Cardboard and Masking Tape Airflow Measurement

March 7, 2012
First of all, a big thank you to those of you who took the time to sign the online petition urging the California Energy Commission to continue allowing the use of traditional balancing hoods in the Title 24 energy regulations.

First of all, a big thank you to those of you who took the time to sign the online petition urging the California Energy Commission to continue allowing the use of traditional balancing hoods in the Title 24 energy regulations. Our united voice of over 600 strong was heard. In honor of this occasion we thought we would make and test a fan powered capture hood and report what we found.

The basis for the changes proposed in California’s Title 24 revolve around a report that was issued over a decade ago by a national government energy laboratory attempting to discredit the use of traditional balancing hoods. To quote one representative of the building science industry “This report has more holes in it than Swiss cheese.” Although a skewed report, it created the widespread idea that unless a balancing hood was powered by a fan to compensate for backpressure and non-uniform airflow it’s of little value.

So We Made One and Tested It

David Richardson and I took some time in our Cleveland training center to build and test a fan powered capture hood. Here are some of our findings and conclusions.

Cardboard and blue masking tape was in abundant use as we carefully crafted a transition according to the instructions provided by the Energy Conservatory Duct Blaster™ user’s manual. It took approximately a half hour to attach the Duct Blaster© fan to the traditional balancing hood we were using to capture airflow.

We then connected a series of hoses to the manometer and programmed it for testing supply registers. David Richardson is a big, strong Kentucky boy and I heard him grunt as he struggled to simultaneously keep the fan powered capture hood assembly on a register, adjust the fan speed controller of the Duct Blaster and read the detached manometer until he was able to document an airflow reading.

We concluded very quickly that a field assembled fan powered capture hood was not a one man job.

The Testing

The testing was performed on a system that had a variable speed fan. Testing requires adjustment of the fan speed using the rheostat provided with the Duct Blaster attached to a capture device until a stable reading could be found. The variable speed fan and the fan powered capture hood fought each other for about three minutes until a stable reading could be taken.

Unfortunately, we found this was a typical register test time, and is far too long when holding a device weighing nearly 20 pounds to a ceiling register. Especially when compared to about 15 seconds, with a traditional balancing hood weighing in at almost half the weight.

The Test Results

We were surprised and somewhat excited to find the airflow readings of the fan powered capture hood actually matching our traditional balancing hood. The readings were within a few percent on a series of ceiling supply registers with the duct blaster fan attached directly to the base of the balancing hood.

We concluded that making and using a fan powered capture hood from a Duct Blaster may be a valid test method for an energy rater who did not have access to a traditional balancing hood. However, we doubted if it would be a sustainable test procedure as it’s simply too hard and too time consuming, especially considering the same results can be achieved with a traditional balancing hood.

Floor Resister Testing

To measure floor registers, we adapted the rig to another recommended configuration by attaching a flexible duct to the discharge of the hood and attached the Duct Blaster to the end of the flexible duct.

Although this test can be done far easier by a single person, we found far less accurate readings in this configuration. We were testing duct runs with 20 duct diameters or more of straight metal duct and we traversed the ducts according to ASHRAE Standard 111. In this configuration the powered capture hood reading did not match the traverses or the traditional balancing hood readings as close as it had the previous configuration.

Return Grilles

We also measured return grille airflows with the fan powered capture hood. It took another 20 minutes to remove the duct blaster fan from the hood and flip it over to provide for the change in direction of the airflow, reconnect the hoses and reconfigure the manometer.

On a variable speed fan programmed to move 1200 CFM, a clean and accurate traverse established airflow at the return grille is measured as 1124 CFM. The return is constructed with solid metal ducting between the traverse point and the return grille with zero opportunity for leakage. The traditional balancing hood measured 1144 CFM following NCI testing protocol.

The fan powered capture hood measured 1067 CFM into the central return grille. When compared to the traditional balancing hood which measured 1144 CFM there was a discrepancy of 6.8%. Close enough for government work.

We were satisfied that the field assembled fan powered capture hood had the ability to measure return airflow if a traditional balancing hood was not available. The testing was difficult, the risk of error is high and if anyone was to make a business out of it, they would end up buying a traditional balancing hood and anemometer in a very short period of time due to ease of use and field variables.

The Hood Backpressure Issue

The primary complaint by the decade old skewed balancing hood report claims backpressure of the hood is one of the culprits for its failure to deliver accurate test results. So we measured the backpressure caused by the traditional balancing hood under similar airflows.

A series of tests were completed and the pressures at the grille and register face were measured and recorded with normal flow, and then pressure was measured and recorded once the hood was placed over the grille or register.

While most of the tests showed no measurable difference in pressure, a very slight difference of .0004-in. w.c. to .0012-in. w.c. was measured twice at the highest airflow rates. We wondered what happened to the backpressure issues of the traditional balancing hood?

We did discover backpressure however when we placed a powered flow hood on a register. Under this condition, natural airflow from a register was reduced over 70% until the powered hood was turned on. Our traditional hood has an opening of 1.23 square feet compared to the opening in the powered flow hood of .08 square feet. Under these test conditions, back pressure is a critical issue that’s solved by the powered flow hood.


Field airflow measurement is not perfect. It doesn’t need to be in a field application. What we’re all looking for is a reasonable indication if the system is performing well or not. This series of tests showed us that a fan powered flow hood, works well enough for field-testing under low flow conditions.

If an energy rater doesn’t have a traditional balancing hood, he or she can effectively measure airflow by field adapting test instruments they currently own to do the job. If an energy rater decides to build a fan powered flow hood they will need a helper, as the field assembled fan powered flow hood is not a one person test procedure.

If an energy rater already has a Duct Blaster assembly, adding the new skirt and accessory package offered by The Energy Conservatory will get them into the airflow measurement business and will do the job for low flow residential supply registers and grilles. It also allows for one-person operation.

To be truthful, we had our concerns about how well the fan powered hood would work, but were delighted to find that it did. The bottom line however is that the range and ease of a traditional balancing hood will continue to make it the go-to instrument for airflow measurement at registers and grilles.

Every test instrument has its place. It’s the use of each tool combined with related tests using additional test instruments that can effectively measure and rate the performance of an HVAC system. In the rapidly changing HVAC testing industry we’re looking anxiously for what’s next as long as it truly works. Meanwhile we can accurately test and rate the performance of any air conditioning and heating system with traditional air balancing instruments that meet the demands of what we are testing.

Will we be able to do it better in a few years? Sure we will. The key is to ensure testing is practical enough to do in the field and not just in a laboratory environment.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC based training company and membership organization. If you're an HVAC contractor or technician interested in a free report on how to accurately use a traditional balancing hood, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at for free information, articles and downloads.