FAQ Category: Duct Blaster

When doing a Total Duct Leakage test, should you open the attic hatch? Duct leakage standards and protocols say that if ducts run through unconditioned spaces, those unconditioned spaces should … Read more

A static pressure probe is pictured at the right.  It includes a magnet to help hole it to a duct, a 90⁰ bend, and a sealed bullet  like tip. The shorter side of the tube has four tiny holes in it.  The static pressure is the outward or bursting pressure in a duct.  You can think of it as like measuring tire pressure.

A pitot tube is pictured at the lower right.  It measures both total pressure and static pressure and has locations for connecting two tubes.  When you connect the two tubes to one side of the DG-700, it will give you a reading of the pressure difference between total pressure and static pressure, which is velocity pressure.  If you connect the tubes to the B channel and put the DG-700 in the PR / V mode, it will read out in velocity in feet per minute instead of velocity pressure in Pa. 

Click here to watch a video with more information.

A theoretical question here. I tested a small building that’s around 110 CFM50. I have ring C for the blower door, and that’s what I used for the entire test. Tec-Tite asked to switch to ring D near the end but I don’t have that. During the test the fan speed moved up and down quite a bit as it tried to hold each pressure point. That doesn’t hapen during more normal tests with higher flows.

Any chance this test would be more accurate if done with a duct blaster fan?

Answer
Good question. If you had Ring D or if you used a Duct Blaster, either would have allowed you get those lower data points. So in that sense, yes the Duct Blaster may have led to a slightly more accurate test. So would Ring D have helped. 

But the issue you were experiencing with the fan speed not stabilizing is a curable one regardless of which fan you are using. In TECTITE you want to decrease the Fan Adjust Rate (one of the Auto Test Parameters found on the Test Settings screen). What is happening is that you have a very tight building (and I would guess from your symptoms it is NOT a very small volume). The combination of very tight and medium to large volume leads to fundamentally slow response times for the building pressure to stabilize. What happens is that the auto fan adjust logic starts making corrections even before the final result of the previous correction has been seen. This leads to overshooting and undershooting of the target (oscillations). By decreasing the fan adjust rate (try 0.5 instead of 1.0) you can likely solve the problem. In extreme cases you may need to resort to using “Semi-Auto” mode which is a mode where YOU are controlling the fan, yet the data are all being electronically captured for you. It is found on the same settings screen under “Method”. This allows you to test almost no matter how tight and how large the building is.

Q: Why am I getting weird test results when I am depressurizing ductwork?

We have received a few calls lately from testers that have been seeing unusual readings with a Duct Blaster System while trying to perform a depressurization test on the duct work.  In fact one person tested a sealed length of duct and then measured 30 cfm of leak.  Obviously this raises questions on the validity of other tests taken when depressurizing duct work.  One way we help users with their tests is to review how the system is set up for the test.  We ask them to describe where and how each component is connected together along with the settings of the Pressure and Flow Gauge.  What we have found recently is that people are not using the air straightener and are placing the flow rings in backwards. Here are a few pictures to ensure that your Duct Blaster is set-up correctly for testing.

Question

I have been doing duct leakage tests on single story homes that have all of the ductwork is in the attic. I am testing both total duct leakage and duct leakage to the outside. Quite often am seeing 60 to 80 CFM differences between the tests with the total duct leakage being higher. Is it normal to see a difference in these two numbers? I am wondering if there is anything I could be doing wrong in my set up, or is this a normal occurrence? 

Answer

I suspect that what you are experiencing is an attic space that is not well vented to the outside and / or has large bypasses between the house and attic.  In these instances, the blower door will pressurize the attic and reduce the duct to attic pressures resulting in lower duct leakage to outside numbers.  If you run across this again, try doing the duct leakage to outside test by measuring the duct to attic pressure on channel A instead of the duct to house pressure and your total leakage and leakage to outside numbers should be the same. 

Below is a copy of a section from Chapter 7 of our Duct Blaster manual that describes this situation. 

7.1.a Building Pressure Measurements: 

During the test, you will need to monitor the change in building pressure caused by the Blower Door system. Typically the Blower Door building pressure gauge will be setup to measure building pressure with reference to the outside (this is the typical set up for a Blower Door test). 

However, if you are testing a duct system that is located primarily in one unconditioned zone (e.g. a single attic or single crawlspace), you have the option of setting up the building pressure gauge to measure building pressure with reference to that zone, rather than with reference to outside. The purpose of making this change is to ensure that the duct leaks located in that zone are subjected to the full test pressure. 

For example, it is possible that a crawlspace containing most of the ductwork may be significantly pressurized by air being forced into that zone from the Blower Door fan (through air leaks between the building and the crawlspace). In this case, you may underestimate the duct leakage rate if you are measuring building pressure with respect to outside during your test because the leaks in the crawlspace ductwork will not be subjected to the full test pressure (i.e. they will be subjected to the test pressure minus the crawlspace pressurization caused by the Blower Door fan). Changing the reference tap on the Blower Door building pressure gauge from outside to the crawlspace would eliminate the underestimation problem in this building.

Yes, there is. 

