 50 Pascals, change the time averaging period to Long and allow it to display the long term average for 30 to 60 seconds before recording the CFM50 number. If the wind speed is greater than 5 mph, we recommend that you do a multi-point test using our TECTITE software. The TECTITE software has the ability to calculate the accuracy and repeatability of the test, so you will have an added confidence in your readings.
You will need to turn the fan around so the flow rings and flow sensor are outside. Older fans have a reverse switch, but the switch cannot be used when you are measuring air flow. If the DG-700 is inside the house, you will also need to install a third tube on your DG-700. That tube will go from the reference tap of channel B to the outdoors near the side of the fan, but not in the fans airstream.
A good solution is to bring along a piece of 1.5” or 2” thick rigid foam board. When you get to the house, cut it to size so it will fill in above the blower door frame and bring the height down to about 7’. You can tape any cracks between the foam board and the door frame as needed.
With automated Blower Door testing, you will be able to perform more accurate and repeatable airtightness tests in windy weather conditions where manual testing is extremely difficult or sometimes impossible. By automating the test procedure, the DG-700 (along with the TECTITE software) is able to quickly gather and analyze hundreds of times more readings during a single test sequence than would be practical with a manual Blower Door test. Quickly collecting large samples of data in windy conditions greatly improves the repeatability of your test results.
Automated operation also eliminates many common operator errors and ensures that tests are performed the same way every time. During the automated test, the TECTITE software provides a series of on-screen messages to the operator to ensure that proper testing procedures are followed. It even tells you when to switch flow rings on the Blower Door fan. The software also detects common set-up and equipment problems (e.g. tubing connected to the wrong pressure tap) and provides appropriate warning messages to the operator.
If you are a frequent Blower Door user, automated testing should save you significant time. Automated operation eliminates the need to zero pressure gauges, adjust and tweak the fan speed, write down test data, and manually enter data into an analysis program. And the cruise control feature makes it much easier for 1 person to perform other test procedures such as pressure pan testing or zone pressure diagnostics.
The temperature of air affects its density. When air heats up it expands, and when it cools it contracts. Because of this and other effects, we need to make an air density adjustment. For example, if you performed a test on a house when the inside temp was 70 and the outside temp was 0, and then performed another test on the same house (in the same physical condition) when the inside temp was 70 and the outside temp was 90, you would expect to see test results differences of about 8%, if you didn’t make corrections for temperature.
Corrections for differences in air density due to temperature are automatically made in our TECTITE software using either the CGSB or RESNET test standards. Because of the way the CGSB test standard deals with air density, you do not need to correct for altitude. The RESNET test standard in TECTITE will have you enter the altitude for the test location so it can make corrections for altitude.
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