Channel B of the DG-700 is actually measuring a pressure at the flow sensor of the fan and converting the pressure to flow based on the Devise and Configuration settings on the gauge. Appendix B of the Duct Blaster manual has Flow Conversion Charts that allow you to determine what fan pressure the gauge was reading during the test. Here is an example that will demonstrate the process. Let’s say you recorded a reading of 260 CFM and had the DG-700 configuration set to Ring 1. On your way home you are thinking that the home should have been much tighter and you realize that you actually had Ring 2 installed on the Duct Blaster fan.

Here is the process to find the correct reading: 

• Follow down the Ring 1 column until you reach 260 CFM
• Follow across to the Fan Pressure column and you will see that you were measuring a fan pressure of 40 Pascals during the test
• Follow across to the Ring 2 column and see that 99 CFM is the correct reading

Ever wondered how to clean your Duct Blaster fan after using a glycol-based smoke generator or fogger? Simply follow these instructions to get rid of that residue.

Unplug the fan from the controller.  Clean all parts of the fan using soft shop cloth. You can use paper towel, however we recommend a soft cloth. Clean the fan housing, motor mount hardware, exit guard, fan blades, as well as the outside of the motor, and the flow sensor.  You can remove the exit fan guard to reach more of the internal parts.

With any remaining residue, wipe down with a diluted solution of Simple Green or other general purpose cleaner.  Dilution can be as little as a 4 to 1 but no more than 2 to 1.  Using a soft shop cloth with the solution sprayed on the cloth, wipe the fan housing, motor mount hardware, exit guard, fan blades, as well as the outside of the motor, and the flow sensor.

Make sure the holes of the flow sensor are unobstructed.  You can use a pin or paperclip to clear the holes.

Make sure that everything is reassembled and dry before plugging in the controller and using the Duct Blaster fan.

If you have any questions please contact technical support at (612) 827-1117.

Recently we received a question from one of our customers.  It is not a Frequently Asked Question (FAQ) but a good one and thought you should know about this.

“I was wondering if there are products or guidance for sealing a toe kick plate duct during a duct blaster test. Specifically, when conducting a test when the home is complete and the cabinets are already installed, such as a test for a retrofit application. I have seen very little information on potential solutions for getting a good seal on these types of duct registers – especially when taping the inside face of the duct is not sufficient. Any guidance would be appreciated.”

Many times, the HVAC contractor will run the duct to the area of the kitchen cabinet and connect the boot to the subfloor or floor and then the cabinet is placed over the opening.  After the cabinet is installed an opening will be created so that a small register is placed at the toe kick recess of the cabinet.  This creates a number of duct leakage issues.

Usually the water lines are also brought up from the floor.  To do so there are two holes in the floor and then two holes in the bottom of the cabinet.  In addition, there are supply lines and a return drain for the dishwasher drilled through a hole in the side of the cabinet.  Then there is the vent stack for the sink drain that is placed in the outside wall and little care is taken to seal it up properly.  When air moves out of the boot and meets the bottom of the cabinet it must go somewhere.  Positive pressure is created, and air is supposed to follow the arrows on the designer’s drawings by going out the toe kick register.  But it also goes in a lot of other places; through the holes in the floor, through the holes in the cabinet, extend around the cabinet to the back of the dishwasher, and out the wall at the drain stack.  This is then adding to the total duct leakage number as well as a small amount of air that is duct leakage to the outside.

When performing a duct leakage test with a toe kick, place the register sealing tape on the register.  The register is the intentional opening.  Everything behind the register is unintentional leakage and is part of the total duct leakage calculation.

So, when you are baffled by a duct leakage number that seems too high, take a look under the sink.  What you find might surprise you.

ASTM E1554, RESNET Chapter 8, the Duct Blaster® manual and the Retrotec duct tester manual are all in agreement that supply registers and return grilles must be sealed during the duct leakage to outside test.  If you are not taping registers, you are going to have a hard time justifying it.

There has also been research that is in agreement with this requirement.  The Energy Center of Wisconsin did a study for Focus on Energy in August 2008.  The title of the study is A Field Study of Exterior Duct Leakage in New Wisconsin Homes.  Here is a link to that study:  http://www.ecw.org/publications/field-study-exterior-duct-leakage-new-wisconsin-homes.  Among its findings are the following:

“The reliability of measured leakage to the outside from duct pressurization tests depends on the level of leakage to the interior. High interior leakage makes for less precise measurements of exterior leakage.  This reinforces the need to properly (temporarily) seal registers before conducting leakage-to-outside test using duct pressurization.  It also indicates that exterior leakage measurements in homes that use building cavities or panned joists as duct runs are subject to substantial error.

Another aspect of the duct pressurization tests that was investigated was the impact of leakage to inside on the resolution of the estimates of leakage to outside.  Since leakage to outside estimates require that the pressure difference between the home and the ducts to be zero, and small error in adjusting this pressure difference could result in a large error in the leakage estimate if the leakage if the inside is large.  For example, it the pressure between the house and the ducts was actually 0.2 Pascals instead of 0 Pascals, and the leakage to the inside was large, the flow to the inside at 0.2 Pascals could be large, and would be read as additional leakage to the outside.”